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µ-PES Studies on TiNCl and Quasi-two-dimensional Superconductor Na-intercalated TiNCl
... For the first data of the PES study in α-form samples, micro-photoemission spectroscopy (µ-PES) with highly focused synchrotron soft X-ray was performed by Kataoka et al. [30]. In the µ-PES, the angle integrated PES spectra corresponding to the electronic density of states (DOS) of the valence band of TiNCl and Na x TiNCl were observed, as shown in Figure 3 [30]. ...
... For the first data of the PES study in α-form samples, micro-photoemission spectroscopy (µ-PES) with highly focused synchrotron soft X-ray was performed by Kataoka et al. [30]. In the µ-PES, the angle integrated PES spectra corresponding to the electronic density of states (DOS) of the valence band of TiNCl and Na x TiNCl were observed, as shown in Figure 3 [30]. The shape of the valence band of TiNCl roughly corresponds to that of band structure calculations. ...
... The shape of the valence band of TiNCl roughly corresponds to that of band structure calculations. The peaks around 6 eV are hybridized states of Ti 3d, N 2p, and Cl 3p orbitals, and the 4 eV shoulder structure is dominated by the Cl 3p component [30]. ...
Layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively. These compounds are band insulators and become metals and show superconductivity after electron doping by intercalating alkali metals between the layers. The superconductivity of β-form had been extensively characterized from decades ago, but it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schriefer mechanisms. The titanium compound TiNCl crystallizes only in the α-form structure. TiNCl also exhibits superconductivity as high as ~16 K after electron doping by intercalating metals and/or organic basis. It is important to compare the superconductivity of different M–N networks. However, α-form compounds are vulnerable to moisture, unlike β-form ones. The intercalation compounds are even more sensitive to humid air. Thus, there are few experimental studies on the superconducting mechanism of α-form, although it has been discussed for exotic Cooper-pairing mechanisms. This short review gathers the recent progress in experimental studies of TiNCl.
... Upon Na intercalation between TiNCl layers, TiNCl becomes a superconductor with a relatively high superconducting transition temperature of ~16 K [7]. As electron-doped -form HfNCl and ZrNCl [8][9][10][11][12][13][14][15][16][17][18][19][20], electron-doped TiNCl is considered a candidate for unconventional superconductors, where exotic mediation forces for Cooper pairing other than phonon are discussed [21][22][23][24][25]. ...
... The initial valence band spectrum of TiNCl had a peak at 6 eV with a shoulder structure at 4 eV, and there was almost no intensity in the near-EF region. The overall spectral shape is consistent with that of the previous study [25], and thus the states of approximately 6 and 4 eV can be ascribed to Cl 3p and N 2p orbitals, respectively. ...
... The peaks at 457.5 eV and 463.5 eV are spin-orbit split Ti 2p, namely Ti 2p3/2 and Ti 2p1/2, respectively [43]. The initial Ti 2p spectrum showed a sharp Ti4+ peak at 457.5 eV [25] and a small shoulder structure at 456.5 eV. In the Ti 2p spectrum, as the irradiation time increased, the peak area of Ti4+ decreased compared to the initial value. ...
We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (EF) and the Ti3+ component increased, while the Cl 2p intensity decreased. Ti 2p-3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.
... Upon Na intercalation between TiNCl layers, TiNCl becomes a superconductor with a relatively high superconducting transition temperature of ∼16 K [7]. As electron-doped β-form HfNCl and ZrNCl [8][9][10][11][12][13][14][15][16][17][18][19][20], electron-doped TiNCl is considered a candidate for unconventional superconductors, where exotic mediation forces for Cooper pairing other than phonon are discussed [21][22][23][24][25]. * Author to whom any correspondence should be addressed. ...
... The initial valence band spectrum of TiNCl had a peak at 6 eV with a shoulder structure at 4 eV, and there was almost no intensity in the near-E F region. The overall spectral shape is consistent with that of the previous study [25], and thus the states of approximately 6 and 4 eV can be ascribed to Cl 3p and N 2p orbitals, respectively. ...
... The peaks at 457.5 eV and 463.5 eV are spin-orbit split Ti 2p, namely Ti 2p 3/2 and Ti 2p 1/2 , respectively [43]. The initial Ti 2p spectrum showed a sharp Ti 4+ peak at 457.5 eV [25] and a small shoulder structure at 456.5 eV. In the Ti 2p spectrum, as the irradiation time increased, the peak area of Ti 4+ decreased compared to the initial value. ...
We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (EF) and the Ti3+component were increased, while the Cl 2pintensity was decreased. Ti 2p-3dresonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3dcharacter. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.
... The material pri-TiNCl has several interesting physical properties. For example, the photoinduced local metallization of semiconducting pri-TiNCl is observed in photo-emission experiments [7,8], and local nanoscale superconducting areas appear inside the semiconducting background as observed by the scanning tunneling microscopy/spectroscopy (STM/STS) experiments [9]. In these experiments, the line profiles of dI/dV spectra exhibited small noticeable gap peaks around |V| ~ -10 mV within the scale of < 1 nm, suggesting local superconductivity induced by carrier doping because of Cl desorption at the nanoscale. ...
... The BJs were formed by cracking the sample at the liquid helium temperature [14,15]. The conductance curves dI/dV(VBJ) were measured by an ac modulation technique using a four-probe method in the range 4.2 K ≤ T ≤ 214 K. pri-TiNCl FFT b 0 ≈ 0.325 nm for pri-TiNCl [2,7]), the observed atomic constants were slightly shorter but almost of the same values. The typical example of the dI/dV (V) curves for pri-TiNCl obtained by averaging over whole areas of the STS measurements is shown in Fig. 1 (a). ...
Scanning tunneling microscopy and spectroscopy (STM/STS) and break-junction tunnel spectroscopy (BJTS) measurements are performed on the pristine layered α-TiNCl semiconductor (pri-TiNCl), being the precursor of superconducting nitride chlorides. The STM topography of pri-TiNCl shows basic crystal structures with the lattice periods of a 0 ≈ 0.38 nm and b 0 ≈ 0.31 nm, ensuring that a clean a-b surface of micro-single crystals is obtained. From the STS measurements, the averaged conductance versus voltage dependence dI/dV(V) reveals kink structures at relatively high bias voltages of |V| ~-50 mV and-100 mV. The spatial (r-) conductance distributions dI/dV (V, r) (dI/dV maps) proportional to those of quasiparticle local densities of states (LDOSes) exhibit the bias-independent streak structures with the period of (4.8±0.2) b 0 , which are predominantly observed within the range of |V| < 95 mV. The temperature dependence of the dI/dV BJTS spectra shows the gap structure with the energy scale of 4Σ = 180 meV vanishing at T s ~ 120 K. Thus, the ratio of the gap Σ to the transition temperature T s is 2Σ/k B T s ~ 10. Here, k B is the Boltzmann constant. Such a ratio is typical of the pseudogap features in cuprate superconductors and dielectric gap characteristics in layered chalcogenides with charge-density waves.
... Several classes of the "unconventional" superconductors, such as cuprates [1][2][3][4][5], layered nitride chlorides [6], metal diborides [7] and iron superconductors [8,9], possess similar two-dimensional (2D) conductive layer structures and demonstrate certain characteristic gap features including the so-called pseudogap. Among them, layered nitride chloride superconductors MNCl (M = Zr, Hf, Ti) constitute the unique kind of materials that has the maximum T c ~ 25.5 K, with the 2D conducting layers with rectangular lattice (-type) or honeycomb lattice (-type) of MN networks [10][11][12][13][14][15][16][17][18][19][20]. Generally, the gap features are one of the most important properties for the superconductivity. ...
... Both images clearly show atomic lattice of rectangular shape with the periodic length of a 0 ≈ 0.38 nm, b 0 ≈ 0.32 nm (Na x TiNCl) and a 0 ≈ 0.38 nm, b 0 ≈ 0.31 nm (pri-TiNCl), indicating that the cleaved surface was a-b surface of the crystal. The lengths were slightly shorter than those given by the XRD analysis (a 0 ≈ 0.402 nm and b 0 ≈ 0.327 nm for Na x TiNCl, a 0 ≈ 0.393 nm and b 0 ≈ 0.325 nm for pri-TiNCl) [11,16]. As shown by the dashed lines in Fig. 1 (a), the STM image of the Na-doped sample additionally possessed a different streak pattern along the b-axis direction with the longer period of the 4 ~ 5 b 0 . ...
... First, the top of the valence band of TiNCl_A lies at ∼1.5 eV, comparable to the previously reported value ( Figure S3). 33 There is a tiny intensity at the E F , appearing as a localized state. This fact is compatible with the semiconductor-like properties of hopping conduction suggested by the resistivity measurement. ...
... The Ti 2p 3/2 peak has a broad shoulder-like component in the lower binding energy side, similar to the valence splitting of previously reported electron-doped samples. 33,34 Third, the prominent peaks of TiNCl_A shift to the higher binding energy side about 0.3 eV compared to TiNCl_C, even though the sample was not a Na-doped one. The same chemical shift is also observed in the core-level spectrum of the other elements N 1s and Cl 2p ( Figure S4). ...
A synthetic route in a closed system for layered titanium nitride chloride TiNCl has been developed using sodium amide NaNH2 as a nitrogen source. A highly crystalline sample is obtained by an appropriate thermal decomposition of aminated titanium chloride. The obtained TiNCl was also characterized using electronic resistivity measurement and photoemission spectroscopy. TiNCl showed hopping conduction compatible with an in-gap state revealed by photoelectron spectroscopy. However, it appeared highly electron-doped, albeit without showing superconductivity. Comparison with the spectrum of superconducting sodium-doped samples suggests the presence of the microstructure required to exhibit superconductivity.
Layered titanium nitride chloride (TiNCl) exhibits a relatively high superconducting transition of ∼16 K, and it is a candidate for the unconventional superconductor. To better understand the superconductivity, we modified the synthesis process for TiNCl. Highly crystalline TiNCl was prepared by chemical vapor transport at different transport temperatures and intercalated with sodium (Na) metal. Although the crystallographic parameters obtained through Rietveld refinement are similar, the superconducting properties strongly depend on the treatment temperature of pristine TiNCl. Optical and photoemission spectroscopy revealed an in-gap state below the Fermi level, indicating hydrogen incorporation, similar to the case of β−ZrNCl. Hydrogen temperature-programmed desorption revealed hydrogen emission against the heat treatment of the obtained pristine TiNCl. The in-gap state of hydrogen inhibits the introduction of electron carriers from the intercalated Na metal.
Recently, as a novel technique, electronic double-layer transistors (EDLTs) with ionic liquids have shown strong potential for tuning the electronic states of correlated systems. EDLT induced local carrier doping can always lead to dramatic changes in physical properties when compared to parent materials, e.g. insulating-superconducting (SC) transition. Generally, the modification of gate voltage (V G ) in EDLT devices produces a direct change on the doping level. Here, we report that the processing temperature (T G ) also plays a dominant role in the electric field induced superconductivity in layered ZrNBr single crystals. When applying V G at 250 K, the induced SC state is irreversible in the material, which is confirmed in the zero resistance and diamagnetism after long-time relaxation at room temperature and/or by applying reverse voltage, whereas the solid/liquid interface induced reversible insulating-SC transition occurs at 235 K. These experimental facts support another electrochemical mechanism that electric field induced partial deintercalation of Br ions could cause permanent electron doping into the system. Our findings in this study will extend the potential of electric fields for tuning bulk electronic states in low-dimension systems. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
Recently emerging two-dimensional (2D) superconductors in atomically thin layers and at heterogeneous interfaces are attracting
growing interest in condensed matter physics. Here, we report that an ion-gated zirconium nitride chloride surface, exhibiting
a dome-shaped phase diagram with a maximum critical temperature of 14.8 kelvin, behaves as a superconductor persisting to
the 2D limit. The superconducting thickness estimated from the upper critical fields is ≅ 1.8 nanometers, which is thinner
than one unit-cell. The majority of the vortex phase diagram down to 2 kelvin is occupied by a metallic state with a finite
resistance, owing to the quantum creep of vortices caused by extremely weak pinning and disorder. Our findings highlight the
potential of electric-field–induced superconductivity, establishing a new platform for accessing quantum phases in clean 2D
superconductors.
Electric field tuning of superconductivity has been a long-standing issue in solid state physics since the invention of the field-effect transistor (FET) in 1960. Owing to limited available carrier density in conventional FET devices, electric-field-induced superconductivity was believed to be possible in principle but impossible in practice. However, in the past several years, this limitation has been overcome by the introduction of an electrochemical concept, and electric-field-induced superconductivity has been realized. In the electric double layer (EDL) formed at the electrochemical interfaces, an extremely high electric field is generated and hence high-density charge carriers sufficient to induce superconductivity exist and are collectively used as a charge accumulation device known as an EDL capacitor. Field-induced superconductivity has been used to establish the relationship between T-c and carrier density and can now be used to search for new superconductors. Here, we review electric-field-induced superconductivity using an FET device, with a particular focus on the latest advances in EDL transistors.
In their article, Zhang et al. [Phys. Rev. B 86, 024516 (2012)] present a
remarkable result for A$_x$(S)$_y$TiNCl compounds ($\alpha$-phase TiNCl
partially intercalated with alkali A and optionally co-intercalated molecular
species S), finding the superconducting transition temperature T$_C$ scales
with $d$$^{-1}$, where the spacing $d$ between TiNCl layered structures depends
on intercalant thickness. Recognizing that this behavior indicates interlayer
coupling, Zhang et al. cite, among other papers, the interlayer Coulombic
pairing mechanism picture [Harshman et al., J. Phys.: Condens. Matter 23,
295701 (2011)]. This Comment shows that superconductivity occurs by
interactions between the chlorine layers of the TiNCl structure and the layers
containing A$_x$, wherein the transverse A$_x$-Cl separation distance {\zeta}
is smaller than $d$. In the absence of pair-breaking interactions, the optimal
transition temperature is modeled by T$_{C0}$ $\propto$
({\sigma}/$A$)$^{1/2}$$\zeta$$^{-1}$, where {\sigma}/$A$ is the fractional
charge per area per formula unit. Particularly noteworthy are the rather
marginally-metallic trends in resistivities of A$_x$(S)$_y$TiNCl, indicating
high scattering rates, which are expected to partially originate from remote
Coulomb scattering (RCS) from the A$_x$ ions. By modeling a small fraction of
the RCS as inducing pair-breaking, taken to cut off exponentially with {\zeta},
observations of T$_C$ < T$_{C0}$ are quantitatively described for compounds
with {\zeta} < 4 {\AA}, and T$_C$ $\approx$ T$_{C0}$ for
Na$_{0.16}$(S)$_y$TiNCl with propylene carbonate and butylene carbonate
co-intercalants for which {\zeta} > 7 {\AA}. Since a spatially separated
alkali-ion layer is not formed in Li$_{0.13}$TiNCl, the observed T$_C$ of 5.9 K
is attributed to an intergrowth phase related to TiN (T$_C$ = 5.6 K).
Spectroscopic techniques including X-ray photoelectron spectroscopy (XPS) can identify particular chemical groups of humic acids (HA) from “Terra Preta de Índios” (TPI) or Amazonian dark earth, the highly fertile anthropogenic soil found in the Amazonian region. The high fertility and resilience of these soils cannot be explained by their chemically inert pyrogenic C content alone, but the natural aging of this C generates reactive carboxyl functional groups attached directly to the recalcitrant polycondensed aromatic backbone. Through spectroscopic techniques used in this work, the HA fraction (the alkaline-soluble organic matter that precipitates at low pH) of the TPI soil was compared with humic and fulvic acids, obtained by oxidizing activated charcoal with sodium hypochlorite. The yields recovery of HA-like substances was 12 and 28 wt% by using 10 and 20 cmol L−1 of oxidizing agent, respectively. X-ray photoelectron spectroscopy, energy dispersive X-ray, and solid-state 13C nuclear magnetic resonance (13C NMR) spectroscopies were used to evaluate the elements and structures present in all samples. XPS C 1 s spectra of HA extracted from TPI soil and from prepared HA showed aromatic structures (C = C and π–π* shake-up satellite peak) bounded to carboxyl groups (COOH). The morphology and polycondensation level of aromatic C were evaluated by scanning electron microscopy (SEM). The similarities of the spectra indicated that the used method was efficient to obtain an organic amendment similar to TPI soil organic matter.
Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on
α-(DDA)xTiNCl
(DDA=H2N-(CH2)10-NH2,
Tc=16 K) have been carried out. The STM topography exhibits
simple rectangular shaped atomic lattice with the periods of |a|=0.38 nm
and |b| =0.33 nm. The averaged dI / dV spectrum shows the gap value of
Δ ~ 9 meV, giving an unusual large gap ratio
2Δ/kBTc ≃ 13. The STS results show the
bar-shaped domain structures along the b-axis direction in the bias
range of V = +5 ~ +15 mV, demonstrating the possible existence of
intercalated molecules.
We have performed photoemission (PE) spectroscopy of β-ZrNCl and NaxZrNCl to study intercalation-induced change in the electronic structure. The valence band PE spectrum of β-ZrNCl shows a small intensity region within ∼2.5 eV of EF with a very small hump at EF that can be ascribed to hydrogen impurity states. These provide spectroscopic evidence that β-ZrNCl is an n-type semiconductor. Upon Na intercalation, new states possibly involving hybridized Zr4d and N2p systematically evolve at EF, with a negligible shift in the valence band structures. Similar results in β-HfNCl and NaxHfNCl indicate that these results represent common electronic features of the layered nitride chloride superconductors.
We use photoemission (PE) and x-ray absorption (XA) spectroscopy to study intercalation-induced changes in the electronic structure of beta-HfNCl and Na0.22HfNCl. PE shows appearance of new states in Na0.22HfNCl within the gap of beta-HfNCl, with negligible changes in the energy positions of valence-band features of the parent, suggesting a non-rigid-band behavior. N 1s XA results show that the tail of the conduction band, with a quasi-two-dimensional character, extends to lower energy in Na0.22HfNCl than in beta-HfNCl. The study provides information on the electronic structure and its change across the insulator-to-metal transition of the new class of superconductors.
We have performed soft X-ray emission (SXE) and high-resolution photoemission (PE) spectroscopy to study the electronic structures of β-HfNCl and Na-intercalated HfNCl. Both the PE and SXE spectra of Na0.37HfNCl show an additional structure within the band gap of β-HfNCl, without an apparent shift in the energy position of valence-band features. These results clearly reveal intercalant-induced changes in the electronic structure of the layered nitride chlorides. From these results, we propose non-rigid band like behavior for this system in contrast to generally expected rigid-band filling upon intercalation.
Scanning tunneling microscopy and spectroscopy (STM-STS) measurements have been carried out on the α (FeOCl)–type KxTiNCl (x∼0.5, Tc=16 K) and β (SmSI)–type HfNCly (y∼0.7, Tc=24 K) layered nitride superconductors. The STM images at 5 K showed clear atomic arrangements for both the compounds, namely, the rectangular lattice on α-KxTiNCl and the triangular lattice on β-HfNCly. The tunneling spectra in the superconducting states at low temperatures demonstrate qualitatively different features between these superconductors. For α-KxTiNCl, the spatial distributions of the density of states and the superconducting gap structures are very inhomogeneous, while those on β-HfNCly are found to be almost homogeneous. The nanoscale electronic features between these compounds correlate with the different lattice structures of the M (=Ti or Hf) N conducting layers, which are caused by the lattice symmetry difference itself or induced by the difference in the local doping distributions in these chemically reactive compounds. The averaged gap magnitudes in the superconducting states, Δ̅ ≃10.2 meV and 7.5 meV for α-KxTiNCl and β-HfNCly, corresponding to the gap ratios 2Δ̅ /kBTc≃ 15 and 7.2, respectively, indicate the unusually strong coupling effects of the superconductivity.
Based on a virtual crystal treatment, we show that alkali doping introduces electron carriers into a single light mass (m* = 0.6), low density of states band in LixZrNCl and NaxHfNCl (which have superconducting Tc up to 25 K) that is a symmetric interlayer combination of dxy,dx2 − y2 character. Doping leads to simple K-centered three-fold symmetrical cylindrical Fermi surfaces that concentrate low-energy scattering into distinct regions of momentum. The lack of any observed magnetic tendencies, together with our calculation of small electron-phonon strength, suggests a pairing mechanism that is distinct from known superconductors.
Tunneling measurements have been carried out on layered superconductors of
the β(SmSI)-type – Li0.48(THF)xHfNCl (THF = C4H8O)
and HfNCl0.7 – by means of break-junction and scanning tunneling
spectroscopy. Break-junction technique reveals Bardeen-Cooper-Schrieffer (BCS) – like gap structures with typical gap values
of 2Δ (4.2K) = 11–12meV for Li0.48(THF)xHfNCl with the highest
Tc = 25.5 K. Some of our measurements revealed multiple gaps and
dip-hump structures, the largest gap 2Δ (4.2 K) ≈
17–20meV closing at Tc. This was shown both by break-junction and
scanning-tunneling spectroscopy. From these experiments it stems that the
highest obtained gap ratio 2Δ/kBTc ~ 8 substantially
exceeds the BCS weak-coupling limiting values: ≈3.5 and ≈4.3 for s-wave and d-wave order parameter symmetry, respectively.
Such large
2Δ/kBTc ratios are rather unusual for conventional
superconductors but quite common to high-Tc cuprates, as well as to
organic superconductors. Our studies allowed to collect much more evidence
concerning the huge pairing energy in those materials and to investigate in
detail the complexity of their superconducting gap spectra. An origin of the
observed phenomena still remains to be clarified.
To explore conditions underlying the superconductivity in electron-doped
TiNCl where T$_c$ = 16 K, we calculate the electronic structure, Wannier
functions and spin and charge susceptibilities using first-principles density
functional theory. TiNCl is the first high-temperature superconductor
discovered in the $\alpha $-structure of the layered transition-metal nitride
family MNCl (M=Ti, Zr, Hf). We construct a tight-binding model based on Wannier
functions derived from the band structure, and consider explicit electronic
interactions in a multi-band Hubbard Hamiltonian, where the interactions are
treated with the random phase approximation (RPA) to calculate spin and charge
susceptibility. The results show that, consistent with TiNCl being a
nonmagnetic material, spin fluctuations do not dominate over charge
fluctuations and both may have comparable impact on the properties of the doped
system.
Superconductivity mostly appears in high carrier density systems and sometimes exhibits common phase diagrams in which the critical temperature Tc continuously develops with carrier density. Superconductivity enhanced in the lightly doped regime has seldom been reported, although it is an ideal direction towards the crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensate (BEC) limits, where the behavior of Cooper pairs changes dramatically. Here we report transport properties and superconducting gaps in single-crystalline lithium-intercalated layered nitrides (LixHfNCl and LixZrNCl) down to the low-doping regime, enabled by a combination of ionic gating device and tunneling spectroscopy. Upon the reduction of doping, both systems display the increase of Tc, up to 24.9 K especially in LixHfNCl, with the concomitant enhancement of two-dimensionality leading to a pseudogap state below 35.5 K as well as the increase of superconducting coupling strengths 2Δ/kBTc reaching 5.9. Such behavior in the carrier density region as low as 1020cm−3 indicates that lightly doped two-dimensional superconductors exhibit the unprecedented nature that is distinct from that in conventional superconductors, and offer a new route to access BCS-BEC crossover.
We present first self-consistent modelling of x-ray photoelectron spectroscopy (XPS) Ti 2p, N 1s, O 1s, and C 1 s core level spectra with a cross-peak quantitative agreement for a series of TiN thin films grown by dc magnetron sputtering and oxidized to different extent by varying the venting temperature Tv of the vacuum chamber before removing the deposited samples. So-obtained film series constitute a model case for XPS application studies, where certain degree of atmosphere exposure during sample transfer to the XPS instrument is unavoidable. The challenge is to extract information about surface chemistry without invoking destructive pre-cleaning with noble gas ions. All TiN surfaces are thus analyzed in the as-received state by XPS using monochromatic Al Kα radiation (hν = 1486.6 eV). Details of line shapes and relative peak areas obtained from deconvolution of the reference Ti 2p and N 1 s spectra representative of a native TiN surface serve as an input to model complex core level signals from air-exposed surfaces, where contributions from oxides and oxynitrides make the task very challenging considering the influence of the whole deposition process at hand. The essential part of the presented approach is that the deconvolution process is not only guided by the comparison to the reference binding energy values that often show large spread, but in order to increase reliability of the extracted chemical information the requirement for both qualitative and quantitative self-consistency between component peaks belonging to the same chemical species is imposed across all core-level spectra (including often neglected O 1 s and C 1 s signals). The relative ratios between contributions from different chemical species vary as a function of Tv presenting a self-consistency check for our model. We propose that the cross-peak self-consistency should be a prerequisite for reliable XPS peak modelling as it enhances credibility of obtained chemical information, while relying entirely on reference binding energy values introduces large ambiguity.
We have studied the electron-doped α-form TiNCl (FeOCl orthorhombic type) in detail and have found
the transition temperature Tc of 18.0 K decreases monotonically with increasing the interlayer spacing d
upon co-intercalation of various kinds of organic solvent molecules with alkali atoms. We conclude that
such kinds of experimental results imply the importance of interlayer Coulomb coupling [Phys. Rev. B 86,
024516 (2012)], which is a strong contrast to the behavior previously found in the electron doped β-form
ZrNCl and HfNCl. In their Comment, Harshman and Fiory [Phys. Rev. B 90, 186501 (2014)] discuss a
detailed model with remote Coulomb scattering to scale the optimized Tc, which is consistent with our
conclusion.
X-ray photoelectron spectroscopy in the core and valence band region was used to study the formation of hydroxyapatite films on the surface of titanium. The approach used achieves the adhesion of hydroxyapatite by the initial formation of a thin, mainly oxide-free, etidronate film on the metal. In this approach, it was not possible to prepare hydroxyapatite films of any reasonable thickness on the titanium surface without prior treatment with etidronic acid. Because hydroxyapatite is a principal component of teeth and bones, it is likely that the coated metals will have desirable biocompatible properties. The hydroxyapatite film showed no changes when the film was exposed to air, water, and 1 m sodium chloride solution as representative components of the environment of the film in the human body. These films formed on titanium may find application in medical implants. The thin hydroxyapatite and etidronate film on the metal show differential charging effects that caused a doubling of some of the spectral features. Copyright © 2012 John Wiley & Sons, Ltd.
Layer-structured metal nitride chlorides MNCl (M=Zr, Hf) were
electron-doped by the intercalation of alkali metals (A=Li and Na) to
form AxMNCl, and then swelled with organic solvent molecules
[tetrahydrofuran (THF) and propylene carbonate (PC)]. Upon swelling, the
organic molecules were cointercalated with the alkali metals, resulting
in the basal spacing of AxMNCl being expanded by 5.6-18.5
Å and the superconducting transition temperatures (Tcs)
increasing by ˜2 K. When the magnetization data were plotted as a
function of temperature, an irreversibility temperature
(Tirr) was observed, where the zero-field-cooled (ZFC) and
field-cooled (FC) magnetization superimpose. The reversible temperature
range (Tc-Tirr) was markedly increased upon
swelling. The slope of the plot of log H (applied magnetic field) vs
log(1-Tirr/Tc) was increased for the swelled
system. The slope n in the power-law relationship for
Na0.28(PC)yHfNCl and
Li0.5(THF)yHfNCl is obtained to be n = 4.8--5.4.
This finding implies a significant weakening of vortex pinning in the
swelled phase. To investigate the irreversibility microscopically,
nuclear magnetic resonance (NMR) measurements were carried out on
Li0.5(THF)yHfNCl. The 1H- and
7Li-NMR relaxation and chemical shift suggested the relevance
of vortex motions to the depinning at Tirr, which is the
depinning temperature between the thermally excited pancake vortex
liquid state at higher temperatures and the pancake vortex glass state
at lower temperatures.
Criterion for occurrence of the high-temperature superconductivity by the pair-hopping mechanism in layered materials is given. Based on the multi-reference density functional theory, vanishing inter-layer single particle hopping processes around the Fermi level and appearance of high-energy unoccupied orbitals mediating the Coulomb-repulsion driven pair scattering among the layers in the ordinal band-structure calculation certify appearance of the super-pair-hopping processes as the off-diagonal process of the quantum density–density fluctuation. The criterion is certified for α-K0.25TiNClα-K0.25TiNCl in the band structure calculation using the generalized gradient approximation.
Intercalation compounds of TiNCl with n-alkyl monoamines (CnH2n+1NH2, 3 ≤ n ≤ 12) and alkylene diamines (H2NCnH2nNH2, 2 ≤ n ≤ 12) have been prepared. Upon intercalation of monoamines, the basal spacing of TiNCl expanded from 7.8 Å to a value in the range of 12.0 to 37 Å, with the alkyl chains oriented in various ways; the stacking of TiNCl layers became turbostratic disordered. Diamines show only a limited expansion of the basal spacing in a range of 11.1 to 12.3 Å with the alkylene chains oriented parallel to TiNCl layers which are mutually shifted along the a- and b-axes in the ab plane, and stacked to form new polytypes with the space group Bmmb or Immm different from Pmmn of the pristine TiNCl. Only diamine intercalated compounds showed superconductivity with the transition temperatures (Tc's) ranging from 6.8 to 17.1 K. The intercalation compounds with diamine molecules with longer alkylene chains tend to exhibit higher Tc's; the compounds with diamines with an even number of carbon atoms tend to have larger superconducting volume fractions. Superconductivities of the intercalation compounds with N,N′-substituted alkylene diamines have been examined, which also showed superconductivity with Tc's in the range of 8.6–15.8 K. It is likely that the charge transfer of the lone pair electrons of amino groups may change the semiconductor TiNCl to superconductors with ordered stacking structures.
Break-junction tunneling spectra have been measured on MgB2 and Li0.5(THF)yHfNCl to investigate the nature of the superconducting gap. The observed largest gap values at 4K are /2Delta=18-20 and 11-12meV for MgB2 (Tc=39K) and Li0.5(THF)yHfNCl (26K), respectively. These values lead to the similar ratio 2Delta/kBTc=5-6, which is extremely large in comparison with that of the conventional strong-coupling superconductors.
Highly crystalline TiNCl with the FeOCl structure was prepared by chemical vapor transport, and intercalated with pyridine (Py) and alkali metals (A = Li, Na, K, and Rb). The structure of each intercalation compound was determined by the Rietveld refinement. Pyridine molecules were taken up to a composition Py0.25TiNCl, and arranged with the molecular plane perpendicular to the TiNCl layers. The pyridine intercalated compound showed superconductivity at a transition temperature (Tc) of 8.6 K. The alkali metal intercalated compounds AxTiNCl also became superconductors with much higher Tcs of 16.3 K. Upon intercalation the TiNCl crystalline layers are mutually shifted in such a way that the primitive cell was changed into different kinds of polytypes with centered cells, depending on the dimensions of the intercalants between the layers. The ab initio electrical band calculation suggested that the pyridine and alkali metal intercalated compounds have different density of states (DOS) profiles, and should have different characters in superconductivity.
Break junction tunneling spectroscopy measurements have been carried out on the novel layered nitride superconductor ZrNCl1-x (x\cong 0.3) with Tc=12--14 K. Two kinds of gap structures are reproducibly observed, both of which can be described by the BCS density of states. Values of the gap parameters at 4 K are obtained to be Delta=1.6--2.3 meV and 4.0-5.1 meV. Upon warming, these gap structures disappear at Tc, thereby giving the ratio 2Delta/kBTc=2.7--3.8 and 6.6-8.4, respectively. The former is within the value of conventional superconductors, while the latter is extremely large in comparison with that of the conventional strong-coupling superconductors. The larger ratio is similar to the value of high-Tc cuprates. The origins of such two kinds of gaps are discussed.
Specific heat has been investigated in a layered nitride superconductor, Li0.12ZrNCl, with Tc=12.7 K. The obtained data have shown a marked dichotomy: The specific heat jump at Tc (DeltaC/gammanTc=1.8) and the superconducting gap ratio (2Delta/kBTc=4.6 5.2) have indicated an intermediate to a strong coupling of electrons, while the upper limit of the electron-phonon coupling constant lambda has directly been estimated to be 0.22, which belongs to a weak coupling regime. Furthermore, the rapid increase of gamma as a function of magnetic field suggests that the present material has an anisotropic s wave gap.
Intercalation compounds of TiNCl with ethylenediamine (EDA) and hexamethylenediamine (HDA) were prepared. The basal spacing of TiNCl increased by 3.3-3.9 angstrom upon intercalation, implying that the molecules are lying with the alkyl chains parallel to the TiNCl layers in both compounds. The intercalated compounds showed superconductivity with transition temperatures (T(c)s) of 10.5 and 15.5 K for EDA and HDA, respectively, which are higher than 8.6 K of pyridine (Py) intercalated compound, Py025TiNCl.
A new series of superconductors based on layer structured nitrides has been developed. The general compositions of the nitrides are MNX (M = Zr, Hf; X = Cl, Br, I). The beta-type polymorph consists of MN double layers sandwiched between close-packed halogen layers, which are characterized as semiconductors with a band gap of 3-4 eV. Electrons can be doped to the nitride layers by intercalation of alkali metals between the layers. Upon the intercalation, the compounds become superconductors with the transition temperatures (Tcs) as high as 13 and 25.5 K for beta-ZrNCl and beta-HfNCl systems, respectively. The Tc of the electron doped beta-HfNCl is higher than that observed in any intermetallic compound and suggests that layered nitrides may exhibit Tcs comparable to those observed in layer structured complex copper oxide superconductors. The layer structured nitrides can be variously modified by the amounts of doping, the types of alkali metals, and the interlayer separation, which can be controlled by co-intercalation of organic molecules with alkali metals. This article dicusses topics including the synthesis and structure of the transition metal nitride halides, intercalation, superconductivity, and band structures.
The first layered nitride superconductor is reported! The intercalation of lithium into the layered compound β-ZrNCl (which is shown on the inside front cover of this issue) results in electron transfer from the lithium to the ZrN layers and the conversion of semiconducting β-ZrNCl to a black metallic compound with a superconducting transition temperature of around 12.5 K. Interesting parallels with layered copper oxide superconductors and alkali-metal doped fullerene superconductors should provide insight into the mechanism of high-Tc superconductivity.
Specific heat has been investigated in electron-doped LixMyZrNCl superconductors, in which interlayer spacing d can be controlled by the intercalation of organic molecules M. It has been revealed that, upon enlargement of d by molecule cointercalation, Tc is enhanced concomitantly with the superconducting coupling strength whereas the density of states at the Fermi level is kept almost constant. Structural and spectroscopic analysis proved that the molecules are intercalated without affecting electronic nor vibrational states, simply increasing the interlayer spacing. These results lead us to conclude that the enhancement of Tc is ascribed to the improvement of two dimensionality, which is consistent with a recent theory that discussed superconductivity in a band insulator on a honeycomb lattice with disconnected Fermi surfaces.
High-resolution gold-valence-band photoemission spectra were obtained by the use of monochromatized Al Kα radiation and a single-crystal specimen. After background and scattering corrections were made, the results were compared directly with broadened theoretical density-of-states functions. The following conclusions were drawn: (i) Relativistic band-structure calculations are required to fit the spectrum. (ii) Both the Korringa-Kohn-Rostoker calculation of Connolly and Johnson and the relativistic-augmented-plane-wave calculation by Christensen and Seraphin give density-of-states results that (after broadening) follow the experimental curve closely. (iii) Of the theoretical functions available to date, those with full Slater exchange agree best with experiment (perhaps because of a cancellation of errors). Fractional (2/3 or 5/6) exchange gives d bands that are too wide. (iv) Eastman's 40.8-eV ultraviolet photoemission spectrum is similar to the x-ray spectrum, suggesting little dependence on photon energy above 40 eV. (v) Both (ii) and (iv) imply an absence of strong matrix-element modulation in the photoemission spectrum of gold.
Titanium nitride chloride (TiNCl), a band semiconductor with the α-form layered structure, becomes a superconductor with a transition temperature Tc≈ 18.0 K by electron doping via alkali-metal intercalation. Upon cointercalation of various kinds of organic solvent molecules with alkali atoms, the superconducting layered crystals are swelled to different extents adjusting to the size of the molecules, and the Tc decreases linearly down to 6.5 K as a function of 1/d, where d is the interlayer separation (basal spacing) of the expanded nitride layers, implying the importance of the Coulomb interlayer coupling for superconductivity. This is in strong contrast to a previous finding that the Tc of the electron-doped ZrNCl and HfNCl with the β-form layered structure rather increases with the increase of d upon a similar cointercalation of solvent molecules.
Pressure dependence of critical temperature, lattice constant, and phonon frequency has been investigated for layered nitride superconductors, ZrNCl0.7 and Li0.5(THF)yHfNCl. The analysis of the data in terms of MacMillan’s theory indicated that the relevant phonon frequencies are low (≈50 and 100 cm−1, respectively), and that the electron-phonon coupling constant λ is larger than 3 in both compounds in sharp contrast with previous experimental and theoretical results. This result may suggest a possibility that other bosonic excitation than phonon additionally contributes to the pairing interaction in these materials.
We report 15N nuclear magnetic resonance (NMR) and the nitrogen-isotope effect studies on c-axis oriented sample of Li-doped HfNCl with superconducting transition temperature Tc∼25.5K. 15N NMR Knight shift decreases toward zero below Tc, giving evidence that the pairing symmetry is an even-parity spin singlet, and that the nitrogen site plays an important role in the occurrence of superconductivity. A nitrogen isotope shift is found to be quite small, ΔTc∼0.1K (αN=0.07±0.02), which cannot reproduce such a high Tc within the traditional BCS framework.
The electronic properties of crystals with a layered structure can be radically altered by the intercalation, between the layers, of guest species that act as electron donors or acceptors. Such studies have been performed extensively on graphite, transition-metal dichalcogenides and oxide bronzes. Interest in redox intercalation reactions has increased recently because the high-transitiontemperature (high-T(c)) superconductors based on copper oxide also have layered structures, the superconductivity occurring within two- dimensional CuO 2 planes separated by charge-reservoir oxide layers. Similarly, in metal-doped fullerenes, which show relatively high transition temperatures, the electron donor atoms sit in the interstitial sites between adjacent fullerene balls. In a previous study, we described a layered nitride, β-ZrNCl, consisting of Zr-N double layers sandwiched between two closepacked chlorine layers. On lithium intercalation, the crystal changed from a semiconductor to a metal, and became a superconductor at 13 K. Here we report the properties of the isostructural compound β-HfNCl. After electron- doping the crystal by lithium intercalation, we observe superconductivity with a T(c) of up to 25.5 K. This transition temperature is higher than that observed in any intermetallic compound, and suggests that layered nitride structures may offer transition temperatures comparable to those observed in layered copper oxide structures.
Alkali metal intercalated compounds AxZrNCl (A = Li, Na, K) were synthesized by the reaction of β-zirconium nitride chloride with n-butyllithium or the respective alkali metal naphthalenes. All of the compounds became superconductors with a transition temperature (Tc) of ca. 15 K. Polar solvent molecules such as tetrahydrofuran, dimethylformamide, and propylene carbonate were co-intercalated with the alkali metals, expanding the interlayer spacing. The Tc was dependent solely on the amount of intercalated alkali metals, or the doping level; the kind of intercalated alkali metals and the interlayer separation were not influential to the Tc. Although β-ZrNCl is a semiconductor, only a small amount of doping of alkali metals was necessary for the superconductivity. Too much doping with alkali metals resulted in the decrease of Tc down to ca. 9 K.
The layer structured crystals β-MNCl (M = Zr, Hf) consist of rhombohedrally stacked six-layer [Cl−M−N−N−M−Cl] slabs, each of which is composed of a strongly bonded honeycomb-like double MN layer sandwiched by two close-packed chlorine layers. A part of the chlorine atoms was deintercalated by the reaction with alkali metals (A) formed by the thermal decomposition of the azides AN3 (A = Na, K, Rb). Upon the partial deintercalation of chlorine atoms, electrons were doped into the MN layers, and the semiconducting β-MNCl changed into superconductors with the transition temperatures (Tc's) of 13−14 and 23−24 K for Μ = Zr and Hf, respectively. The thermal stability and the structure of the deintercalated phase β-MNCl1-x are discussed.
Various data analysis techniques of importance in X-ray photoelectron spectroscopy (XPS) are discussed. Inelastic background determination and subtraction and the use of derivative and difference spectra are illustrated with particular bias toward O 1s and C 1s spectra obtained from carbon fibers. Derivative spectra, as a relatively quick method of resolution enhancement, are shown to provide a fairly accurate, though qualitative guide to the makeup of convoluted peak envelopes. The importance of spectral alignment and normalization in difference spectra are illustrated, and procedures for determining optimal results are shown for complex C 1s spectra. Correct spectral alignment is found to be more important than correct normalization for difference spectra.
1Es wurden TiNJ und je zwei Modifikationen von ZrNCl und ZrNBr, ferner TiNCl und TiNBr hergestellt. Die Präparate wurden durch Ammonolyse der Tetrahalogenide bei erhöhter Temperatur erhalten.2Die Nitridhalogenide sind leicht hydrolisierbar und thermisch wenig beständig, das Minimum der Beständigkeit liegt beim TiNJ. Eine Sonderstellung haben die Hochtemperaturmodifikationen β-ZrNCl und β-ZrNBr, die thermisch und chemisch sehr widerstandsfähig sind.3Die Kristallstrukturbestimmung ergab für α-ZrCl, α-ZrNBr, TiNCl, TiNBr und TiNJ den EO5-Typ Raumgruppe D (FeOCl).1TiNCl, TiNBr, TiNJ and (both having two modifications) ZrNCl and ZrNBr have been prepared ammonolyzing the corresponding tetrahalides at elevated temperatures.2The nitride halides are easily hydrolized and are not very stable against thermal decomposition; TiNJ is the least stable compound. A special case are the high-temperature modifications β-ZrNCl and β-ZrNBr which are very stable against thermal and chemical attack.3The EO5-type, space group D (FeOCl-type) is found for α-ZrNCl, α-ZrNBr, TiNCl, TiNBr and TiNJ.
Band dispersion of the layer-structured nitride β-HfNCl, the host insulator of a two-dimensional electron-doped superconductor of Tc=25.5 K, has been revealed by angle-resolved photoemission spectroscopy (ARPES). The dispersion of the highest occupied band suggests that the magnitudes of the direct and indirect band gaps are ∼ 3.7 and ∼ 3.4 eV, respectively. Characteristic features of the overall valence-band dispersion are approximately consistent with the calculation within the local density approximation, while the observed valence-band width ∼ 6.3 eV is 10% larger than calculation. The absence of strong renomalization in the valence-band structure also implies that the high-Tc occurrence in the layer-structured nitrides is unusual.
The layer structured α-HfNBr is isotypic with FeOCl, and forms a Li intercalated compound by the reaction with n-butyllithium. The lithiated compound was swelled with tetrahydrofuran (THF), increasing the basal spacing from 8.6 to 17.3 Å. Similar alkali metal and THF co-intercalated compounds could be prepared by a one-step reaction with Li and Na naphthalene solutions in THF. X-ray structural study showed that the two-dimensional structure of the host crystal was maintained after the swelling with THF. On Li intercalation the non-magnetic α-HfNBr was changed into a paramagnetic compound having localized electron spins down to 2 K.
Three furnace carbon blacks, a thermal black, and an electrically conductive sample were submitted as received to chlorination, at 450 °C, with a mix of chlorine and carbon tetrachloride vapors. The treated samples were examined using chemical and spectroscopic (XPS) methods. The interpretation of the XPS spectra is facilitated by comparison with spectra of reference materials. XPS detects various types of carbon-chloride bonds formed essentially by addition or hydrogen substitution processes. The amount of fixed chlorine is dependent on the origin of the blacks, and there is no direct relationship between specific surface area and chlorine uptake. It is shown that chlorination is not limited to the external surface, nor is it to a sub-superficial layer. In fact, it proceeds inside the carbon black particles to a depth depending on their origin.
DC plasma nitriding was applied to titanium metal sheets in a commercial cell using Ar + NH3 as the admixture and the nitrided surface investigated by means of XPS. As a comparison, in situ nitriding of a chemically pure Ti surface at room temperature was performed by N2+ ion bombardment (1–5 keV) in the electron spectrometer. Synthesis of the complex Ti2p envelope was accomplished using two sets of loss peaks, separated by 1.6 and 3.0 eV from the major TiN-type Ti2p and Ti2p components. A doublet at 458.8 and 464.5 eV was also included to account for a TiO2-type oxide. The sum of the main Ti TiN peak and the two loss peaks were taken to be representative of nearly stoichiometric TiNx with x ranging from 0.85 to 1.15. Further components derived from the peak synthesis were assigned to TiNxOy and Ti2O3. The stoichiometric nitride is represented by a Ti2p doublet at binding energies of 454.7 and 460.6 eV and a single sharp N 1s peak at 396.7 ± 0.1 eV. On the superstoichiometric samples, especially after N2+ bombardment, a second peak appears at about 395.8 eV with a positive correlation between this peak concentration and the relative amounts of species (TiNxOy, Ti2O3, TiO2) derived from Ti2p components and the surface O and N content. Consecutive Ar+, N2+ and (N2+ + O2+) bombardment leads to significant changes in composition together with rearrangement of short-range chemical structure which is reflected in peak-shape changes of the Ti2p and N 1s lines.
Atomic subshell photoionization cross sections and asymmetry parameters are calculated with the Hartree-Fock-Slater one-electron central potential model (dipole approximation) for all elements Z = 1–103. The cross-section results are plotted for all subshells in the energy region 0–1500 eV, and cross sections and asymmetry parameters are tabulated for selected energies in the region 10.2–8047.8 eV. In addition, more detailed graphs are given for the 4d (Z = 39–71) and 5d (Z = 64–100) subshell cross sections in the vicinity of the Cooper minimum. These data should be particularly useful for work based on spectroscopic investigations of atomic subshells using synchrotron radiation and/or discrete line sources.
We have systematically investigated the global phase diagram for Li{x}M{y}HfNCl (M: molecule), demonstrating the independent controllability of carrier density x and interlayer spacing d. In LixHfNCl, the superconducting phase with almost constant T{c} of 20 K prevails for 0.15<or<or.50. Accompanied by an increase in d upon the cointercalation, an enhancement of T{c} up to 30% was observed with its x independence preserved. This result indicates that pairing interaction strongly depends on the Fermi surface warping along the k{z} direction, implying the relevance of charge and/or spin fluctuation.
We have calculated simple approximate expressions for the normalized specific-heat difference between superconducting (S) and normal (N) states at and below Tc and for the ratio gammaT2c/H2c(0), using a square-well model for the gap. While our approach is very different, the formulas obtained have the same general form as those that have already appeared in the literature. Each contains two parameters, which we determine through a phenomenological fit to exact numerical results from Eliashberg theory. The strong-coupling variable is Tc/omegaln, where omegaln is the Allen-Dynes expression for the average phonon energy.
We have studied Y1-xCaxTiO3 by photoemission and inverse-photoemission spectroscopy. Valence-band photoemission spectra show a d-band peak ~1.4 eV below the Fermi level (EF), which evolves into the lower Hubbard band in the x= 0 (d1) limit. The spectra show quasiparticle emission at EF with an extremely small spectral weight, z~0.01, which vanishes as the system approaches either the Mott insulator limit (x=0) or the band insulator limit (x=1). Correspondingly, inverse-photoemission spectra show the upper Hubbard band and a quasiparticle feature in the unoccupied state. The fact that the observed quasiparticle spectral weight is smaller than that of La1-xSrxTiO3 is attributed to the larger U/W, where U is the on-site d-d Coulomb energy and W is the d-band-width. The presence of the ~1.4-eV peak for a nearly empty d band (x~ 1) and the small spectral weight at EF cannot be explained within the Hubbard model, indicating the importance of interactions which are not included in the model, such as the long-range Coulomb interaction and the electron-phonon interaction.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
We report magnetic susceptibility measurements on a layered superconductor Li0.48(THF)0.3HfNCl having Tc approximately 26 K. The present study revealed that (a) the Fermi level density of states is small, N*(EF) approximately 0.25 states/(eV spin f.u.), (b) mass enhancement is negligible, gamma; approximately 1, (c) electron-phonon coupling is weak, lambda(ep)<1, (d) exchange enhancement is negligible, 1/(1+F(a)0) approximately 1, and (e) electronic density parameter is large, r(2D)s approximately 10.3 (i.e., low-carrier density). It is difficult to explain the origin of the high Tc in terms of the conventional phonon (BCS) mechanism of superconductivity.
Specific heat has been investigated in a layered nitride superconductor, Li(0.12)ZrNCl, with T(c)=12.7 K. The obtained data have shown a marked dichotomy: The specific heat jump at T(c) (DeltaC/gamma(n)T(c)=1.8) and the superconducting gap ratio (2Delta/k(B)T(c)=4.6-5.2) have indicated an intermediate to a strong coupling of electrons, while the upper limit of the electron-phonon coupling constant lambda has directly been estimated to be 0.22, which belongs to a weak coupling regime. Furthermore, the rapid increase of gamma as a function of magnetic field suggests that the present material has an anisotropic s wave gap.
We revealed a detailed phase diagram of the very lightly doped regime in Li-intercalated superconductors, LixZrNCl, to which previous studies have never gained access owing to the difficulty in synthesizing single-phase samples. A continuous and uniform Li intercalation without any indication of phase separation was carefully confirmed by means of synchrotron x-ray diffraction and Raman scattering experiments. Upon reducing the carrier density below x=0.12, we found a rapid increase in the superconducting transition temperature (Tc) immediately followed by the superconductor-to-insulator transition (SIT). Such an increase in Tc on the verge of SIT seems to be difficult to explain by the conventional theory, but may be indicative of the charge fluctuation contribution to superconductivity in low-carrier-density systems.