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(color online) Band structure of TiNCl along the orthorhombic symmetry lines, calculated with the GGA exchange-correlation functional. The thick "fatbands" are the tight-binding representation determined by the Wannier functions labeled in the inset, chosen to represent accurately the bands at and around the Fermi level after doping.
Source publication
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 tra...
Contexts in source publication
Context 1
... calculated band structure of pristine TiNCl is shown in Fig. 2 and is generally consistent with that presented by Yamanaka et al. 8 plotted along other lines in the zone. It is an insulator with a calculated energy gap of 0.5 eV. The real band gap may be as large as 1 eV, based on the common observation that LDA and GGA underestimates gaps in insulators. The band structure exhibits clearly a ...
Context 2
... expressed in terms of the orthorhombic coordinate axes, d xy , d z 2 , d xz have most of the weight in the 0.5 − 3 eV region, and d yz , d x 2 −y 2 are in a higher energy window of 4 − 6 eV. Thus it is feasible, in a low-energy tight- binding model, to include only Ti d xy , d z 2 , d xz and N 2p x states. Plotted on top of the DFT bands in Fig. 2 is the tight-binding fit using the Wannier functions. The representation of the full t 2g complex is excellent, as is that of the top of the upper valence ...
Context 3
... and there are 2 Ti and 2 N atoms per unit cell with different z coordinates. the actual tight-binding model contains 8 bands and 8 WFs, but WFs on symmetry related ions are symmetry equivalent. Overall the Wannier orbitals generate a well represented band structure compared to the DFT bands within the energy window of interest, as shown in Fig. ...
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Citations
... They might be related to the band structure of pri-TiNCl. In fact, according to the band calculations [1,16], there is a noticeable kink in the 2p band of N near E ~ -100 meV (the semiconducting gap edge in the occupied state side just below the Fermi level). ...
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.
... In order to understand the pairing mechanism of this possible exotic superconductor, it is important to clarify how electronic structure evolves with carrier doping. The band structure of pristine TiNCl has been calculated by first-principle calculations [3,65]. It exhibits a two-dimensional feature, with the general flatness of the band along the Ґ-Z direction perpendicular to the layers. ...
... It exhibits a two-dimensional feature, with the general flatness of the band along the Ґ-Z direction perpendicular to the layers. The top valence band primarily has a N 2p x character, and the lowest conduction band has a strong Ti 3d xy character [65]. There is 3d weight in the valence bands and N weight in the conduction bands, respectively, reflecting substantial N 2p-Ti 3d hybridization in addition to the ionic character reflected in their formal charges [65]. ...
... The top valence band primarily has a N 2p x character, and the lowest conduction band has a strong Ti 3d xy character [65]. There is 3d weight in the valence bands and N weight in the conduction bands, respectively, reflecting substantial N 2p-Ti 3d hybridization in addition to the ionic character reflected in their formal charges [65]. The β-form compounds have disconnected cylindrical Fermi surfaces favorable for nesting [66,67], while the electron-doped TiNCl has a single oval Fermi surface centered at the Ґ point [65]; the possible nesting between Fermi surfaces discussed for the β-form compounds should be excluded from TiNCl. ...
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.
... Among the TMNHs, group-IVB nitride halides are of great interest since they are environmentally friendly materials. 20 Several studies have been done on layered TMNHs to investigate properties such as superconductivity, 21,22 photocatalyst, 20 energy storage capability, 23 and electronic properties. 24,25 Moreover, we can envision materials like HfNCl to be compatible with existing semiconductor technology since HfCl 4 is a known Atomic Layer Deposition (ALD) precursor 26,27 and nitrogen is omnipresent in semiconductor processing. ...
Using first-principles calculations, we investigate six transition-metal nitride halides (TMNHs): HfNBr, HfNCl, TiNBr, TiNCl, ZrNBr, and ZrNCl as potential van der Waals (vdW) dielectrics for transition metal dichalcogenide (TMD) channel transistors. We calculate the exfoliation energies and bulk phonon energies and find that the six TMNHs are exfoliable and thermodynamically stable. We calculate both the optical and static dielectric constants in the in-plane and out-of-plane directions for both monolayer and bulk TMNHs. In monolayers, the out-of-plane static dielectric constant ranges from 5.04 (ZrNCl) to 6.03 (ZrNBr) whereas in-plane dielectric constants range from 13.18 (HfNBr) to 74.52 (TiNCl). We show that the bandgap of TMNHs ranges from 1.53 eV (TiNBr) to 3.36 eV (HfNCl) whereas the affinity ranges from 4.01 eV (HfNBr) to 5.60 eV (TiNCl). Finally, we estimate the dielectric leakage current density of transistors with six TMNH bilayer dielectrics with five monolayer channel TMDs (MoS2, MoSe2, MoTe2, WS2, and WSe2). For p-MOS TMD channel transistors 25 out of 30 combinations have a smaller leakage current than hexagonal boron nitride (hBN), a well-known vdW dielectric. The smallest bilayer leakage current of 1.15 × 10-2 A cm-2 is predicted for a p-MOS MoSe2 transistor with HfNCl as a gate dielectric. HfNBr, ZrNBr, and ZrNCl are also predicted to yield small leakage currents in certain p-MOS TMD transistors.
... Among the TMNHs, Group-IVB nitride halides are of great interest since they are environmentally friendly materials [18]. Several studies have been done on layered TMNHs to investigate properties such as superconductivity [19,20], photocatalyst [18], energy storage capability [21], and electronic properties [22,23]. Moreover, we can envision materials like HfNCl to be compatible with existing semiconductor technology since HfCl4 is a known Atomic Layer Deposition (ALD) precursor [24,25] and nitrogen is omnipresent in semiconductor processing. ...
Using first-principles calculations, we investigate six transition-metal nitride halides (TMNHs): HfNBr, HfNCl, TiNBr, TiNCl, ZrNBr, and ZrNCl as potential van der Waals (vdW) dielectrics for transition metal dichalcogenide (TMD) channel transistors. We calculate the exfoliation energies and bulk phonon energies and find that the six TMNHs are exfoliable and thermodynamically stable. We calculate both the optical and static dielectric constants in the in-plane and out-of-plane directions for both monolayer and bulk TMNHs. In monolayers, the out-of-plane static dielectric constant ranges from 5.04 (ZrNCl) to 6.03 (ZrNBr) whereas in-plane dielectric constants range from 13.18 (HfNBr) to 74.52 (TiNCl). We show that the bandgap of TMNHs ranges from 1.53 eV (TiNBr) to 3.36 eV (HfNCl) whereas the affinity ranges from 4.01 eV (HfNBr) to 5.60 eV (TiNCl). Finally, we estimate the dielectric leakage current density of transistors with six TMNH monolayer dielectrics with five monolayer channel TMDs (MoS2, MoSe2, MoTe2, WS2, and WSe2). For p-MOS TMD channel transistors, 19 out of 30 combinations have a smaller leakage current compared to monolayer hexagonal boron nitride (hBN), a well-known vdW dielectric. The smallest monolayer leakage current of 2.14*10-9 A/cm2 is predicted for a p-MOS WS2 transistor with HfNCl as a gate dielectric. HfNBr, HfNCl, ZrNBr, and ZrNCl are also predicted to yield small leakage currents in certain p-MOS TMD transistors.
... 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 KF model involves the excitation of core electrons and is applicable to ion bonding. Since the TiNCl is considered to be an ionic insulator composed of TiN + and Cl − [23], we assumed that this mechanism is involved in the Cl desorption of TiNCl. In the case of TiO 2 , photoinduced desorption is explained by the KF model in terms of Coulomb expulsion between Ti 4+ and O + that are induced by emitting Auger electrons after the relaxation of electrons in O 2p levels to create photo-holes in the Ti 3p levels [48]. ...
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.
... 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 KF model involves the excitation of core electrons and is applicable to ion bonding. Since the TiNCl is considered to be an ionic insulator composed of TiN+ and Cl-[23], we assumed that this mechanism is involved in the Cl desorption of TiNCl. In the case of TiO2, photoinduced desorption is explained by the KF model in terms of Coulomb expulsion between Ti4+ and O+ that are induced by emitting Auger electrons after the relaxation of electrons in O 2p levels to create photo-holes in the Ti 3p levels[48]. ...
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.
... [28][29][30] Theoretical studies considering spin and charge fluctuations reported that charge fluctuation is superior to spin fluctuation. 31) More-recent theoretical studies further elaborated the possibility of a charge-fluctuation-mediated model for the superconductivity in electron-doped TiNCl. [32][33][34] To understand the pairing mechanism of this possible exotic superconductor, it is important to clarify how the electronic structure evolves with carrier doping. ...
... MNX compounds are found in two different types of structure. The first type is the FeOCl structure α-type (orthorhombic) and the second type is the SmSI structure β-type (hexagonal) [11]. The most widely studied intercalated superconductor metal nitride halide β-ZrNCl has a superconducting transition temperature T c = 13−15 K [6]. ...
... In the orthorhombic phase, MNX consists of M-N layer net which is topologically equivalent to a single NaCl layer. There is strong buckling of this layer [11] perpendicular to the b axis of the crystal. Due to this buckling, the neighbouring chains of M-N atoms running along the adirection differ in height [11]. ...
... There is strong buckling of this layer [11] perpendicular to the b axis of the crystal. Due to this buckling, the neighbouring chains of M-N atoms running along the adirection differ in height [11]. The M ions are twofold coordinated by the halogen ions residing in the bc-plane. ...
Titanium nitride halides, TiNX (X = F, Cl, Br, I) in the α-phase (orthorhombic) are exciting quasi two-dimensional (2D) electronic systems exhibiting a fascinating series of electronic ground states under different conditions. Pristine TiNX are semiconductors with varying energy gaps and possess attractive properties for potential applications in the fields of optoelectronics, photovoltaics, and thermoelectrics. Alkali metal intercalated TiNCl becomes superconducting at reasonably high temperature in the α-phase. We have revisited the electronic band structure of these compounds using density functional theory (DFT) based first-principles calculations. The atomic species and orbital resolved partial electronic energy density of states are calculated together with the total density of states (TDOS). The structural and elastic properties have been investigated in details for these layered compounds for the first time. The elastic anisotropy has been explored. The optical properties, including energy dependent real and imaginary parts of the dielectric constant, optical conductivity, reflectivity, and loss function of TiNX are studied for the first time. The Debye temperatures of these compounds have been calculated and the related thermal and phonon parameters are discussed. The calculated physical parameters are compared with existing theoretical and experimental results and show fair agreement, where available. All these compounds are found to reflect electromagnetic radiation strongly in the mid ultraviolet region. The elastic properties show high degree of anisotropy. The lattice is highly compressible along the crystallographic c-direction. The effect of halogen atoms on various structural, elastic, electronic, and thermal properties in TiNX are also discussed in detail.
... MNX compounds are found in two different types of structure. The first type is the FeOCl structure α-type (orthorhombic) and the second type is the SmSI structure β-type (hexagonal) [11]. The most widely studied intercalated superconductor metal nitride halide β-ZrNCl has a superconducting transition temperature T c = 13−15 K [6]. ...
... In orthorhombic phase, MNX consists of M-N layer net which is topologically equivalent to a single NaCl layer. There is strong buckling of this layer [11] perpendicular to the b axis of the crystal. Due to this buckling, the neighboring chains of M-N atoms running along the a-direction differ in height [11]. ...
... There is strong buckling of this layer [11] perpendicular to the b axis of the crystal. Due to this buckling, the neighboring chains of M-N atoms running along the a-direction differ in height [11]. The M ions are twofold coordinated by the halogen ions residing in the bc-plane. ...
Titanium nitride halides, TiNX (X = F, Cl, Br, I) in the {\alpha}-phase (orthorhombic) are exciting quasi two-dimensional (2D) electronic systems exhibiting a fascinating series of electronic ground states under different conditions. Pristine TiNX are semiconductors with varying energy gaps and possess attractive properties for potential applications in the fields of optoelectronics, photovoltaics, and thermoelectrics. Alkali metal intercalated TiNCl becomes superconducting at reasonably high temperature in the {\alpha}-phase. We have revisited the electronic band structure of these compounds using density functional theory (DFT) based first-principles calculations. The atomic species and orbital resolved partial electronic energy density of states are calculated together with the total density of states (TDOS). The structural and elastic properties have been investigated in details for these layered compounds for the first time. The elastic anisotropy has been explored. The optical properties, including energy dependent real and imaginary parts of the dielectric constant, optical conductivity, reflectivity, and loss function of TiNX are studied for the first time. The Debye temperatures of these compounds have been calculated and the related thermal and phonon parameters are discussed. The calculated physical parameters are compared with existing theoretical and experimental results and show fair agreement, where available. All these compounds are found to reflect electromagnetic radiation strongly in the mid ultraviolet region. The elastic properties show high degree of anisotropy. The lattice is highly compressible along the crystallographic c-direction. The effect of halogen atoms on various structural, elastic, electronic, and thermal properties in TiNX are also discussed in detail.
... The small anisotropy on H c2 and the Coulomb coupling between the superconducting layers should be the relevant nature of the superconductivity of the α-form layered nitrides [57]. For more discussion on the superconducting mechanisms and the anisotropy of the two different kinds of layered nitrides, a theoretical study including the electric band structure is required [328]. ...
This paper reviews the highlights of a 4-years-long research project
supported by the Japanese Government to explore new superconducting materials
and relevant functional materials. The project found several tens of new
superconductors by examining ~1000 materials, each of which was chosen by
Japanese team member experts with a background in solid state chemistry. This
review summarizes the major achievements of the project in newly found
superconducting materials, and the wire and tape fabrication of iron-based
superconductors. It is a unique feature of this review to incorporate a list of
~700 unsuccessful materials examined for superconductivity in the project. In
addition, described are new functional materials and functionalities discovered
during the project.
