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Measuring the Spin Polarization of a Metal with a Superconducting Point Contact
Abstract
A superconducting point contact is used to determine the spin polarization at the Fermi energy of several metals. Because the process of supercurrent conversion at a superconductor-metal interface (Andreev reflection) is limited by the minority spin population near the Fermi surface, the differential conductance of the point contact can reveal the spin polarization of the metal. This technique has been applied to a variety of metals where the spin polarization ranges from 35 to 90 percent: Ni0.8Fe0.2, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2.
... A singlet Cooper pair simply cannot enter such a material. Nevertheless, there are experiments [8,9] observing a significant triplet supercurrent in half-metallic samples of CrO 2 brought in proximity with singlet superconductors. Extremely strong spin polarization of this material (up to 97%) was also experimentally confirmed in a related work [10]. ...
... Superconducting current was detected between two tungsten electrodes through a cobalt nanowire of length 600 nm. While spin polarization of cobalt is [8] as high as 42%, the range of proximity effect in this materials is normally about few nanometers [4]. ...
... On the other hand, relatively clean superconductors with Δ ≳ 1 cannot provide necessary spin-orbit scattering since SO ≫ always. In this case, the triplet component is very small and the magnitude of sub-gap Andreev conductance will rather indicate the amount of spin polarization in the ferromagnet [8]. ...
We calculate the conductance of a junction between a disordered superconductor and a very strong half-metallic ferromagnet admitting electrons with only one spin projection. A usual mechanism of Andreev reflection is strongly suppressed in this case since Cooper pairs are composed of electrons with opposite spins. However, this obstacle can be overcome if we take into account spin-orbit scattering inside the superconductor. Spin-orbit scattering induces a fluctuational (zero on average) spin-triplet component of the superconducting condensate, which is enough to establish Andreev transport into a strong ferromagnet. This remarkably simple mechanism is quite versatile and can explain long-range triplet proximity effect in a number of experimental setups. One particular application of the suggested effect is to measure the spin-orbit scattering time τSO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\tau _{\text {SO}}$$\end{document} in disordered superconducting materials. The value of Andreev conductance strongly depends on the parameter ΔτSO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta \tau _\text {SO}$$\end{document} and can be noticeable even in very disordered but relatively light metals like granular aluminum.
... Obviously, the valley-and spin-resolved bandgap with the magnitude 2|∆ ησ | can be controlled by using the antiferromagnetic exchange field and electric field. The valley and spin polarizations of the AF region can be given by [48,49] ...
... Interestingly, for a small λ AF , the gap-edge conductance G(∆ 0 ) appears as a sharp peak in figure 3(a), indicating the superconducting DOS coherence peak [53,54]. Particularly, the peak is attributed to the formation of Andreev bound states at the gap edge, which can be used to estimate the superconducting energy gap through transport experiments [48]. However, the sharp coherence peak at the gap edge gradually becomes indistinct with increasing λ AF and is eventually replaced by broadened features at λ AF = 5∆ 0 . ...
We theoretically study the valley-polarized subgap transport and intravalley pairing states in silicene-based antiferromagnet/superconductor (AF/SC) junctions. It is found that in the absence of an electric field, the antiferromagnetic order induced in silicene can give rise to valley-polarized states that strongly affect the subgap conductance. With the increasing antiferromagnetic exchange field, the gap-edge Andreev-resonant peak is replaced by broadened features for the Homo-SC model whereas by a sharp conductance dip for the Bulk-SC one. This significant difference arises from the intravalley Andreev reflection caused by the valley-mixing scattering in the Bulk-SC model, which can be enhanced by the antiferromagnetic order. Particularly, this intravalley pairing process can be switched on or off by adjusting the spin polarization through the electric field applied in the AF region. Our findings not only pave a new road to employ antiferromagnetic materials in valleytronics, but also facilitate the verification and detection of potential intravalley pairing state and valley polarization in silicene.
... The rapid advancement of spintronics [9] has recently further heightened interest in Heusler alloys [10,11]. These ternary metals hold immense potential in spintronics due to the generation of spin-polarized currents crucial for devices that rely on a spin and utilize the full-spin polarization at the Fermi energy [12][13][14]. Magnetic Heusler alloys have also garnered considerable attention for spintronic applications due to their high Curie (T C ) temperatures and adjustable structural, electronic, and magnetic characteristics [4,5]. ...
... In 1983, HMF behavior was observed by De Groot et al. for the very first time, by studying band structures of half-Heusler compound such as PtMnSb and NiMnSb [9]. Several researchers have been predicted theoretically as well as experimentally, the behavior of HMF in various types of materials such as perovskite compounds La 0.7 Sr 0.3 MnO 3 [10], Heusler alloys Co 2 MnSi [11], double perovskites Sr 2 FeMoO 6 [12], also in binary compound like Cr-doped ZnTe [13], Cr-based BeTe, BeSe [14], V doped MgSe/MgTe [15], BeTe [16], ZnSe [17] and ZnTe [18]. ...
In this study, spin polarized density functional theory (DFT) is implemented to predict physical characteristic of Be1-xCrxSe (x = 6.25%, 12.5%, 18.75%, 25%) compound. The electronic characteristics of pure BeSe compound show semiconductor behavior but after Cr doping BeSe elucidate half-metallic ferromagnetism (HMF) for all doping concentrations. The outcomes elucidate the total magnetic moment MTot per Cr-atom are 4.0028, 4.0027, 4.0021 and 4.0002 μB for 6.25%, 12.5%, 18.75%, 25% concentrations, respectively and the magnetism mainly originated from d-state of the impurity atom which is further ensured from the magnetic spin density. Furthermore, the optical parameters are also computed to determine the effect of doping on the material’s response to incident light of energy spanning from 0 to 10 eV. The optical study depict that the studied systems possess maximum absorbance and optical conductivity in UV-range with minimal reflection. The overall outcomes illustrate that the Cr doped beryllium selenide (BeSe) is promising material for spintronic and optoelectronic devices.
... As seen from Fig. 2(h), p = ±100% in the regions of a finite N ↑,↓ for one spin and zero N ↑,↓ for another. Unlike RuO 2 (001), the net-spin polarization is nonvanishing for RuO 2 (110), namely p = 31%, which is comparable to the spin polarization of representative FM metals like Fe, Co, and Ni [61,62]. Thus, RuO 2 (110) can be used as a spin detector in MTJs with a single FM electrode. ...
Magnetic tunnel junctions (MTJs) are key components of spintronic devices, such as magnetic random-access memories. Normally, MTJs consist of two ferromagnetic (FM) electrodes separated by an insulating barrier layer. Their key functional property is tunneling magnetoresistance (TMR), which is a change in MTJ's resistance when magnetization of the two electrodes alters from parallel to antiparallel. Here, we demonstrate that TMR can occur in MTJs with a single FM electrode, provided that the counterelectrode is an antiferromagnetic (AFM) metal that supports a spin-split band structure and/or a Néel spin current. Using RuO2 as a representative example of such antiferromagnet and CrO2 as a FM metal, we design all-rutile RuO2/TiO2/CrO2 MTJs to reveal a nonvanishing TMR. Our first-principles calculations predict that magnetization reversal in CrO2 significantly changes conductance of the MTJs stacked in the (110) or (001) planes. The predicted giant TMR effect of about 1000% in the (110)-oriented MTJs stems from spin-dependent conduction channels in CrO2 (110) and RuO2 (110), whose matching alters with CrO2 magnetization orientation, while TMR in the (001)-oriented MTJs originates from the Néel spin currents and different effective TiO2 barrier thickness for two magnetic sublattices that can be engineered by the alternating deposition of TiO2 and CrO2 monolayers. Our results demonstrate a possibility of a sizable TMR in MTJs with a single FM electrode and offer a practical test for using the antiferromagnet RuO2 in functional spintronic devices.
... Due to the fact that their contribution is quite prominent near the fermi level in both spin-states. The following equation can be used to determine the electron's spin polarization at Fermi energy [46], ...
... an ideal material for building spintronic devices. According to previous reports, HMs are found in transition-metal oxides [1][2][3], transition-metal chalcogenides [4][5][6][7][8] or Hessler alloys [9,10]. The rutile structure of CrO 2 is the first experimentally synthesized HM material [11]. ...
Bipolar magnetic semiconductor (BMS) is a class of magnetic semiconductors, whose valence band maximum and conduction band minimum are fully spin-polarized with opposite spin directions. Due to the special energy band, half-metallicity can be easily obtained in BMS by gate voltage, and the spin polarization can be reversed between spin-up and down when the gate voltage switches from positive to negative. BMSs have great potential applications in spintronic devices, such as the field-effect spin valves, spin filters and spin transistors, etc.. With the rapid progress of the two-dimensional (2D) magnetic materials, more and more 2D BMSs have been explored. In this paper, we review the research progress in 2D BMSs.
... † schatterjee@bose.res.in tunneling microscopy [15], and point-contact Andreev reflection measurements [16,17] are utilized to identify an HMF. However, temperature-dependent electrical resistivity and magnetoresistance (MR) are the bulk measurements and can, in principle, precisely identify a true HMF because of the absence of spin-flip electron-magnon scattering at the E F . ...
Half-metallic ferromagnets (HMFs) are ferromagnetic metallic compounds with 100% spin polarization at the Fermi level (EF), and thus they are of particular interest in the field of spintronics, but the identification of HMFs with experiments is a challenging task. In principle, temperature-dependent electrical transport measurements should sensitively probe the half-metallic ferromagnetism since the spin-flip electron-magnon scattering mechanism is expected to be absent. In this work, we perform a systematic temperature-dependent electrical transport measurement on the ferromagnetic full Heusler compound Co2MnGe. Our experimental results reveal that Co2MnGe exhibits an exponential suppression of spin-flip electron-magnon scattering below a characteristic crossover temperature (Δ∼79 K), which suggests that this material possesses a perfect spin polarization at the low temperature. The energy gap (kBΔ) characterizing the suppression of spin-flip electron-magnon scattering is ∼7meV. This key finding is further established by a sign change in magnetoresistance at T≳Δ. Moreover, we have also studied the anomalous Hall effect (AHE) of the present compound. We find a notable change in the ordinary Hall coefficient (R0) and in the carrier density (n) at T≲Δ, which is consistent with the complete absence of minority-spin states at the EF. The anomalous Hall resistivity (ρyxA) is observed to scale near quadratically with the longitudinal resistivity (ρxx), and further experimental analysis indicates that the AHE in Co2MnGe is dominated by the intrinsic Karplus-Luttinger Berry phase mechanism. These findings imply that the Co2MnGe is a rare ferromagnetic full Heusler compound in which half-metallicity coexists with the topology-driven AHE.
... It shows HMF behavior, as spin-up version has semiconducting nature while spin-dn version illustrates the metallic behavior showing 100% spin polarization. The following formula is commonly used to compute spin-polarization in half metallic ferromagnetic (HMF) materials [33]. ...
In present work, the magneto-electronic and optical features of Sr1-xNixTiO3 (x = 12.5%, 25%, 50% and 75%) compounds are calculated using full potential linearized augmented plane wave (FP-LAPW) scheme within density functional theory (DFT) as employed in WIEN2k software. The electronic band structures (BS) and density of states (DOS) interpret the induced half metallic ferromagnetism mainly originating from highly spin polarized Ni-d states. The computed value of total magnetic moment of Sr1-xNixTiO3 is 1.99998, 1.99991, 2.00003 and 2.00005 µB at 12.5%, 25%, 50% and 75% concentration respectively, which emerge primarily due to Ni-3d electrons. Furthermore, the optical features (refraction, dielectric function, absorption, and reflectivity) have also been computed within energy range of 0-10 eV. Sr1-xNixTiO3 is optically active in visible to ultraviolet (UV) region owing to low reflectivity and high absorption. Results portray that the studied compound is a potential contender for its usage in the development of spintronic and optoelectronic devices.
... The spinpolarized currents are typically extracted from the laser-induced charge currents through superdiffusive spin scattering, 20,21 resulting in a spin polarization rate of 0.2 to 0.4 within the spin diffusion length. 22,23 As a result, an external magnetic field is usually required to saturate the magnetization of the FM materials, although field-free emitters have recently been realized by utilizing exchange bias between antiferromagnetic and FM nanofilms. 24 In the second step, the efficiency of the relativistic spin-to-charge conversion is characterized by the spin-Hall angle γ. ...
... 38 Table II shows the half-metallic gap (g h ) too, which represents the difference (ΔE) between the unoccupied and occupied energy levels above and below the Fermi energy in the spin-down channel, respectively. 39 The half-metallic gap assisted us in determining the exchange mechanism nature in the spinels under investigation. Figures 5-6 show the calculated PDOS used for the magnetic behaviour analysis of Cs 2 VCl 6 and Cs 2 VBr 6, respectively. ...
Spintronics is an emerging field based on the usage of electronic-spin for various technological applications. Herein, we explored the ferromagnetic characteristics of novel double-perovskite compounds Cs2VX6 (X=Cl, Br) within the framework of density functional theory. In search of the most stable ground state, we found the ferromagnetic state is at the lowest possible energy state. The structural stability in the cubic phase was assessed from Goldschmidt tolerance factor and thermodynamic stability was confirmed from negative values of formation energies for both compounds. Existence of all positive frequencies in phonon spectra further validates the thermodynamic stability. Spin-polarized density of states and band structures revealed the half-metallic ferromagnetism nature of the elapsolite under investigation. The 100% spin polarization factor along with the reasonable magnetic moment values suggests the potential of these compounds in spintronic applications. Additionally, the thermoelectric response, in terms of high power factors, suggests the suitability of these materials for thermoelectric device applications.
... Semi-metallic ferromagnetism is a new class of materials based on the spin of the electron. The concept of spintronics has been applied in many magneto electronics such as magnetic memory devices, magnetic sensors, tunnel junctions, Very useful in the increase in data processing speed increases the integration speed and it also reduces the power consumption [18][19][20][21]. This paper presents the results of all fundamental physical properties of Co 2 CrZ (Z = As, B, Ga, Pb) compounds using the WIEN2k code and the Atomic Tool Kit-Virtual Nanolab (ATK-VNL) code. ...
Two separate computational algorithms have been used in the current investigation of Full-heusler compounds. One is the pseudo-potential approach used in the Atomistic Tool Kit-Virtual Nanolab, while the other is FP-LAPW method as implemented in WIEN2k. In the computational code, these compounds exhibit mattelic character in both the majority and minority spin channels. According to the WIEN2k and ATK-VNL codes, the computed magnetic moments of these compounds Co2CrZ (Z = As, B, Ga, and Pb) are 4.93 µB and 5.02 µB, 3.00 µB and 3.08 µB, 3.02 µB and 3.16 µB, and 4.07 µB and 4.30 µB, respectively. Between the estimated value and the Slater-Pauling rule, we discovered excellent agreement. These compounds' optical characteristics include reflectivity, refractive index, excitation coefficient, and absorption coefficient, among others. An analysis has been done on the electron energy loss and optical conductivity. Both the electron energy-loss function and the absorption coefficient increase as the energy value increases. As per the results of elastic properties, Co2CrZ compounds with Z = As, Pb are ductile by nature, whereas those with Z = B, Ga are brittle. Co2CrZ (Z = As, B, Ga, and Pb) compounds exhibit metallic behaviour when their Cauchy pressure (CP = C12 - C44) value is positive.
... The lowest-energy spin configuration is then analyzed further. Furthermore, we analyze the magnetic configurations for the percentage of spin 054431-2 polarization (SP) [57,58] using ...
Considering the vast compositional space of Heusler alloys, first-principles-based calculations are ideally suitable for predicting the ground state structure and tailoring the magnetic properties of these alloys. We perform density-functional-theory-based calculations for step-by-step identification of the most stable phase of Fe2MnSn, taking into account all the different structural phases exhibited for this alloy (viz., cubic L21, cubic XA, tetragonal L21, tetragonal XA, and hexagonal D019), followed by the calculations of magnetic properties. We identify the magnetic ground state of each phase and then the most stable structural phase by taking into account electronic and geometric relaxation, spin polarization, and vibrational free energy contributions. The ferromagnetic configuration of all the phases is found to be energetically the most favorable magnetic state, while the ferromagnetic hexagonal phase is identified to be the stable structural phase of Fe2MnSn, with a sizable magnetization of 6.45µB/f.u. Furthermore, the exchange interactions in the hexagonal phase are calculated using the Liechtenstein approach, and this phase shows a high Curie temperature of 729 K attributed to the strong Fe-Fe exchange coupling. The stable hexagonal phase reveals an in-plane magnetic anisotropy of −1.24 MJ/m3. The large magnetization and high Curie temperature of this phase can make this material suitable for desired magnetic applications. Computational investigations such as this one, in addition to being a cost effective pathway, may provide many valuable insights for the experimental realization and application of a given alloy.
... To estimate the spin polarisation in the chemically ordered L1 0 and L1 1 CoPt samples, we perform point contact Andreev reflection (PCAR) spectroscopy experiments 30,36,[73][74][75][76] . In the PCAR technique, spin polarisation in the ballistic transport regime can be determined from fitting the bias dependence of the conductance with the a modified Blonder-Tinkham-Klapwijk (BTK) model 77 . ...
Ferromagnetic films with perpendicular magnetic anisotropy are of interest in spintronics and superconducting spintronics. Perpendicular magnetic anisotropy can be achieved in thin ferromagnetic multilayer structures, when the anisotropy is driven by carefully engineered interfaces. Devices with multiple interfaces are disadvantageous for our application in superconducting spintronics, where the current perpendicular to plane is affected by the interfaces. Robust intrinsic PMA can be achieved in certain Co[Formula: see text]Pt[Formula: see text] alloys and compounds at any thickness, without increasing the number of interfaces. Here, we grow equiatomic Co[Formula: see text]Pt[Formula: see text] and report a comprehensive study on the structural, magnetic, and spin-polarisation properties in the [Formula: see text] and [Formula: see text] ordered compounds. Primarily, interest in Co[Formula: see text]Pt[Formula: see text] has been in the [Formula: see text] crystal structure, where layers of Pt and Co are stacked alternately in the [100] direction. There has been less work on [Formula: see text] crystal structure, where the stacking is in the [111] direction. For the latter [Formula: see text] crystal structure, we find magnetic anisotropy perpendicular to the film plane. For the former [Formula: see text] crystal structure, the magnetic anisotropy is perpendicular to the [100] plane, which is neither in-plane or out-of-plane in our samples. We obtain a value for the ballistic spin polarisation of the [Formula: see text] and [Formula: see text] Co[Formula: see text]Pt[Formula: see text] to be [Formula: see text].
The method of linearized full-potential augmented plane waves based on density functional theory (DFT) is employed to investigate the structural, elastic, magnetic electronic, and thermoelectric properties of the cerium-based half-Heusler alloy FeCeSi. The exchange correlation functional is treated with the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) and the Tran-Blaha-modified Beck-Johnson (TB-mBJ) as implemented in the Wien2k package. According to our results, we have discovered that the material studied is mechanically stable, which means that this compound can be synthesized experimentally. Furthermore, FeCeSi exhibits a half-metallic behavior obeying the Slater-Pauling rule with an integer magnetic moment of 2 μB. The electronic band structures and density of states confirm the half-metallic character with an indirect band gap equals to 0.51 eV and 0.59 eV for GGA-PBE and TB-mBJ approximation, respectively. For the study of the thermoelectric parameters, such as Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), and figure of merit (ZT), the Boltzmann transport equations within the framework of DFT have been used. Significant values for the figure of merit and Seebeck coefficient indicate promising candidate for useful thermoelectric applications for FeCeSi alloy. So far, no experimental or theoretical investigations have been carried out on the half-Heusler alloy FeCeSi. Accordingly, our theoretical results concerning structural, elastic, electronic, magnetic, and thermoelectric properties will probably be confirmed by experimental investigations.
Heusler compounds belong to a large family of materials and exhibit numerous physical phenomena with promising applications, particularly ferromagnetic Weyl semimetals for their use in spintronics and memory devices. Here, anomalous Hall transport is reported in the room‐temperature ferromagnets NiMnSb (half‐metal with a Curie temperature (TC) of 660 K) and PtMnSb (pseudo half‐metal with a TC of 560 K). They exhibit 4 µB/f.u. magnetic moments and non‐trivial topological states. Moreover, NiMnSb and PtMnSb are the first half‐Heusler ferromagnets to be reported as Weyl semimetals, and they exhibit anomalous Hall conductivity (AHC) due to the extended tail of the Berry curvature in these systems. The experimentally measured AHC values at 2 K are 1.8 × 10² Ω−1 cm⁻¹ for NiMnSb and 2.2 × 10³ Ω⁻¹ cm⁻¹ for PtMnSb. The comparatively large value between them can be explained in terms of the spin‐orbit coupling strength. The combined approach of using ab initio calculations and a simple model shows that the Weyl nodes located far from the Fermi energy act as the driving mechanism for the intrinsic AHC. This contribution of topological features at higher energies can be generalized.
In this study, we investigate the electronic, magnetic, and optical properties of the Ba2FeReO6 double perovskite using density functional theory within the framework of the full-potential linearized augmented plane Wave method. We employ various approximations including GGA-PBE, GGA + U (Hubbard Coulomb correction), and GGA plus Spin-Orbit coupling (GGA + SOC). The densities of states and spin-polarized band structure reveal a half-metallic behavior with a magnetic moment of 3 μ B for all approximations, with iron and rhenium atoms contributing the most to the overall magnetic moment. Regarding optical properties, it can be stated that the Ba2FeReO6 material exhibits strong absorption and holds potential for application across a wide energy range spanning from visible to ultraviolet light spectra in future spintronics and optoelectronic technologies.
Typically, organic multiferroic junctions (OMFJs) are formed of an organic ferroelectric layer sandwiched between two ferromagnetic electrodes. The main scientific interest in OMFJs focuses on the magnetoresistive properties of the magnetic spin valve combined with the electroresistive properties associated with the ferroelectric junction. In consequence, memristive properties that couple magnetoelectric functionalities, which are one of the most active fields of research in material sciences, are opening a large spectrum of technological applications from nonvolatile memory to elements in logic circuits, sensing devices, energy harvesting and biological synapsis models in the emerging area of neuromorphic computing. The realization of these multifunctional electronic elements using organic materials is presenting various advantages related to their low-cost, versatile synthesis and low power consumption functioning for sustainable electronics; green disintegration for transient electronics; and flexibility, light weight and/or biocompatibility for flexible electronics. The purpose of this review is to address the advancement of all OMFJs including not only the achievements in the charge and spin transport through OMFJs together with the effects of electroresistance and magnetoresistance but also the challenges and ways to overcome them for the most used materials for OMFJs.
A theoretical study of magnetic, electronic structure, mechanical and thermodynamic properties of Cr2TaZ (Z = Al, Ga and Sb) Heusler alloys has been extensively investigated by the full-potential linearized augmented plane wave (FP-LAPW) method with the Generalized Gradient Approximation (GGA). Structural parameters such as lattice constant \((a)\), bulk modulus \((B)\) and first pressure derivative of bulk modulus \(({B}^{,})\) were obtained by using the Murnaghan equation. The calculated lattice constants for Cr2TaAl, Cr2TaGa and Cr2TaSb alloys are for 61,029, 61,170 and 60,795 Å, respectively. It is found that the L21-type (AlCu2Mnl-type) structure is energetically more stable than the X-type (CuHg2Ti-type) structure due to the lower total energy. The results of elastic constants robustness show that the mechanical stability of Cr2TaZ (Z = Al, Ga and Sb) can be well-maintained. The calculated electronic band structure reveals the metallic nature of Cr2TaZ (Z = Al, Ga and Sb) and its total magnetic moment of 2.00 \({\mu }_{B}\) is mainly contributed by Cr atom from strong spin splitting effect, as indicated with the distinctive distributions of the density of states in two spin directions. Furthermore temperature and pressure dependence of thermodynamic properties of these materials have been examined in the ranges (0–1400 K) and (0–25 GPa), respectively.
Nontrivial topological superconductivity has received enormous attention due to its potential applications in topological quantum computing. The intrinsic issue concerning the correlation between a topological insulator and a superconductor is, however, still widely open. Here, we systemically report an emergent superconductivity in a cross junction composed of a magnetic topological insulator MnBi2Te4 and a conventional superconductor NbSe2. Remarkably, the interface indicates the existence of a reduced superconductivity at the surface of NbSe2 and a proximity-effect-induced superconductivity at the surface of MnBi2Te4. Furthermore, the in-plane angular-dependent magnetoresistance measurements unveil distinctive features indicative of unconventional pairing symmetry in these superconducting gaps. Our findings extend our views and ideas of topological superconductivity in the superconducting heterostructures with time-reversal symmetry breaking, offering an exciting opportunity to elucidate the cooperative effects on the surface state of a topological insulator aligning a superconductor.
Odd-frequency pairing is an unconventional type of Cooper pairing in superconductors related to the frequency dependence of the corresponding anomalous Green's function. We show by a combination of analytical and numerical methods that odd-frequency pairing is ubiquitously present in the current of Andreev-scattered particles across a junction formed by a normal metal (N) and a superconductor (S), even if the superconducting pairing is of conventional s-wave, spin-singlet type. We carefully analyze the conductance of NS junctions with different pairing symmetries (s wave, p wave, d wave). In all cases, we identify a generic equal balance of even- and odd-frequency pairing to the contributions related to Andreev reflection. This analysis shows in retrospect that the presence of odd-frequency pairing in electric currents across NS junctions is rather the rule, not the exception. This insight stems from an alternative approach of analyzing the transport problem of hybrid structures. It is based on the Kubo-Greenwood formula with direct access to symmetries of the anomalous Green's functions characterizing the superconducting pairing. We expect that our predictions substantially enrich the interpretation of transport data across NS junctions in many material combinations.
In this study, we employed density functional theory coupled with the full-potential linearized augmented plane-wave method (FP-LAPW) to investigate the structural, electronic, and magnetic properties of the Ti2FeAs alloy adopting the Hg2CuTi-type structure. Our findings demonstrate that all the examined structures exhibit ferromagnetic (FM) behaviour. By conducting electronic band structure calculations, we observed an energy gap of 0.739 eV for Ti2FeAs in the spin-down state and metallic intersections at the Fermi level in the spin-up state. These results suggest the half-metallic (HM) nature of Ti2FeAs, where the Ti-d and Fe-d electronic states play a significant role near the Fermi level. Additionally, the obtained total magnetic moments are consistent with the Slater–Pauling rule (Mtot = Ztot − 18), indicating 100% spin polarization for these compounds. To explore their optical properties, we employed the dielectric function to compute various optical parameters, including absorption spectra, energy-loss spectra, refractive index, reflectivity, and conductivity. Furthermore, various thermodynamic parameters were evaluated at different temperatures and pressures. The results obtained from the elastic parameters reveal the anisotropic and ductile nature of the Ti2FeAs compound. These findings suggest that Ti2FeAs has potential applications in temperature-tolerant devices and optoelectronic devices as a UV absorber.
Half-metallic ferromagnetic Heusler alloys attracted much attention in the last few decades due to their significant physical properties as well as its potential applications in spintronic devices. To explore and predict the various physical properties of Co-based Heusler alloy, Co2MnSi (CMS), First principles calculations are of interest and has been used in the present study. Optimized E–V curves represent that CMS alloy is most stable in its magnetic L21 structural phase. The electronic charge density plot represents the ionic bonding along with co-valent character for the compound. DOS and BS calculations shows 100% spin polarization at Fermi level (EF) which confirms the half metallic character of the compound. DOS exhibited the prominent role of Co-3d and Mn-3d electronic states along with Si-2p electronic states. Estimated magnetic moment of Co2MnSi Heusler alloy was found to be ~ 5μB per f. u. which is in line with the Slater-Pauling behaviour. Dynamical stability of Co2MnSi Heusler alloy has also been verified by the phonon dispersion curves which revealed that compound is dynamically stable. Additionally, the physical properties have also been explored as a consequence of SOC and significant changes in electronic and optical properties were observed. The observed significant results of electronic, magnetic, and optical properties of CMS Heusler alloy makes it promising towards potential application in optical and spintronic devices.
The mechanical and electronic properties of perovskite compounds Cs2KFeF6, Cs2KVF6, Cs2TiVF6 and Cs2TlVF6 were comprehensively studied. It was found that the four compounds have 100% spin polarizability in the equilibrium state, Cs2KFeF6 exhibits bipolar magnetic semiconductor with a band gap of 1.72 eV, and Cs2KVF6, Cs2TiVF6 and Cs2TlVF6 show half-metallic properties with half-metallic gap of 1.75 eV, 0.94 eV and 1.76 eV, respectively. All four compounds show mechanical stability and varying degrees of anisotropy, and calculations of the ideal tensile strength show that they resist well in one direction at different strains. It is found that Cs2KFeF6 exhibits stable semiconductor characteristics under strain and strong light absorption in visible region under tensile strain. Cs2KVF6, Cs2TiVF6 and Cs2TlVF6 maintain robust half-metallic properties throughout the strain interval. The total magnetic moments of the equilibrium and strains are always kept at integer values, and the transition metal atoms contribute the most to the total magnetic moments. The high spin polarizability, modulable visible light absorption and wider half-metallic gap of these four perovskite compounds indicate that have demonstrated significant potential and wide-ranging application prospects in the exploration of next-generation optoelectronic devices and spintronics.
Using first-principles quantum-transport calculations, we investigate spin-dependent electronic and transport properties of antiferromagnetic tunnel junctions (AFMTJs) that consist of (110)-oriented antiferromagnetic (AFM) metal RuO2 electrodes and an insulating TiO2 tunneling barrier. We predict the emergence of a giant tunneling magnetoresistance (TMR) effect in a wide energy window, a series of barrier layer thicknesses, and different interface terminations, indicating the robustness of this effect. We show that the predicted TMR cannot be explained in terms of the global transport spin-polarization of RuO2 (110) but is well understood based on matching the momentum-dependent spin-polarized conduction channels of the two RuO2 (110) electrodes. We predict oscillations of TMR with increasing barrier thickness, indicating a non-negligible contribution from the perfectly epitaxial interfaces. Our work helps the understanding of the physics of TMR in AFMTJs and aids in realizing efficient AFM spintronic devices.
We studied the topological features in Kondo insulator SmB6 by point-contact Andreev reflection spectroscopy with a Nb probe tip. Below the superconducting transition of Nb, the spectra exhibited a narrow dip at around zero bias superposed on a broad asymmetric background caused by the Kondo resonance. The contact-size dependence of the spectra revealed that the width of the resonance is suppressed near the surface. The spin polarization in the surface state was ∼0.56, which is smaller than that in Bi-based topological insulators. These indicate that the Kondo breakdown plays a crucial role for the topological features, not only in the bulk state near the surface but also in the surface state on SmB6.
Semiconductor nanowires coupled to superconductors can host Andreev bound states with distinct spin and parity, including a spin-zero state with an even number of electrons and a spin-1/2 state with odd-parity. Considering the difference in spin of the even and odd states, spin-filtered measurements can reveal the underlying ground state. To directly measure the spin of single-electron excitations, we probe an Andreev bound state using a spin-polarized quantum dot that acts as a bipolar spin filter, in combination with a non-polarized tunnel junction in a three-terminal circuit. We observe a spin-polarized excitation spectrum of the Andreev bound state, which can be fully spin-polarized, despite strong spin-orbit interaction in the InSb nanowires. Decoupling the hybrid from the normal lead causes a current blockade, by trapping the Andreev bound state in an excited state. Spin-polarized spectroscopy of hybrid nanowire devices, as demonstrated here, is proposed as an experimental tool to support the observation of topological superconductivity.
We present computational results on electronic, magnetic, and structural properties of CoVMnSb, a quaternary Heusler alloy. Our calculations indicate that this material may crystallize in two energetically close structural phases: inverted and regular cubic. The inverted cubic phase is the ground state, with ferrimagnetic alignment, and around 80% spin polarization. Despite having a relatively large bandgap in the minority-spin channel close to the Fermi level, this phase does not undergo a half-metallic transition under pressure. This is explained by the “pinning” of the Fermi level at the minority-spin states at the Γ point. At the same time, the regular cubic phase is half-metallic and retains its perfect spin polarization under a wide range of mechanical strain. Transition to a regular cubic phase may be attained by applying uniform pressure (but not biaxial strain). In practice, this pressure may be realized by an atomic substitution of non-magnetic atoms (Sb) with another non-magnetic atom (Si) of a smaller radius. Our calculations indicate that 25% substitution of Sb with Si results in a half-metallic regular cubic phase being the ground state. In addition, CoVMnSb0.5Si0.5 retains its half-metallic properties under a considerable range of mechanical pressure, as well as exhibits thermodynamic stability, thus making this alloy attractive for potential spintronic applications. We hope that the presented results will stimulate experimental efforts to synthesize this compound.
We investigate superconducting hybrid nanowires defined by patterned gates on a two-dimensional heterostructure of InAs and Al, with lateral quantum dots operating as single-level spectrometers along the side of the nanowire. Applying magnetic field along the wire axis spin splits dot levels, providing spin-resolved spectroscopy. We investigate spin and charge polarizations of subgap states in the nanowire and their evolution with magnetic field and gate voltage.
By employing the full-potential linearized augmented plane wave (FP-L/APW[Formula: see text]lo) technique based on density functional theory (DFT), the structural, electronic and magnetic properties of zincblende W[Formula: see text]Ag x Ge alloys ([Formula: see text] and 0.25) are scrutinized thoroughly. Based on the generalized gradient approximation (GGA), the exchange-correlation energy functional is included in the current simulation. For computing the structural features, the GGA approximation is applied, whereas GGA, [Formula: see text] and the modified Burke–Johnson of [Formula: see text] (TB-mBJ-[Formula: see text] approximations are incorporated to perform the calculations of electronic and magnetic behaviors of these alloys. The structural analysis of the alloys indicated that the total energy of the W[Formula: see text]Ag[Formula: see text]Ge alloy was favorable in the ferromagnetic ground state. The spin-polarized electronic structure shows the half-metallic behavior of the W[Formula: see text]Ag[Formula: see text]Ge alloy, while the WGe compound is identified as a metal. The magnetic results obtained from the half-metallic W[Formula: see text]Ag[Formula: see text]Ge alloy increasingly support the full half-metallicity of this compound because an integer value is acquired for the total magnetic moment. The strong hybridization between 4p-Ge and 3d-W states brings forth weak local magnetic moments at the nonmagnetic Ge atomic sites and reduces the local magnetic moment of the W atomic sites from its free space charge.
Careful tunneling studies in high quality Co/Al2O3/Ni80Fe20 junctions show a junction magnetoresistance (JMR) of 20.2% and 27.1% at 295 and 77 K, respectively, where the latter is in agreement with Julliere's model. The temperature dependence of the JMR can be explained by the temperature dependence of surface magnetization. The decrease of the JMR with increasing de bias is intrinsic to ferromagnetic junctions. The strong disagreement with recent theories in the low de bias region can be attributed to magnetic excitations in these junctions, as seen in inelastic tunneling measurements.
We have performed X-ray photoemission spectroscopy (XPS) on thin films
of (001) and (200) oriented La 0.67Ca 0.33MnO
3 grown on (100) and (110) SrTiO 3 substrates by
off-axis sputtering. The films were examined by XPS without exposing
them to air. We have compared the core levels and the valence spectra
between the two different orientations, as well as after the effects of
air exposure and annealing in UHV. We find that the surfaces are very
stable against exposure to air. Comparing the measured intensity ratios
to a model for the uniform termination of the film shows the terminating
layer to be MnO 2 for both the (001) and (200) oriented La
0.67Ca 0.33MnO 3 films.
A new field that has come to be called “spin‐polarized transport” is growing dramatically. Although its roots are in the quantum description of solids, only recently have new material fabrication techniques permitted widespread study of the phenomenon and the development of device applications (see figure 1). Electrons have spin as well as charge, and this may make all the difference in future electronics.
We propose a simple theory for the I-V curves of normal-superconducting
microconstriction contacts which describes the crossover from metallic
to tunnel junction behavior. The detailed calculations are performed
within a generalized semiconductor model, with the use of the Bogoliubov
equations to treat the transmission and reflection of particles at the
N-S interface. By including a barrier of arbitrary strength at the
interface, we have computed a family of I-V curves ranging from the
tunnel junction to the metallic limit. Excess current, generated by
Andreev reflection, is found to vary smoothly from
4Δ3eRN in the metallic case to zero for the tunnel
junction. Charge-imbalance generation, previously calculated only for
tunnel barriers, has been recalculated for an arbitrary barrier
strength, and detailed insight into the conversion of normal current to
supercurrent at the interface is obtained. We emphasize that the
calculated differential conductance offers a particularly direct
experimental test of the predictions of the model.
We consider spin-dependent tunneling between two ferromagnets separated by a simple step barrier, and examine four models for the magnetoconductance ratio ΔG/G: A model due to Julliere which characterizes the magnetoconductance solely in terms of the tunneling spin polarization, a model due to Slonczewski which provides an approximate expression for the magnetoconductance of free electrons tunneling through a barrier, the exact expression for the magnetoconductance of free electrons tunneling through a barrier, and the numerical calculation of the magnetoconductance of band electrons in iron tunneling through a barrier. We find that the Julliere model does not accurately describe the magnetoconductance of free electrons tunneling through a barrier. Although Slonczewski’s model provides a good approximation to the exact expression for free electrons in the limit of thick barriers, we find that the tunneling of band electrons shows features that are not described well by any free electron picture and which reflect the details of the band structure of iron at the Fermi energy.
CrO2 films were deposited on Al2O3 and TiO2 by high-pressure decomposition of CrO3. They are metallic, with a residual resistivity ρ0 in the range 0.2–10 μΩ m. The resistivity below Tc varies as ρ0+αT2+βT7/2 as expected for a correlated fully spin-polarized d band. At Tc the resistivity changes slope but the sign remains positive. There is a small negative magnetoresistance (∼−0.5%/ T) in a wide temperature range below Tc. Coercivity of the films varies according to the substrates from 7 mT for Al2O3 (110) to 15 mT for TiO2 (110). © 1997 American Institute of Physics.
In a recent paper we proposed a detailed theory for the I-V curves of a
normal-superconductor microconstriction, predicting a smooth crossover
from metallic to tunnel-junction behavior. This paper describes
experiments on Cu-Nb point contacts which span the crossover regime. We
find excellent quantitative agreement between theory and experiment over
a substantial range of contact resistances.
Molecular beam epitaxy methods have been used to grow good quality magnetic single‐crystal films of bcc Fe on fcc GaAs substrates. These films were characterized by Auger, reflection high energy electron diffraction (RHEED) and ferromagnetic resonance (FMR) techniques. Both RHEED and FMR indicate that the best crystal quality occurs for substrate temperatures in the 175–225 °C range. For 200‐Å films the Fe surface lattice constant agrees with that of bulk α‐Fe.
Values of the electron spin-polarization and their proportionality to bulk moments are easily explained by realizing that they are due to the itinerant d-like electrons. The proportionality of the tunneling current of superconductors and ferromagnets to the three-dimensional density-of-states is seen to occur naturally in the independent particle model. In contrast to tunneling, photoelectrons are dominated by localized electrons; these are also seen to be in agreement with calculated band structure.
The band structure of Mn-based Heusler alloys of the C1b crystal structure (MgAgAs type) has been calculated with the augmented-spherical-wave method. Some of these magnetic compounds show unusual electronic properties. The majority-spin electrons are metallic, whereas the minority-spin electrons are semiconducting.
Spin-resolved photoemission from polycrystalline Cr${\mathrm{O}}_{2}$ films shows a spin polarization ($P$) of nearly +100% for binding energies near 2 eV below the Fermi level (${E}_{\mathrm{F}}$). Despite the metallic resistivity behavior, extremely low intensity is observed in the photoemission spectra near ${E}_{\mathrm{F}}$. Our findings are in contrast to recent band-structure calculations, predicting Cr${\mathrm{O}}_{2}$ to be a half-metallic ferromagnet with $P=+100%$ at ${E}_{\mathrm{F}}$.
The transport properties of a ferromagnet-superconductor (FS) junction are studied in a scattering formulation. Andreev reflection at the FS interface is strongly affected by the exchange interaction in the ferromagnet. The conductance G_FS of a ballistic point contact between F and S can be both larger or smaller than the value G_FN with the superconductor in the normal state, depending on the ratio of the exchange and Fermi energies. If the ferromagnet contains a tunnel barrier (I), the conductance G_FIFS exhibits resonances which do not vanish in linear response -- in contrast to the Tomasch oscillations for non-ferromagnetic materials.