Multiband Character Research Articles

Extremely large magnetoresistance and non-trivial band topology in YSb semimetal

The topologically protected states in a material lead to the occurrence of spectacular properties like massless fermions, high carrier mobility, extremely large magnetoresistance (XMR) and nontrivial Berry phase. In this context, we report the experimental realization of non-trivial band topology and nonsaturating XMR at low temperatures in a rock-salt type rare-earth monopnictide YSb semimetal which is ∼ two million percent at 2 K and 9 T. The Hall resistivity and Shubnikov-de Hass (SdH) oscillations confirm the multiband character of YSb crystal along with high carrier mobility. The analysis of SdH oscillations reveal a π-Berry phase for the α-band, indicating the non-trivial band topology in YSb. This non-trivial band topology is further confirmed by the observation of the topologically protected state, which is demonstrated by a high transport lifetime (τtr) to the quantum lifetime (τq) ratio, r = τtrτq = 300 times. The topological protection results in a very low zero-field resistivity at low temperatures. Here, we show that the topological protection mechanism, charge compensation and high carrier mobility are the key factors for obtaining XMR in YSb crystal.

Discovery of a Superconductor Bi5 O4 S3 Cl Containing the Unique BiS3 Layer.

The BiS2 -based layered superconductors with structures similar to those of cuprates and iron-based superconductors have stimulated much research interest. Here, a new quaternary compound is reported, Bi5 O4 S3 Cl, which crystalizes in a tetragonal structure with P4/mmm (No. 123) space group having alternately stacking unique BiS3 layers and Bi2 O2 layers along the c-axis with a Cl atom located at the center of the unit cell. A superconducting transition above 3 K is observed for both electrical transport and magnetic measurements. Hall resistivity measurements show its multiband character with a conduction dominated by electron-like charge carriers. The first-principles calculations exhibit that the semiconducting parent phase Bi5 O4 S3 Cl becomes metallic when sulfur vacancies are introduced, which hints the origin of superconductivity in Bi5 O4 S3 Cl. The findings will inspire the exploration of new BiS-based superconductors.

Electronic properties and surface states of RbNi[formula omitted]Se[formula omitted

Iron-based superconductors, with the ThCr2Si2-type tetragonal structure (122 family), due to the iron arsenide/selenide layers exhibit several characteristic electronic properties. For example, multiband character mostly associated with the d-orbitals of iron and the quasi-two-dimensional (2D) cylindrical Fermi surface. Moreover, external hydrostatic pressure leads to the isostructural phase transition from the tetragonal to collapsed-tetragonal phase. In this paper, in relation to the iron-based superconductors, we discuss the electronic properties of novel 122-family member RbNi2Se2 (Liu et al., 2022). We show that the two Fermi pockets exhibit quasi-2D character. Calculation of the surface spectral function for the (001) surface shows that the surface states are realized independently on the surface termination. Additionally, contrary to the iron-based 122 compounds, RbNi2Se2 exhibits extraordinary multiple isostructural phase transitions under pressure. Moreover, the Lifshitz transition occurs under external pressure, which results into a strong modification of the shapes of the Fermi pockets.

Multiband character revealed from weak antilocalization in platinum thin films

Platinum (Pt) has been very much used for spin-charge conversion in spintronics research due to its large intrinsic spin-orbit interaction. Magnetoconductance originating from weak antilocalization in the quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and the coherence phase breaking scattering process among itinerant electrons. To acquire the knowledge of different types of scattering processes, we have performed a magnetoconductance study on Pt thin films which manifests multiband (multichannel) conduction. An extensive analysis of quantum-interference-originated weak antilocalization reveals the existence of strong (weak) interband scattering between two similar (different) orbitals. Coherence phase breaking lengths $({l}_{\ensuremath{\phi}})$ and their temperature dependence are found to be significantly different for these two conducting bands. The observed effects are consistent with the theoretical prediction that there exist three Fermi sheets with one $s$ and two $d$ orbital character. This study provides the evidence of two independent nonsimilar conducting channels and the presence of anisotropic spin-orbit interaction along with $e\text{\ensuremath{-}}e$ correlation in Pt thin films.

Effect of Various Electron and Hole Transport Layers on the Performance of CsPbI3-Based Perovskite Solar Cells: A Numerical Investigation in DFT, SCAPS-1D, and wxAMPS Frameworks.

CsPbI3 has recently received tremendous attention as a possible absorber of perovskite solar cells (PSCs). However, CsPbI3-based PSCs have yet to achieve the high performance of the hybrid PSCs. In this work, we performed a density functional theory (DFT) study using the Cambridge Serial Total Energy Package (CASTEP) code for the cubic CsPbI3 absorber to compare and evaluate its structural, electronic, and optical properties. The calculated electronic band gap (E g) using the GGA-PBE approach of CASTEP was 1.483 eV for this CsPbI3 absorber. Moreover, the computed density of states (DOS) exhibited the dominant contribution from the Pb-5d orbital, and most charges also accumulated for the Pb atom as seen from the electronic charge density map. Fermi surface calculation showed multiband character, and optical properties were computed to investigate the optical response of CsPbI3. Furthermore, we used IGZO, SnO2, WS2, CeO2, PCBM, TiO2, ZnO, and C60 as the electron transport layers (ETLs) and Cu2O, CuSCN, CuSbS2, Spiro-MeOTAD, V2O5, CBTS, CFTS, P3HT, PEDOT:PSS, NiO, CuO, and CuI as the hole transport layers (HTLs) to identify the best HTL/CsPbI3/ETL combinations using the SCAPS-1D solar cell simulation software. Among 96 device structures, the best-optimized device structure, ITO/TiO2/CsPbI3/CBTS/Au, was identified, which exhibited an efficiency of 17.9%. The effect of the absorber and ETL thickness, series resistance, shunt resistance, and operating temperature was also evaluated for the six best devices along with their corresponding generation rate, recombination rate, capacitance-voltage, current density-voltage, and quantum efficiency characteristics. The obtained results from SCAPS-1D were also compared with wxAMPS simulation results.

On the Superconducting Critical Temperature of Heavily Disordered Interfaces Hosting Multi-Gap Superconductivity

LaAlO3/SrTiO3 interfaces are a nice example of a two-dimensional electron gas, whose carrier density can be varied by top- and back-gating techniques. Due to the electron confinement near the interface, the two-dimensional band structure is split into sub-bands, and more than one sub-band can be filled when the carrier density increases. These interfaces also host superconductivity, and the interplay of two-dimensionality, multi-band character, with the possible occurrence of multi-gap superconductivity and disorder calls for a better understanding of finite-bandwidth effects on the superconducting critical temperature of heavily disordered multi-gap superconductors.

Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m-1 K-1 ), a high power factor (11.6µW cm-1 K-2 ), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe4 tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl+ lone-pair electrons, boosts phonon-phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl+ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

Interacting holes in Si and Ge double quantum dots: From a multiband approach to an effective-spin picture

The states of two electrons in tunnel-coupled semiconductor quantum dots can be effectively described in terms of a two-spin Hamiltonian with an isotropic Heisenberg interaction. A similar description needs to be generalized in the case of holes due to their multiband character and spin-orbit coupling, which mixes orbital and spin degrees of freedom and splits $j=3/2$ and $j=1/2$ multiplets. Here we investigate two-hole states in prototypical coupled Si and Ge quantum dots via different theoretical approaches. Multiband $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ and configuration-interaction calculations are combined with entanglement measures in order to thoroughly characterize the two-hole states in terms of band mixing and justify the introduction of an effective spin representation, which we analytically derive a from generalized Hubbard model. We find that, in the weak interdot regime, the ground state and first excited multiplet of the two-hole system display---unlike their electronic counterparts---a high degree of $J$ mixing, even in the limit of purely heavy-hole states. The light-hole component additionally induces $M$ mixing and a weak coupling between spinors characterized by different permutational symmetries.

Pressure-induced superconductivity and modification of Fermi surface in type-II Weyl semimetal NbIrTe4

Weyl semimetals (WSMs) hosting Weyl points (WPs) with different chiralities attract great interest as an object to study chirality-related physical properties, topological phase transitions, and topological superconductivity. Quantum oscillation measurements and theoretical calculations imply that the type-II WPs in NbIrTe4 are robust against the shift of chemical potential making it a good material for pressure studies on topological properties. Here we report the results of electrical transport property measurements and Raman spectroscopy studies under pressures up to 65.5 GPa accompanied by theoretical electronic structure calculations. Hall resistivity data reveal an electronic transition indicated by a change of the charge carrier from multiband character to hole-type at ~12 GPa, in agreement with the calculated Fermi surface. An onset of superconducting transition is observed at pressures above 39 GPa, with critical temperature increasing as pressure increases. Moreover, theoretical calculations indicate that WPs persist up to highly reduced unit cell volume (−17%), manifesting that NbIrTe4 is a candidate of topological superconductor.

Irreversible Multi‐Band Effects and Lifshitz Transitions at the LaAlO3/SrTiO3 Interface Under Field Effect

AbstractIn this work, the irreversible effects of an applied electric field on the magnetotransport properties of LaAlO3/SrTiO3 conducting interfaces are investigated, with focus on their multiband character. Samples of different types, namely with either crystalline or amorphous LaAlO3 overlayer, are studied. The two‐band analysis highlights the similarity of the electronic properties of crystalline and amorphous interfaces, regardless much different carrier densities and mobilities. Furthermore, filling and depletion of the two bands follow very similar patterns, at least in qualitative terms, in the two types of samples. In agreement with previous works on crystalline interfaces, it is observed that an irreversible charge depletion takes place after application of a first positive back gate voltage step. Such charge depletion affects much more, in relative terms, the higher and three‐dimensional dyz, dzx bands than the lower and bidimensional dxy, driving the system through the Lifshitz transition from two‐band to single band behavior. The quantitative analysis of experimental data evidences the roles of disorder, apparent in the depletion regime, and temperature. Noteworthy, filling and depletion of the two bands follow very similar patterns in crystalline and amorphous samples, at least in qualitative terms, regardless much different carrier densities and mobilities.

Anisotropy of upper critical field and surface superconducting state in the intermediate-valence superconductor CeIr3

${\mathrm{CeIr}}_{3}$ is a Ce-based superconductor with a superconducting transition temperature ${T}_{\mathrm{c}}=3.4$ K in an intermediate-valence state. We grew high-quality single crystals of ${\mathrm{CeIr}}_{3}$ using the Czochralski method, and measured the electrical resistivity, magnetic torque, and specific heat. The anisotropy of the superconducting upper critical field ${H}_{\mathrm{c}2}$ was determined. The temperature dependence of ${H}_{\mathrm{c}2}$, obtained from the resistivity measurements, suggests the multiband character of the superconductivity in ${\mathrm{CeIr}}_{3}$. Different field-angle dependencies of ${H}_{\mathrm{c}2}$ in electrical transport and thermodynamic measurements indicate the robust surface effect in the bulk superconductivity of ${\mathrm{CeIr}}_{3}$. We analyzed the surface superconductivity based on simple models to reveal the bulk superconducting properties. Our results support the isotropic bulk superconducting state and robust surface superconductivity in ${\mathrm{CeIr}}_{3}$ single crystals.

Anisotropic field dependence of the electronic transport in superconducting FeSe thin films

Electronic transport measurements were carried out on c-axis oriented epitaxial FeSe thin films in static magnetic fields up to 14 T parallel to the crystallographic c-axis and parallel to the ab-plane. The temperature dependence of the upper critical fields and was determined from the field dependence of the resistivity. While was orbitally limited, the larger was paramagnetically limited. The anisotropy was decreasing with decreasing temperature and revealed a pseudoisotropic behaviour towards zero temperature. This temperature dependence is indicative of the multiband character of FeSe. From measurements of the temperature- and field-dependent critical current densities the effective depinning or irreversibility field was obtained, and its angular dependence yielded the effective mass anisotropy . The temperature dependence of , which was different from that of , can be considered as another signature of multiband FeSe.

Electronic transport through correlated electron systems with nonhomogeneous charge orderings

The spinless Falicov-Kimball model exhibits outside the particle-hole symmetric point different stable nonhomogeneous charge orderings. These include the well known charge stripes and a variety of orderings with phase separated domains, which can significantly influence the charge transport through the correlated electron system. We show this by investigating a heterostructure, in which the Falicov-Kimball model on a finite two-dimensional lattice is located between two noninteracting semi-infinite leads. We use a combination of nonequilibrium Green's functions techniques with a sign-problem-free Monte Carlo method for finite temperatures or a simulated annealing technique for the ground state to address steady-state transport through the system. We show that different ground-state phases of the central system can lead to simple metallic-like or insulating charge transport characteristics, but also to more complicated current-voltage dependencies reflecting a multi-band character of the transmission function. Interestingly, with increasing temperature, the orderings tend to form transient phases before the system reaches the disordered phase. This leads to nontrivial temperature dependencies of the transmission function and charge current.

Physical properties and pressure-induced superconductivity in the single-crystalline band insulator SnO

We report the growth and physical properties of high-quality single-crystal SnO via electrical transport, specific heat, Hall coefficients, and the high-pressure effect. Apart from polycrystalline SnO showing an insulating behavior in the whole temperature range, the in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}$ of single-crystal SnO exhibits a metal-insulator transition around the characteristic temperature ${T}_{\mathrm{M}\ensuremath{-}\mathrm{I}}$. The anisotropic resistivity ratio ${\ensuremath{\rho}}_{\mathrm{c}}/{\ensuremath{\rho}}_{\mathrm{ab}}$ is \ensuremath{\sim}1 for $T\ensuremath{\ge}{T}_{\mathrm{M}\ensuremath{-}\mathrm{I}}$ and increases quickly up to \ensuremath{\sim}400 for $T<{T}_{\mathrm{M}\ensuremath{-}\mathrm{I}}$, which implies the enhanced anisotropic electronic structures and electronic correlations. Its multiband electronic character with dominant hole-type carriers is revealed via the Hall coefficient and the appearance of low-lying phonon models, evidenced by specific heat showing an evident peak at \ensuremath{\sim}10 K in $({C}_{\mathrm{p}}\ensuremath{-}{\ensuremath{\gamma}}_{\mathrm{n}}T)/{T}^{3}$ vs $T$. The appearance of a metal-insulator phase transition in single-crystal SnO was attributed to the slight difference in the lattice parameters ratio $c/a$, the atomic coordinate of $Z(\mathrm{Sn})$, and the chemical pressure effect. Under hydrostatic pressures generated in a cubic-anvil pressure cell, the insulating state melts at the critical pressure ${P}_{\mathrm{c}}\ensuremath{\sim}3\ensuremath{-}4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, and the temperature exponent of resistivity $\ensuremath{\rho}\ensuremath{\propto}{T}^{n}$ in the metallic state increases gradually from $n=2$ to 3 with increasing the pressure. A domelike superconductivity is achieved in the diamond pressure cell with the pressure up to 13.5 GPa and the temperature to 80 mK, with the superconducting transition temperatures and the upper critical fields close to those of polycrystals. Several possible physical mechanisms are proposed.

Low-energy excitations in type-II Weyl semimetal Td−MoTe2 evidenced through optical conductivity

Molybdenum ditelluride, MoTe2, is a versatile material where the topological phase can be readily tuned by manipulating the associated structural phase transition. The fine details of the band structure of MoTe2, key to understanding its topological properties, have proven difficult to disentangle experientially due to the multi-band character of the material. Through experimental optical conductivity spectra, we detect two strong low-energy interband transitions. Both are linked to excitations between spin-orbit split bands. The lowest interband transition shows a strong thermal shift, pointing to a chemical potential that dramatically decreases with temperature. With the help of ab initio calculations and a simple two-band model, we give qualitative and quantitative explanation of the main features in the temperature-dependent optical spectra up to 400 meV.

Persistent antiferromagnetic order in heavily overdoped Ca1−xLaxFeAs2

In the Ca1−xLaxFeAs2 (1 1 2) family of pnictide superconductors, we have investigated a highly overdoped composition (x = 0.56), prepared by a high-pressure, high-temperature synthesis. Magnetic measurements show an antiferromagnetic transition at TN = 120 K, well above the one at lower doping (0.15 < x < 0.27).Below the onset of long-range magnetic order at TN, the electrical resistivity is strongly reduced and is dominated by electron–electron interactions, as evident from its temperature dependence. The Seebeck coefficient shows a clear metallic behavior as in narrow band conductors. The temperature dependence of the Hall coefficient and the violation of Kohler’s rule agree with the multiband character of the material. No superconductivity was observed down to 1.8 K. The success of the high-pressure synthesis encourages further investigations of the so far only partially explored phase diagram in this family of Iron-based high temperature superconductors.

Galvanomagnetic properties of the putative type-II Dirac semimetal PtTe2

Platinum ditelluride has recently been characterized, based on angle-resolved photoemission spectroscopy data and electronic band structure calculations, as a possible representative of type-II Dirac semimetals. Here, we report on the magnetotransport behavior (electrical resistivity, Hall effect) in this compound, investigated on high-quality single-crystalline specimens. The magnetoresistance (MR) of PtTe2 is large (over 3000% at T = 1.8 K in B = 9 T) and unsaturated in strong fields in the entire temperature range studied. The MR isotherms obey a Kohler’s type scaling with the exponent m = 1.69, different from the case of ideal electron-hole compensation. In applied magnetic fields, the resistivity shows a low-temperature plateau, characteristic of topological semimetals. In strong fields, well-resolved Shubnikov – de Haas (SdH) oscillations with two principle frequencies were found, and their analysis yielded charge mobilities of the order of 103 cm2 V−1 s−1 and rather small effective masses of charge carriers, 0.11 me and 0.21 me. However, the extracted Berry phases point to trivial character of the electronic bands involved in the SdH oscillations. The Hall effect data corroborated a multi-band character of the electrical conductivity in PtTe2, with moderate charge compensation.

Multiband Mechanism for the Sign Reversal of Coulomb Drag Observed in Double Bilayer Graphene Heterostructures.

Coupled 2D sheets of electrons and holes are predicted to support novel quantum phases. Two experiments of Coulomb drag in electron-hole (e-h) double bilayer graphene (DBLG) have reported an unexplained and puzzling sign reversal of the drag signal. However, we show that this effect is due to the multiband character of DBLG. Our multiband Fermi liquid theory produces excellent agreement and captures the key features of the experimental drag resistance for all temperatures. This demonstrates the importance of multiband effects in DBLG: they have a strong effect not only on superfluidity, but also on the drag.

Orthorhombic vs. hexagonal epitaxial SrIrO3 thin films: Structural stability and related electrical transport properties

Metastable orthorhombic SrIrO3 (SIO) is an arch-type spin-orbit coupled material. We demonstrate here a controlled growth of relatively thick (200 nm) SIO films that transform from bulk “6H-type” structure with monoclinic distortion to an orthorhombic lattice by controlling growth temperature. Extensive studies based on high-resolution X-ray diffraction and transmission electron microscopy infer a two distinct structural phases of SIO. Electrical transport reveals a weak temperature-dependent semi-metallic character for both phases. However, the temperature-dependent Hall-coefficient for the orthorhombic SIO exhibits a prominent sign change, suggesting a multiband character in the vicinity of EF. Our findings thus unravel the subtle structure-property relation in SIO epitaxial thin films.

Rhombohedral to Cubic Conversion of GeTe via MnTe Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence, and High Thermoelectric Performance.

In this study, a series of Ge1-xMnxTe (x = 0-0.21) compounds were prepared by a melting-quenching-annealing process combined with spark plasma sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge1-xMnxTe is profound. With increasing content of MnTe, the structure of the Ge1-xMnxTe compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and Fm-3m structures, enhancing a multiband character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m0 (GeTe) to 6.15 m0 (Ge0.85Mn0.15Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 W m-1 K-1 at 823 K. Subsequent doping of the Ge0.9Mn0.1Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yields a maximum ZT value of 1.61 at 823 K (for Ge0.86Mn0.10Sb0.04Te). The average ZT value of the compound over the temperature range 400-800 K is 1.09, making it the best GeTe-based thermoelectric material.