Hole Pockets Research Articles

Thermoelectric properties of Fe2VAl in the temperature range 300–800 K: A combined experimental and theoretical study

Here, the experimentally observed thermoelectric (TE) properties of Fe2VAl are understood through electronic structure calculations in the temperature range of 300–800 K. The Seebeck coefficient (S) is observed as ∼−138μV/K at 300 K. Then, the |S| decreases with increase in temperature, with a value of ∼−18μV/K at 800 K. The temperature dependence of electrical conductivity, σ (thermal conductivity, κ) exhibits the increasing (decreasing) trend with values of ∼1.2× 105Ω−1 m−1 (∼23.7 W/m K) and ∼2.2× 105Ω−1 m−1 (∼15.3 W/m K) at 300 K and 800 K, respectively. In order to understand these transport properties, the DFT based semi-classical Boltzmann theory is used. The contributions of multi-band electron and hole pockets are found to be mainly responsible for the temperature dependent trend of these properties. The present study suggests that DFT based calculations provide reasonably good explanations of experimental TE properties of Fe2VAl in the high-temperature range of 300–800 K.

Emerging Nontrivial Topology in Ultrathin Films of Rare-Earth Pnictides.

Thin films of rare-earth monopnictide (RE-V) semimetals are expected to turn into semiconductors due to quantum confinement effects (QCE), lifting the overlap between electron pockets at Brillouin zone edges (X) and hole pockets at the zone center (Γ). Instead, using LaSb as an example, we find the emergence of the quantum spin Hall (QSH) insulator phase in (001)-oriented films as the thickness is reduced to 7, 5, or 3 monolayers (MLs). This is attributed to a strong QCE on the in-plane electron pockets and the lack of quantum confinement on the out-of-plane pocket projected onto the zone center, resulting in a band inversion. Spin-orbit coupling (SOC) opens a sizable nontrivial gap in the band structure of ultrathin films. Such effect is anticipated to be general in rare-earth monopnictides and may lead to interesting phenomena when coupled with the 4f magnetic moments present in other members of this family of materials.

Bilayer Two-Orbital Model of La_{3}Ni_{2}O_{7} under Pressure.

The newly discovered Ruddlesden-Popper bilayer La_{3}Ni_{2}O_{7} reaches a remarkable superconducting transition temperature T_{c}≈80 K under a pressure of above 14GPa. Here we propose a minimal bilayer two-orbital model of the high-pressure phase of La_{3}Ni_{2}O_{7}. Our model is constructed with the Ni-3d_{x^{2}-y^{2}}, 3d_{3z^{2}-r^{2}} orbitals by using Wannier downfolding of the density functional theory calculations, which captures the key ingredients of the material, such as band structure and Fermi surface topology. There are two electron pockets, α, β, and one hole pocket, γ, on the Fermi surface, in which the α, β pockets show mixing of two orbitals, while the γ pocket is associated with Ni-d_{3z^{2}-r^{2}} orbital. The random phase approximation spin susceptibility reveals a magnetic enhancement associated with the d_{3z^{2}-r^{2}} state. A higher energy model with O-p orbitals is also provided for further study.

Revealing a 3D Fermi Surface Pocket and Electron-Hole Tunneling in UTe_{2} with Quantum Oscillations.

Spin triplet superconductor UTe_{2} is widely believed to host a quasi-two-dimensional Fermi surface, revealed by first-principles calculations, photoemission, and quantum oscillation measurements. An outstanding question still remains as to the existence of a three-dimensional Fermi surface pocket, which is crucial for our understanding of the exotic superconducting and topological properties of UTe_{2}. This 3D Fermi surface pocket appears in various theoretical models with different physics origins, but has not been unambiguously detected in experiment. Here for the first time we provide concrete evidence for a relatively isotropic, small Fermi surface pocket of UTe_{2} via quantum oscillation measurements. In addition, we observed high frequency quantum oscillations corresponding to electron-hole tunneling between adjacent electron and hole pockets. The coexistence of 2D and 3D Fermi surface pockets, as well as the breakdown orbits, provide a test bed for theoretical models and aid the realization of a unified understanding of the superconducting state of UTe_{2} from the first-principles approach.

Possible evidence of Weyl fermion enhanced thermal conductivity under magnetic fields in the antiferromagnetic topological insulator Mn(Bi1−xSbx)2Te4

We report thermal conductivity and Seebeck effect measurements on $\text{Mn}{({\text{Bi}}_{1\ensuremath{-}x}{\text{Sb}}_{x})}_{2}{\text{Te}}_{4}$ (MBST) with $x=0.26$ under applied magnetic fields below $50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Our data shows clear indications of the electronic structure transition induced by the antiferromagnetic (AFM) to ferromagnetic (FM) transition driven by applied magnetic field as well as significant positive magnetothermal conductivity in the Weyl semimetal state of MBST. Further, by examining the dependence of magnetothermal conductivity on field orientation for MBST and comparison with the magnetothermal conductivity of ${\text{MnBi}}_{2}{\text{Te}}_{4}$, we see possible evidence of a contribution to thermal conductivity due to Weyl fermions in the FM phase of MBST. From the temperature dependence of the Seebeck coefficient under magnetic fields for MBST, we also observed features consistent with the Fermi surface evolution from a hole pocket in the paramagnetic state to a Fermi surface with coexistence of electron and hole pockets in the FM state. These findings provide further evidence for the field-driven topological phase transition from an AFM topological insulator to a FM Weyl semimetal.

Interlayer coupling in the superconducting state of iron-based superconductors

High-temperature superconducting paring is generally believed to be mediated by the magnetic fluctuations that mostly occur within the two-dimensional conducting layers (Cu-O, Fe-As, or Fe-Se) in copper oxides, iron pnictides, iron chalcogenides, etc. Here, on the basis of inelastic neutron scattering measurements on a superconducting iron pnictide ${\mathrm{Ca}}_{0.33}{\mathrm{Na}}_{0.67}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, we have discovered a highly three-dimensional spin resonance mode with upward V-shape dispersions both in the $ab$ plane and along the $c$ axis. The superconducting gaps exhibit strong ${k}_{z}$ modulations involved with significant changes on the size of one hole pocket and the density state of the ${d}_{{z}^{2}}$ orbital of Fe. The resonance dispersions don't always seem confined by the total gaps summed on those imperfectly nesting Fermi sheets. It is demonstrated that the $c$-axis dependence of both the resonance energy and its intensity in iron pnictides can be universally scaled with the distance between two adjacent Fe-As layers ($d$), and the ${k}_{z}$ modulation of the gap is also anticorrelated with $d$. These results highlight the role of interlayer coupling in iron-based superconductivity, suggesting that the interlayer pairing may also be driven by magnetic fluctuations under certain spin-orbit couplings.

Realization of a Two-Dimensional Checkerboard Lattice in Monolayer Cu2N

Two-dimensional checkerboard lattice, the simplest line-graph lattice, has been intensively studied as a toy model, while material design and synthesis remain elusive. Here, we report theoretical prediction and experimental realization of the checkerboard lattice in monolayer Cu2N. Experimentally, monolayer Cu2N can be realized in the well-known N/Cu(100) and N/Cu(111) systems that were previously mistakenly believed to be insulators. Combined angle-resolved photoemission spectroscopy measurements, first-principles calculations, and tight-binding analysis show that both systems host checkerboard-derived hole pockets near the Fermi level. In addition, monolayer Cu2N has outstanding stability in air and organic solvents, which is crucial for further device applications.

Commensurate and spiral magnetic order in the doped two-dimensional Hubbard model: Dynamical mean-field theory analysis

We develop a dynamical mean-field theory approach for the spiral magnetic order, changing to a local coordinate frame with preferable spin alignment along the $z$-axis, which can be considered with the impurity solvers treating the spin diagonal local Green's function. We furthermore solve the Bethe-Salpeter equations for nonuniform dynamic magnetic susceptibilities in the local coordinate frame. We apply this approach to describe the evolution of magnetic order with doping in the $t-t'$ Hubbard model with $t'=0.15$, which is appropriate for the description of the doped La$_2$CuO$_4$ high-temperature superconductor. We find that with doping the antiferromagnetic order changes to the $(Q,\pi)$ incommensurate one, and then to the paramagnetic phase. The spectral weight at the Fermi level is suppressed near half filling and continuously increases with doping. The dispersion of holes in the antiferromagnetic phase shows qualitative agreement with the results of the $t$-$J$ model consideration. In the incommensurate phase we find two branches of hole dispersions, one of which crosses the Fermi level. The resulting Fermi surface forms hole pockets. We also consider the dispersion of the magnetic excitations, obtained from the non-local dynamic magnetic susceptibilities. The transverse spin excitations are gapless, fulfilling the Goldstone theorem; in contrast to the mean-field approach the obtained magnetic state is found to be stable. The longitudinal excitations are characterized by a small gap, showing the rigidity of the spin excitations. For realistic hopping and interaction parameters we reproduce the experimentally measured spin-wave dispersion of La$_2$CuO$_4$.

Semimetallic, Half-Metallic, Semiconducting, and Metallic States in Gd-Sb Compounds.

The electronic and band structures of the Gd- and Sb-based intermetallic materials have been explored using the theoretical ab initio approach, accounting for strong electron correlations of the Gd-4f electrons. Some of these compounds are being actively investigated because of topological features in these quantum materials. Five compounds were investigated theoretically in this work to demonstrate the variety of electronic properties in the Gd-Sb-based family: GdSb, GdNiSb, Gd4Sb3, GdSbS2O, and GdSb2. The GdSb compound is a semimetal with the topological nonsymmetric electron pocket along the high-symmetry points Γ-X-W, and hole pockets along the L-Γ-X path. Our calculations show that the addition of nickel to the system results in the energy gap, and we obtained a semiconductor with indirect gap of 0.38 eV for the GdNiSb intermetallic compound. However, a quite different electronic structure has been found in the chemical composition Gd4Sb3; this compound is a half-metal with the energy gap of 0.67 eV only in the minority spin projection. The molecular GdSbS2O compound with S and O in it is found to be a semiconductor with a small indirect gap. The GdSb2 intermetallic compound is found to have a metallic state in the electronic structure; remarkably, the band structure of GdSb2 has a Dirac-cone-like feature near the Fermi energy between high-symmetry points Г and S, and these two Dirac cones are split by spin-orbit coupling. Thus, studying the electronic and band structure of several reported and new Gd-Sb compounds revealed a variety of the semimetallic, half-metallic, semiconducting, or metallic states, as well topological features in some of them. The latter can lead to outstanding transport and magnetic properties, such as a large magnetoresistance, which makes Gd-Sb-based materials very promising for applications.

Realization of Z2 Topological Metal in Single‐Crystalline Nickel Deficient NiV2Se4

AbstractTemperature‐dependent electronic and magnetic properties are reported for nickel‐deficient NiV2Se4. Single‐crystal X‐ray diffraction shows it to crystallize in the monoclinic Cr3S4 structure type with space group and vacancies on the Ni site, resulting in the composition Ni0.85V2Se4 in agreement with our electron‐probe microanalysis. Structural distortions are not observed down to 1.5 K. Nevertheless, the electrical resistivity shows metallic behavior with a broad anomaly around 150–200 K that is also observed in the heat capacity data. This anomaly indicates a change of state of the material below 150 K. It is believed that this anomaly could be due to spin fluctuations or charge‐density‐wave fluctuations, where the lack of long‐range order is caused by vacancies at the Ni site of Ni0.85V2Se4. The non‐linear temperature dependence of the resistivity as well as an enhanced value of the Sommerfeld coefficient mJ mol−1 K−2 suggest strong electron–electron correlations in this material. First‐principles calculations performed for NiV2Se4, which are also applicable to Ni0.85V2Se4, classify this material as a topological metal with and coexisting electron and hole pockets at the Fermi level. The phonon spectrum lacks any soft phonon mode, consistent with the absence of periodic lattice distortion in the present experiments.

Role of Doping Effect and Chemical Pressure Effect Introduced by Alkali Metal Substitution on 1144 Iron-Based Superconductors.

CaAFe4As4 with A = K, Rb, and Cs are close to the doped 122 system, and the parent material can reach a superconducting transition temperature of 31-36 K without doping. To study the role of alkali metals, we investigated the induced hole doping and chemical pressure effects as a result of the introduction of alkali metals using density-functional-based methods. These two effects can affect the superconducting transition temperature by changing the number of electrons and the structure of the FeAs conductive layer, respectively. Our study shows that the dxz and dyz orbitals, which are degenerate in CaFe2As2, become nondegenerate in CaAFe4As4 due to two nonequivalent arsenic atoms (As1 and As2). The unusual oblate ellipsoid hole pocket at Γ point in CaAFe4As4 results from a divalent cation Ca2+ replaced by a monovalent cation A+. It shows one of the main differences in fermiology compared to a particular form of CaFe2As2 with reduced 1144 symmetry, due to the enhancement of As2-Fe hybridization. The unusual band appears in CaFe2As2 (1144) and gradually disappears in the change of K to Cs. Further analysis shows that this band is contributed by As1 and has strong dispersion perpendicular to the FeAs layer, suggesting that it is related to the peculiar van Hove singularity below the Fermi level. In addition, various aspects of CaFe2As2 (1144) and CaAFe4As4 in the ground state are discussed in terms of the influence of hole doping and chemical pressure.

Synthesis and physical properties of a new layered ferromagnet Cr1.21Te2

Single crystals of a new layered compound, ${\mathrm{Cr}}_{1.21}{\mathrm{Te}}_{2}$, were synthesized via a vapor transport method. The crystal structure and physical properties were characterized by single crystal and powder x-ray diffraction, temperature- and field-dependent magnetization, zero-field heat capacity, and angle-resolved photoemission spectroscopy. ${\mathrm{Cr}}_{1.21}{\mathrm{Te}}_{2}$, containing two Cr sites, crystalizes in a trigonal structure with a space group P-3 (No. 147). The Cr site in the interstitial layer is partially occupied. Physical property characterizations indicate that ${\mathrm{Cr}}_{1.21}{\mathrm{Te}}_{2}$ is metallic with hole pockets at the Fermi energy and undergoes a ferromagnetic phase transition at $\ensuremath{\sim}173\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The magnetic moments align along the $c$ axis in the ferromagnetic state. Based on low-temperature magnetization, the spin stiffness constant, D, and spin excitation gap, $\mathrm{\ensuremath{\Delta}}$, were estimated according to Bloch's law to be $D=94\ifmmode\pm\else\textpm\fi{}17\phantom{\rule{0.16em}{0ex}}\mathrm{meV}\phantom{\rule{0.16em}{0ex}}{\AA{}}^{2}$ and $\mathrm{\ensuremath{\Delta}}=0.45\ifmmode\pm\else\textpm\fi{}0.33$ meV, suggesting its possible application as a low dimensional ferromagnet.

Effects of lithium intercalation in bilayer graphene

Modulation of the Dirac cone of graphene has been highly desired to enrich the exotic properties of graphene. Intercalation of Li into multilayer graphene could lead to the gap opening of the Dirac cone of graphene. Here, using density functional theory calculations, we identify the stable intercalated structure and the corresponding evolution of band structures in the intercalation process of Li into bilayer graphene (BLG). The generalized Kekul\'e order beyond the traditional $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ supercell has been observed to modulate the Dirac cones of BLG by opening a gap or splitting the electron and hole pocket, contributed by the Kekul\'e-O and the Kekul\'e-Y distortion, respectively. Our work provides a further understanding of the modulation of the Dirac cone of graphene and can serve as a landmark for experiment to investigate the Li-intercalated BLG.

Magnetic and Magnetotransport Properties of the Magnetic Topological Nodal‐Line Semimetal TbSbTe

AbstractThe magnetic properties, magneto‐resistivity, Hall resistivity, Seebeck coefficient, and heat capacity are investigated for a magnetic topological nodal‐line semimetal TbSbTe. The calculated energy‐band structures of TbSbTe show two nodal rings on the kx‐ky planes, and the Fermi surface possesses one electron pocket with an enclosure of one hole pocket around the Γ point. The multiple magnetic phase transitions are induced at critical magnetic fields of Hc1 ≈ 10.5 kOe, Hc2 ≈ 20.8 kOe, and Hc3 ≈ 38.2 kOe at 2 K for the H // ab plane. Temperature dependence of electrical resistivity displays a hump‐like feature around 240 K, a general positive slope above TN = 6 K but a negative slope below TN. The magnetoresistance exhibits a conversion from the semi‐classical H2 dependence at weak fields to the linear‐field dependence at high fields, indicating the presence of a Dirac linear energy dispersion. TbSbTe may provide an ideal platform for studying topological physics and designing devices based on topological quantum materials.

Electronic signatures of successive itinerant, antiferromagnetic transitions in hexagonal La2Ni7

We use high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic and magnetic properties of La2Ni7, an itinerant magnetic system with a series of three magnetic transition temperatures upon cooling, which end in a weak antiferromagnetic ground state. Our APRES data reveal several electron and hole pockets that have hexagonal symmetry near the Γ point. We observe significant reconstruction of the band structure upon successive magnetic transitions at T 1 ∼ 61 K, T 2 ∼ 57 K and T 3 ∼ 42 K. Several features observed in ARPES data were reasonably well reproduced by DFT calculations, while others were not. In particular, the flat band near E F predicted by DFT in antiferromagnet (AFM) state, was seemingly absent in ARPES data. Our results detail the effects of magnetic ordering on the electronic structure in a Ni-based weak AFM and highlight challenges of current computational methods.

Crystal growth, characterization and electronic band structure of TiSeS

Layered semimetallic van der Waals material $\mathrm{Ti}{\mathrm{Se}}_{2}$ has attracted a lot of attention because of interplay of a charge density wave (CDW) state and superconductivity. Its sister compound ${\mathrm{TiS}}_{2}$, being isovalent to $\mathrm{Ti}{\mathrm{Se}}_{2}$ and having the same crystal structure, shows a semiconducting behavior. The natural question rises---what happens at the transition point in $\mathrm{Ti}{\mathrm{Se}}_{2\ensuremath{-}x}{\mathrm{S}}_{x}$, which is expected for x close to one. Here we report the growth and characterization of TiSeS single crystals and the study of the electronic structure using density functional theory (DFT) and angle-resolved photoemission (ARPES). We show that TiSeS single crystals have the same morphology as $\mathrm{Ti}{\mathrm{Se}}_{2}$. Transport measurements reveal a metallic state; no evidence of CDW was found. DFT calculations suggest that the electronic band structure in TiSeS is similar to that of $\mathrm{Ti}{\mathrm{Se}}_{2}$, but the electron and hole pockets in TiSeS are much smaller. The ARPES results are in good agreement with the calculations.

Coupling between Charge Density Wave Ordering and Magnetism in Ho2Ir3Si5

Ho2Ir3Si5 belongs to the family of three-dimensional (3D) R2Ir3Si5 (R = Lu, Er, Ho) compounds that exhibit a first-order, charge-density-wave (CDW) phase transition, where there is a strong orthorhombic-to-triclinic distortion of the lattice accompanied by superlattice reflections. The analysis by single-crystal X-ray diffraction (SXRD) has revealed that the Ir–Ir zigzag chains along c are responsible for the CDW in all three compounds. The replacement of the rare earth element from nonmagnetic Lu to magnetic Er or Ho lowers TCDW, where TCDWLu = 200 K, TCDWEr = 150 K, and TCDWHo = 90 K. Out of the three compounds, Ho2Ir3Si5 is the only system where second-order superlattice reflections could be observed, indicative of an anharmonic shape of the modulation wave. The CDW transition is observed as anomalies in the temperature dependencies of the specific heat, electrical conductivity, and magnetic susceptibility, which includes a large hysteresis of 90 to 130 K for all measured properties, thus corroborating the SXRD measurements. Similar to previously reported Er2Ir3Si5, there appears to be a coupling between CDW and magnetism such that the Ho3+ magnetic moments are influenced by the CDW transition, even in the paramagnetic state. Moreover, earlier investigations on polycrystalline material revealed antiferromagnetic (AFM) ordering at TN = 5.1 K, whereas AFM order is suppressed and only the CDW is present down to at least 0.1 K in our highly ordered single crystal. First-principles calculations predict Ho2Ir3Si5 to be a metal with coexisting electron and hole pockets at the Fermi level. The Ho and Ir atoms have spherically symmetric metallic-type charge density distributions that are prone to CDW distortion. Phonon calculations affirm that the Ir atoms are primarily responsible for the CDW distortion, which is in agreement with the experiment.

Structures of vortex in Co-doped BaFe2As2 iron superconductors with different doping level by scanning Hall probe microscopy

The 122 iron arsenide unconventional superconductors are part of a new class of iron-based superconductors. Co-doped BaFe2xCoxAs2 (Ba-122) iron superconductors sample have been examine by scanning Hall probe microscopy (SHPM) technique to find out the magnetic properties of Ba-122 . Has been completed the evolution of profiles of vortices which has well isolated and it is as function of temperature, then utilized suitable technique to extract the temperature depending on penetration depth, .So, this allowed to deduce the temperature dependent on density of superfluid and it has been compared with α-model consequences for a (2-band) of superconductor. When the superfluid density for the BaFe2xCoxAs2 (Over D x= 0.113) sample. As result, the two gap -model has been fitted to the data with Δ1=4.25kTc, Δ2=1.92kTc , =0.708, 1=0.293 then a2=1. However, When values of for the BaFe2-xCoxAs2 (Over D x= 0.075) sample. Result of the superfluid density for BaFe2-xCoxAs2 with parameters are (Δ1=3.9k, Δ2=1.6k), =0.615 for Δ1, and =0.237, =1. Suitable parameters produce refer into the symmetry of the order parameter at hole pockets with the electron, and then the relative supports of the bands to the density of superfluid in the iron-based crystals.

Exploring the Dirac nature of RbBi2

Characteristics of topological semimetals such as a nonsaturating magnetoresistance (MR), a field-induced metal to semiconducting crossover and a robust resistivity plateau are observed under a magnetic field in type-I ${\mathrm{RbBi}}_{2}$ bulk superconductor with ${T}_{\mathrm{c}}=4.15$ K. The MR exhibits a notable 3500% increase at 2 K and 9 T and the resistivity follows a power law temperature dependence, while the MR $\ensuremath{\propto}\phantom{\rule{4pt}{0ex}}{H}^{1.26}$, indicating weak carrier compensation. First principles calculations provided insights into the dynamical stability of the cubic structure at 0 K. Both hole and electron pockets are observed at the Fermi surface. The electron-phonon interaction constant indicates weak coupling strength ($<1$) that leads to a maximum predicted ${T}_{\mathrm{c}}$ of 2.852 K. Just below the Fermi level, ${\mathrm{E}}_{F}$, the electronic band structure consists of linear band crossings at the $X$ points in the Brillouin zone (BZ) corresponding to massless, symmetry-protected Dirac fermions.

Interference Effect of Beam Splitter Current in Iron-Pnictide Superconductors

We consider a Cooper pair beam splitter for Iron-Pnictide superconductor and calculate the entangled electron-hole current. We investigate the interplay of various physical parameters such as doping at electron and hole pockets as well as non-zero nesting between the electron and hole pocket. We find that in the absence of magnetic order, the current due to hole pocket and electron pocket add up ordinarily. However in the presence of magnetic ordering the two currents take part in characteristic interference effect to modify the resultant current significantly. This interference effect manifests itself in non-monotonous and oscillatory nature of beam splitter current. We investigate in details this non-monotonicity with the chemical potential as well as nesting vectror $|\bf q|$. We also investigate the evolution of density of states with system parameters and correlate it with the beam-splitter current. Further we enumerate the relevant parameter space where the efficiency of such beam splitter set up is enhanced. Our finding can be useful in experimental determination or verification of co-existence phase in Iron-Pnictide superconductors and has potential applications in realizing quantum gates or switches.