Isotropic Fermi Surface Research Articles

Giant Planar Hall Effect in an Ultra-Pure Mercury Selenide Single Crystal Sample

A giant planar Hall effect with an amplitude of about 50 mΩ cm at a temperature ofT= 80 K in a magnetic field of 10 T has been detected in an ultra-pure HgSe single crystal sample with an electron density of 5.5×1015cm–3. Its oscillating dependence on the rotation angle of the sample in various magnetic fields has been determined. Attributes (oscillation period, positions of extrema, correlation between the amplitudes of planar Hall and planar longitudinal magnetoresistance) indicate that the planar Hall effect in this nonmagnetic gapless semimetal with an isotropic Fermi surface originates from the chiral anomaly. This is a solid argument for the topological nature of the electronic spectrum of HgSe.

Giant Planar Hall Effect in an Ultra-Pure Mercury Selenide Single Crystal Sample

A giant planar Hall effect with an amplitude of about 50 mΩ cm at a temperature of T = 80 K in a magnetic field of 10 T has been detected in an ultra-pure HgSe single crystal sample with an electron density of 5.5 × 1015 cm–3. Its oscillating dependence on the rotation angle of the sample in various magnetic fields has been determined. Attributes (oscillation period, positions of extrema, correlation between the amplitudes of planar Hall and planar longitudinal magnetoresistance) indicate that the planar Hall effect in this nonmagnetic gapless semimetal with an isotropic Fermi surface originates from the chiral anomaly. This is a solid argument for the topological nature of the electronic spectrum of HgSe.

DC Transport and Magnetotransport Properties of the 2D Isotropic Metallic System with the Fermi Surface Reconstructed by the Charge Density Wave

We report the ground state stabilization and corresponding electrical transport and magnetotransport properties of a 2D metallic system with an isotropic Fermi surface reconstructed by a charge density wave. The onset of the charge density wave is a spontaneous process, stabilized by the condensation energy gain due to the self-consistent mechanism of topological reconstruction of the Fermi surface and opening of the pseudo-gap around it. We address the signature of the uni-axial reconstruction in terms of the measurable quantities, such as the intra-band transport properties, including the one-particle density of states, the total and effective concentration of electrons, and the Hall coefficient. Additionally, we analyze the magnetotransport properties of the system reconstructed by the bi-axial, checkerboard-like charge density wave, under conditions of magnetic breakdown. It manifests huge quantum oscillations in diagonal components of magnetoconductivity, while the Hall conductivity changes sign, varying the external magnetic field with a finite region of vanishing Hall coefficient in between.

Prediction of topological superconductivity from type-IV, -III, -II, and -I′ nodal points induced by Rashba spin-orbit coupling

Topological superconductivity (TSC) has received great theoretical and experimental attention recently. Type-I Rashba nodal point (RNP) with isotropic band dispersions and point Fermi surface (FS) induced by the Rashba spin-orbit coupling (SOC) provides a promising route to the artificial TSC, because the inherent interspin coupling (ISC) shares identical form as the $p$-wave pairing $({k}_{x}{\ensuremath{\sigma}}_{y}\ensuremath{-}{k}_{y}{\ensuremath{\sigma}}_{x})$ exactly. Here we discuss the potential TSC of other types of RNPs with different ISC forms. By constructing a generic tight-binding model with Rashba SOC, we demonstrate type-IV, -III, -II, and -I\ensuremath{'} RNPs can be achieved on two-dimensional (2D) Bravais lattices, whose FS consists of only a hole (electron) pocket, two contacted hole (electron) pockets, contacted hole and electron pockets, and point of tangency, respectively. With the coorpration of $s$-wave pairing and Zeeman gaps, these new types of RNP will evoke TSC phases with chiral Majorana edge modes (MEMs), where the Chern number will be larger than 1 for multiple symmetry-equivalent RNPs. The Chern number can be further composited when the energies of unequivalent RNPs are equal, leading to edge-dependent MEMs. Moreover, by using first-principles calculations, we demonstrate the BiSb monolayer is an ideal platform for realizing TSC with Chern number 6 from type-II, -I, or -IV RNP. This work enriches the types of nodal point induced by Rashba SOC and offers a generic guidance on realizing multiple and edge-dependent MEMs from the abundantly synthesized 2D surface metal layers.

Anisotropic magnetoresistance and memory effect in bulk systems with extended defects

To describe kinetic phenomena in disordered conductors, various acts of scattering of electrons can be often considered as independent, that is captured by the Boltzmann equation. However, in some regimes, especially, in a magnetic field, it becomes necessary to take into account the correlations between different scattering events of electrons on defects at different times in the past. Such memory effects can have a profound impact on the resistivity of 2D semiconductor systems, resulting in giant negative magnetoresistance and microwave-induced resistance oscillations phenomena. This work opens the discussion of the memory effects in 3D conducting systems featured by the presence of extended one-dimensional defects, such as screw dislocations or static charge stripes. We demonstrate that accounting for the memory effect, that is the capture of electrons on collisionless spiral trajectories winding around extended defects, leads to the strong negative magnetoresistance in case when the external magnetic field direction becomes parallel to the defects axis. This effect gives rise to a significant magnetoresistance anisotropy already for an isotropic Fermi surface and no spin–orbit effects. The proposed resistivity feature can be used to detect one-dimensional scattering defects in these systems.

Profiling spin and orbital texture of a topological insulator in full momentum space

We investigate the coupled spin and orbital textures of the topological surface state in ${\mathrm{Bi}}_{2}$(Te,${\mathrm{Se})}_{3}$(0001) across full momentum space using spin- and angle-resolved photoelectron spectroscopy and relativistic one-step photoemission theory. For an approximately isotropic Fermi surface in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$, the measured intensity and spin momentum distributions, obtained with linearly polarized light, qualitatively reflect the orbital composition and the orbital-projected in-plane spin polarization, respectively. In ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, the in-plane lattice potential induces a hexagonal anisotropy of the Fermi surface, which manifests in an out-of-plane photoelectron spin polarization with a strong dependence on light polarization, excitation energy, and crystallographic direction.

Onset of pseudogap and density wave in a system with a closed Fermi surface

We study the influence of anisotropy, treated as a dimensional crossover between 1D and 3D systems, on the topological instability induced by a (self-consistent) uniaxial periodic potential. The mechanism on which the instability is based involves the topological reconstruction of the Fermi surface, from initially closed pockets to the surface with open Fermi sheets, creating two peculiar points in the band dispersion---the saddle point and elliptical point, between which the pseudogap in electron density of states develops. The self-consistent periodic potential appears as a result of interactions, either electron-phonon or electron-electron, which, linked with the topological instability of the system, results in formation of a new ground state of the system---the density wave provided that the relevant coupling constant is larger than critical. Our analysis shows that the phase transition takes place along the whole continuous interval of a dimensional crossover between 1D and 3D, but that the critical coupling strength significantly increases with the dimensionality of the system. It is our intention to give an initial framework for understanding the nature of charge density waves experimentally observed in a number of materials, like $\mathrm{high}\text{\ensuremath{-}}{T}_{c}$ cuprates or graphite intercalates, both being materials with a closed, rather isotropic Fermi surface far from the nesting regime.

Effect of the electron-phonon interaction on the spin texture in Bi2−ySbySe3−xTex

The electronic surface state of a three-dimensional strong topological insulator (TI) is characterized by an isotropic Dirac cone at the $\mathrm{\ensuremath{\Gamma}}$ point and an ideally perpendicular spin-momentum locking. Away from this point, the lattice anisotropy becomes effective where deviations from the ideal picture are observed. In particular, weak violations of the spin-momentum orthogonality is observed in many experiments. In parallel to these, the existence of the electron-phonon interaction has been verified by a number of experimental groups. In this article we devise an interacting theory of the surface spin. We observe that the interactions, while weakly affecting the energy bands, can yield changes in the spin texture in the presence of a Fermi surface anisotropy. In particular, using the coupling of a specific optical phonon experimentally discovered in the ${\mathrm{Bi}}_{2\ensuremath{-}y}{\mathrm{Sb}}_{y}{\mathrm{Se}}_{3\ensuremath{-}x}{\mathrm{Te}}_{x}$ family to the surface state, we obtain a sixfold nonorthogonal modulations in the in-plane spin-momentum pair. The out-of-plane spin experiments are also discussed in this context. Our results indicate that there is an additionally new mechanism contributing to the deviations in the spin texture beyond the conventional independent particle picture in topological materials where interactions are observed to be important. Recent experimental reports on the spin-momentum nonorthogonality in other symmetries, and even in nearly isotropic Fermi surfaces, indicate that the theory introduced here may be relevant to a larger set of interacting Dirac materials and deserves further exploration.

Density wave and topological reconstruction of an isotropic two-dimensional electron band in external magnetic field

We predict a mechanism of spontaneous stabilization of a uniaxial density wave in a two-dimensional metal with an isotropic Fermi surface in the presence of external magnetic field. The topological transformation of a closed Fermi surface into an open one decreases the electron band energy due to delocalization of electrons initially localized by magnetic field, additionally affected by the magnetic breakdown effect. The driving mechanism of such reconstruction is a periodic potential due to the self-consistently formed electron density wave. It is accompanied with quantum oscillations periodic in inverse magnetic field, similar to the standard de Haas-van Alphen effect, due to Landau-level filling. The phase transition appears as a quantum one at $T=0$, provided the relevant coupling constant is above the critical one. This critical value rapidly decreases, and finally saturates toward zero on the scale of tens of Tesla. Thus, a strong enough magnetic field can induce the density wave in the system in which it was absent in zero field.

Hidden anisotropy in the Drude conductivity of charge carriers with Dirac-Schrödinger dynamics

We show that the conductivity of a two-dimensional electron gas can be intrinsically anisotropic despite isotropic Fermi surface, energy dispersion, and disorder configuration. In the model we study, the anisotropy stems from the interplay between Dirac and Schr\"odinger features combined in a special two-band Hamiltonian describing the quasiparticles similar to the low-energy excitations in phosphorene. As a result, even scalar isotropic disorder scattering alters the nature of the carriers and results in anisotropic transport. Solving the Boltzmann equation exactly for such carriers with point-like random impurities we find a hidden knob to control the anisotropy just by tuning either the Fermi energy or temperature. Our results are expected to be generally applicable beyond the model studied here, and should stimulate further search for the alternative ways to control electron transport in advanced materials.

Electron hydrodynamics with a polygonal Fermi surface

Recent experiments have observed hints of hydrodynamic electron flow in a number of materials, not all of which have an isotropic Fermi surface. We revisit these experiments in $\mathrm{PdCoO}_2$, a quasi-two-dimensional material whose Fermi surface is a rounded hexagon, and observe that the data appears quantitatively consistent with a non-hydrodynamic interpretation. Nevertheless, motivated by such experiments, we develop a simple model for the low temperature kinetics and hydrodynamics of a two-dimensional Fermi liquid with a polygonal Fermi surface. A geometric effect leads to a finite number of additional long-lived quasihydrodynamic "imbalance" modes and corresponding qualitative changes in transport at the ballistic-to-hydrodynamic crossover. In the hydrodynamic limit, we find incoherent diffusion and a new dissipative component of the viscosity tensor arising from the explicit breaking of rotational invariance by the Fermi surface. Finally, we compute the conductance of narrow channels across the ballistic-to-hydrodynamic crossover and demonstrate a modification of the Gurzhi effect that allows for non-monotonic temperature and width dependence in the channel conductance.

Analysis of the diverging effective mass in YaBa2Cu3O6+x for high-Tc mechanism and pairing symmetry

In order to clarify the high-T[Formula: see text] mechanism in inhomogeneous cuprate layer superconductors, we deduce and find the correlation strength which has not revealed before, contributing to the formation of the Cooper pair and the two-dimensional density of state, and demonstrate the pairing symmetry in the superconductors which is still controversial. To the open questions, the fitting and analysis of the diverging effective mass with decreasing doping, extracted from the acquired quantum-oscillation data in underdoped YaBa2Cu3O[Formula: see text] superconductors, using the extended Brinkman–Rice (BR) picture, reveal the nodal constant Fermi energy with the maximum carrier density, a constant Coulomb correlation strength [Formula: see text]=U/U[Formula: see text]0.90, and a growing Fermi arc from the nodal Fermi point to the isotropic Fermi surface with an increasing x. The growing of the Fermi arc indicates that a superconducting gap develops with x from the node (underdoped) to the anti-node (optimally or over-doped). The large [Formula: see text] results from the [Formula: see text]-wave metal–insulator transition for the pseudogap phase in lightly doped superconductors, which can be direct evidence for high-T[Formula: see text] superconductivity. The quantum critical point is regarded as the nodal Fermi point satisfied with the BR picture. The experimentally measured mass diverging behavior is an average effect and the true effective mass is constant. As an application of the nodal constant carrier density, the superconducting node gap analyzed by an angle-resolved photoemission spectroscopy (ARPES) is a precursor of s-wave symmetry in underdoped cuprates. Furthermore, the half-flux quantum, induced by the circulation of d-wave supercurrent and observed by the phase sensitive Josephson-[Formula: see text] junction experiments, is not shown due to “anisotropic or asymmetric effect” appearing in superconductors with trapped flux.

Charging due to Pair-Potential Gradient in Vortex of Type-II Superconductors

Besides the magnetic Lorentz force familiar from the Hall effect in metals and semiconductors, there exists a mechanism for charging peculiar to superconductors that is caused by the pair-potential gradient (PPG). We incorporate it in the augmented quasiclassical equations of superconductivity with the Lorentz force to study charging of an isolated vortex in an equilibrium s-wave type-II superconductor. It is found that the PPG mechanism gives rise to charging concentrated within the core whose magnitude at the core center can be 10 to 100 times larger than that caused by the Lorentz force. Our detailed calculations on the spatial, temperature, and magnetic-penetration-depth dependences of the vortex-core charge reveal that the PPG mechanism contributes dominantly to the core charging of the isolated vortex over a wide parameter range. The two mechanisms are also found to work additively at the core center for the present model with an isotropic Fermi surface.

Hall Effect in the Vortex Lattice ofd-Wave Superconductors with Anisotropic Fermi Surfaces

On the basis of the augmented quasiclassical theory of superconductivity with the Lorentz force, we study the mag- netic field dependence of the charge distribution due to the Lorentz force in a d-wave vortex lattice with anisotropic Fermi surfaces. Owing to the competition between the energy-gap and Fermi surface anisotropies, the charge profile in the vortex lattice changes dramatically with increasing magnetic field because of the overlaps of each nearest vortex-core charge. In addition, the accumulated charge in the core region may reverse its sign as a function of magnetic field. This strong field dependence of the vortex-core charge cannot be observed in the model with an isotropic Fermi surface.

Giant Isotropic Nernst Effect in an Anisotropic Kondo Semimetal.

The "failed Kondo insulator" CeNiSn has long been suspected to be a nodal metal, with a node in the hybridization matrix elements. Here we carry out a series of Nernst effect experiments to delineate whether the severely anisotropic magnetotransport coefficients do indeed derive from a nodal metal or can simply be explained by a highly anisotropic Fermi surface. Our experiments reveal that despite an almost twentyfold anisotropy in the Hall conductivity, the large Nernst signal is isotropic. Taken in conjunction with the magnetotransport anisotropy, these results provide strong support for an isotropic Fermi surface with a large anisotropy in quasiparticle mass derived from a nodal hybridization.

Temperature Dependence of the Resistivity and Seebeck Coefficient of Individual Single-Crystal Bismuth Nanowires of 345-nm and 594-nm Diameters Encased in a Quartz Template

Temperature dependence measurements of resistivity and the Seebeck coefficient were performed using individual bismuth nanowires of 345-nm and 594-nm diameters encased in a quartz template. Each nanowire was confirmed as a single crystal by Laue measurement, and an advanced mean free path model was utilized to explain both temperature dependencies. The model successfully explains the dependence over 100 K, which occurs because the carrier mobility is restricted by boundary scattering at the nanowire surface considering the crystal orientation along the wire length direction, band structure, and isotropic Fermi surface. However, it is difficult to determine the temperature dependence of the resistivity in the low-temperature region, in which a much higher temperature coefficient is indicated, especially at temperatures lower than 50 K. Although we calculated the temperature dependence of the resistivity with the influence of p-type contamination in the nanowire because a positive Seebeck coefficient at low temperatures was observed, an explanation of the resistivity in the low-temperature region has not been developed. Therefore, a hypothesis was introduced in which the hole mobility was not restricted in the nanowire, and the hole mobility was estimated by the mobility ratio from the Seebeck coefficient, the measured resistivity, and the electron mobility using the model. Lastly, the temperature dependence of the resistivity can be explained over the entire temperature region. This result suggests that the scattering mechanism between holes and electrons in the nanowire differs and depends on the crystal orientation in the low-temperature region.

Microwave radiation absorption by 2D-electrons in the type II composite InAs/GaSb/AlSb quantum wells in a magnetic field

Strong Shubnikov - de Haas (SdH) oscillations were observed in the derivative of microwave absorption (f = 10 GHz) in the InAs/GaSb/AlSb composite quantum wells (CQWs) using electron-paramagnetic-resonance spectroscopy at low temperatures (2.7-20 K) and in the magnetic field up to 14 kOe. CQWs were grown on the n-GaSb:Te(100) and n-InAs:Mn(100) substrates with various width of QWs by MOVPE. Predominance contribution of the bulk n- GaSb substrate in SdH oscillations was manifested. Two frequencies of the SdH oscillations were found from Fourier analysis, which is connected to warping of the Fermi surface of GaSb. Unusual angular indicatrix was observed in dependence on orientation of the samples in the magnetic field. Obtained results can be explained by inversion asymmetry, which is a feature of the substances with lack of inversion centres. For CQWs grown on n-InAs:Mn (ns = 1.1 × 1017 cm-3) substrate, only several SdH oscillations with higher period were observed. Taking into account isotropic Fermi surface of bulk InAs, we succeeded to extract a contribution of the 2D carriers of InAs QW ∼ H⊥, where H⊥= HconstcosΘ, from bulk substrate oscillations using special spline interpolation from angular dependence of SdH oscillatory amplitudes in the angle range 0-90°. 2D electron concentration in the InAs QW ns ≈ 1 - 3 × 1011 cm-2 was evaluated from oscillatory period.

Geometric factors in the magnetoresistance of n-doped InAs epilayers

We investigate the magnetoresistance (MR) effect in n-doped InAs and InAs/metal hybrid devices with geometries tailored to elucidate the physical mechanism and the role of geometry in the MR. Despite the isotropic Fermi surface in InAs, we observe a strong intrinsic MR in the InAs epilayer due to the existence of a surface conducting layer. Experimental comparison confirms that the extraordinary MR in the InAs/metal hybrids outperforms the orbital MR in the Corbino disk in terms of both the MR ratio and the magnetic field resolution. The results also indicate the advantage of a two-contact configuration in the hybrid devices over a four-contact one with respect to the magnetic field resolution. This is in contrast to previously reported results, where performance was evaluated in terms of the MR ratio and a four-contact configuration was found to be optimal. By applying Kohler's rule, we find that at temperatures above 75 K the extraordinary MR violates Kohler's rule, due to multiple relaxation rates, whereas the orbital MR obeys it. This finding can be used to distinguish the two geometric effects, the extraordinary MR and the orbital MR, from each other.

Small and nearly isotropic hole-like Fermi surfaces in LiFeAs detected through de Haas–van Alphen effect

LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which find a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin fluctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase diagram is indeed observed in the isostructural NaFeAs compound. Previous angle-resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas--van Alphen (dHvA) measurements which detected a few, electron-like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results reconcile the apparent discrepancy between ARPES and the previous dHvA results. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor an $s$-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP suggest that the nodes in the gap function of LiFeP might be located on the hole pockets. This would be difficult to reconcile with the current understanding of the $s\ifmmode\pm\else\textpm\fi{}$ scenario.

Hartree-Fock Ground State Phase Diagram of Jellium

We calculate the ground state phase diagram of the homogeneous electron gas in three dimensions within the Hartree-Fock approximation and show that broken symmetry states are energetically favored at any density against the homogeneous Fermi gas state with isotropic Fermi surface. At high density, we find metallic spin-unpolarized solutions where electronic charge and spin density form an incommensurate crystal having more crystal sites than electrons. For r(s)→0, our solutions approach pure spin-density waves, whereas the commensurate Wigner crystal is favored at lower densities, r(s)≳3.4. Decreasing the density, the system undergoes several structural phase transitions with different lattice symmetries. The polarization transition occurs around r(s)≈8.5.