Fermi Surface Evolution Research Articles

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.

Odd-frequency pairs and anomalous proximity effect in nematic and chiral states of superconducting topological insulators

We investigate emergent odd-frequency pairs and proximity effect in nematic and chiral states of superconducting topological insulators (STIs), such as ${M}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ ($M=$ Cu, Sr, Nb). The interplay of pairing symmetry, the orbital degrees of freedom, and strong spin-orbit interaction generates a variety of odd-frequency pairs in the bulk and surface of STIs. The nematic and chiral states are the prototypes of topological superconductors with and without time-reversal symmetry, respectively. We find that the Fermi surface evolution from a closed spheroidal to an open cylindrical shape changes the pairing symmetry from the nematic to chiral state, which causes the evolution of the odd-frequency pairings and surface Andreev bound states (SABSs). In addition, spin polarization of odd-frequency pairs and SABSs stems from the nonunitary pairing in the chiral state. The evolution and spin polarization of odd-frequency pairs and SABSs can be captured by tunnel conductance spectroscopy. Furthermore, we study the anomalous proximity effect in various irreducible representations of STIs. The anomalous proximity effect was originally predicted in spin-triplet superconductor junctions without spin-orbit interaction. Odd-frequency spin-triplet $s$-wave pairs penetrate into diffusive normal (DN) metals and induce a pronounced zero-energy peak of the local density of states in the DN region. Here we demonstrate that contrary to the well-known results, the anomalous proximity effect in STIs is not immune to nonmagnetic impurities. The fragility is attributed to the fact that the proximitized odd-frequency even-parity pairs are admixtures of $s$-wave and non-$s$-wave pairs due to strong spin-orbit interaction inherent to the parent materials.

Magnetic Anisotropy Control with Curie Temperature above 400 K in a van der Waals Ferromagnet for Spintronic Device.

The technological appeal of van der Waals ferromagnetic materials is the ability to control magnetism under external fields with desired thickness toward novel spintronic applications. For practically useful devices, ferromagnetism above room temperature or tunable magnetic anisotropy is highly demanded but remains challenging. To date, only a few layered materials exhibit unambiguous ferromagnetic ordering at room temperature via gating techniques or interface engineering. Here, it is demonstrated that the magnetic anisotropy control and dramatic modulation of Curie temperature (Tc ) up to 400 K are realized in layered Fe5 GeTe2 via the high-pressure diamond-anvil-cell technique. Magnetic phases manifesting with in-plane anisotropic, out-of-plane anisotropic and nearly isotropic magnetic states can be tuned in a controllable way, depicted by the phase diagram with a maximum Tc up to 360 K. Remarkably, the Tc can be gradually enhanced to above 400 K owing to the Fermi surface evolution during a pressure loading-deloading process. Such an observation sheds light on the understanding and control of emergent magnetic states in practical spintronic applications.

Evolution of holographic Fermi surface from non-minimal couplings

We study a holographic toy model by considering a probe fermion of finite charge density in an anisotropic background. By computing the fermionic spectral function numerically, we observed that the system exhibits some interesting behaviours in the nature of the Fermi surface (FS) and its evolution when tuning the controlling parameters. We introduced non-minimal interaction terms in the action for holographic fermions along with a complex scalar field but neglecting the backreaction of the fermions field on the background. Suppression in the spectral weight and deformation of FS is observed, which are reminiscent of the results seen in various condensed matter experiments in real materials.

Theory for doping trends in titanium oxypnictide superconductors

A family of titanium oxypnictide materials $\mathrm{Ba}{\mathrm{Ti}}_{2}{Pn}_{2}\mathrm{O}$ ($Pn=\mathrm{pnictogen}$) becomes superconducting when a charge and/or spin-density wave is suppressed. With hole doping, isovalent doping and pressure, a whole range of tuning parameters is available. We investigate how charge doping controls the superconducting transition temperature ${T}_{\mathrm{c}}$. To this end, we use experimental crystal structure data to determine the electronic structure and Fermi surface evolution along the doping path. We show that a naive approach to calculating ${T}_{\mathrm{c}}$ via the density of states at the Fermi level and the McMillan formula systematically fails to yield the observed ${T}_{\mathrm{c}}$ variation. On the other hand, spin-fluctuation theory pairing calculations allow us to consistently explain the ${T}_{\mathrm{c}}$ increase with doping. All alkali-doped materials ${\mathrm{Ba}}_{1\ensuremath{-}x}{A}_{x}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$ ($A=\mathrm{Na},\phantom{\rule{0.28em}{0ex}}\mathrm{K},\phantom{\rule{0.28em}{0ex}}\mathrm{and}\phantom{\rule{0.28em}{0ex}}\mathrm{Rb}$) are described by a sign-changing $s$-wave order parameter. Susceptibilities also reveal that the physics of the materials is controlled by a single Ti $3d$ orbital.

Signatures of Fermi surface topology change in the nodal-line semimetal ZrSiSe1−xTex

The topological nodal-line semimetal (TNLS) $\mathrm{ZrSi}M$ $(M=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ is a promising platform to study the TNLS phase by tuning chalcogens. In this work, we study the evolution of the Fermi surface (FS) by tuning the Se/Te ratio in ${\mathrm{ZrSiSe}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ compounds. Transport properties and magnetometry results present signatures of Fermi surface topology change by the sudden changes in symmetry and carrier densities, as well as singularity of FSs at the critical chemical potentials. A $2\frac{1}{2}$-order Lifshitz transition occurs when 0.20 $\ensuremath{\le}\phantom{\rule{4pt}{0ex}}x\phantom{\rule{4pt}{0ex}}\ensuremath{\le}$ 0.33. Another $3\frac{1}{2}$-order electron topological transition is revealed by a large diamagnetic anomaly of susceptibility at $x\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}$ 0.80. In combination with first-principles calculations, this study reveals the vital roles of spin-orbit coupling, charge transfer, and shifts of the chemical potential in the evolution of FSs in this system. Our results demonstrate how the FSs evolve in ${\mathrm{ZrSiSe}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ compounds, providing fundamental clues for designing topological state switchable devices.

Numerical Study of the Fermi Surface Evolution in Cuprates Using the One-Band Hubbard Model

A Numerical calculation of the Fermi Surface (FS) evolution in cuprate using the one-band Hubbard model by the matrix diagonalization method has been done. This work focusses on the study of the evolution of the FS in the cuprate material, namely , numerically by introducing a specific order parameter in the Hubbard model matrix. In this study, we confirm two evolution types of the FS of as an experimental result. Firstly, the evolution of the antibonding FS topology from the electron-like to the hole-like is generated by the order parameter that has a form of where is the order parameter coefficient that corresponds to the hopping parameter of the atomic neighbor long-range interaction and is the normalized momenta coordinate of the first Brillouin zone. On the contrary, the order parameter that has a form of generates the evolution of the FS from the hole-like topology to the electron-like topology. Secondly, the anisotropic evolution of the FS can be described by an extended d-wave order parameter which generating either the V-shape or U-shape type of the energy gap.

Self-Doping Effect in FeSe Superconductor by Pressure-Induced Charge Transfer

Several unambiguous experimental observations clearly showed that the critical temperature of FeSe superconductor depends significantly on the microstructure. Experiments also showed that the critical temperature can be greatly enhanced by the application of external pressure. The present paper deals with the effect of pressure on charge transfer and self-doping properties of the superconducting compound FeSe, based on the investigation of the pressure dependence of the Fermi surface by means of first-principles methods. From the numerically evaluated electronic and crystalline properties, the pressure-induced modifications of the FeSe Fermi surface’s topology are determined. The Luttinger theorem was also used to evaluate the carrier concentration on the Fe atomic sites from the evolution of the Fermi surface. We have found that the electronic density at the Fe sites increases with the increase in external pressure, following the distortion of Fermi surface. Our simulations reveal that the pressure-induced charge transfer from Se atoms to Fe atoms in the FeSe superconductor can be interpreted as a direct correlation between the electron carrier concentration and the applied pressure. The pressure dependence of superconducting properties of FeSe can be reasonably ascribed to a self-doping effect. The predictions about the electronic density on the Fe sites (reaching a value of 0.1, where the corresponding pressure is about 8.6 GPa) are in good agreement with experimental data available in literature. The interpretation of the pressure-induced Tc enhancement in FeSe caused by the electron transfer and self-doping effect is carried out, which can be the correct approach to explain the Tc behavior under a wide type of mechanical loading.

Thermopower Evolution in Yb(hbox {Rh}_{1-x}hbox {Co}_x)_2hbox {Si}_2 Upon 4f Localization

We present thermopower measurements on Yb(hbox {Rh}_{1-x}hbox {Co}_x)_2hbox {Si}_2. Upon cobalt substitution, the Kondo temperature is decreasing and the single large thermopower minimum observed for hbox {YbRh}_2hbox {Si}_2 splits into two minima. Simultaneously, the absolute thermopower values are strongly reduced due to a weaker exchange coupling between the 4f and the conduction electron states with increasing x. Pure hbox {YbCo}_2hbox {Si}_2 is considered a stable trivalent system. Nevertheless, we still observe two minima in the thermopower indicative of weak residual Kondo scattering. This is in line with results from photoemission spectroscopy revealing a tiny contribution from hbox {Yb}^{2+}. The value at the high-T minimum in S(T) is found to be proportional to the Sommerfeld coefficient for the whole series. This unexpected finding is discussed in relation to recent measurements of the valence and Fermi surface evolution with temperature.

Possibility of a new order parameter driven by multipolar moment and Fermi surface evolution in CeGe

Polycrystalline CeGe is investigated by means of DC and AC susceptibility, non-linear DC susceptibility, electrical transport and heat capacity measurements in the low temperature regime. This compound shows two peaks at low magnetic field around TI ~ 10.7 and TII ~ 7.3 K due to antiferromagnetic ordering and subsequent spin rearrangement respectively. Investigation of non-linear DC susceptibility reveals a presence of higher order magnetization which results in the development of a new order parameter around TI. This leads to a lowering of symmetry of the magnetic state. The order parameter increases with decreasing temperature and stabilizes around TII. Consequently, the symmetry of the magnetic state is preserved below this transition. Heat capacity and resistivity results indicate the presence of a gap opening around TI on portion of Fermi surface, due to evolution of the Fermi surface. Magnetoresistance behavior and violation of Kohler’s rule suggest that the evolution of Fermi surface changes the symmetry of magnetic state. The observation of new order parameter (which is of second order) is also confirmed from the Landau free energy theory.

Evolution of the low-temperature Fermi surface of superconducting FeSe1\u2212xSx across a nematic phase transition

The existence of a nematic phase transition in iron-chalcogenide superconductors poses an intriguing question about its impact on superconductivity. To understand the nature of this unique quantum phase transition, it is essential to study how the electronic structure changes across this transition at low temperatures. Here, we investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition of FeSe1−xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 T in the low temperature regime down to 0.4 K. Most of the Fermi surfaces of FeSe1−xSx monotonically increase in size except for a prominent low frequency oscillation associated with a small, but highly mobile band, which disappears at the nematic phase boundary near x ~ 0.17, indicative of a topological Lifshitz transition. The quasiparticle masses are larger inside the nematic phase, indicative of a strongly correlated state, but they become suppressed outside it. The experimentally observed changes in the Fermi surface topology, together with the varying degree of electronic correlations, will change the balance of electronic interactions in the multi-band system FeSe1−xSx and promote different kz-dependent superconducting pairing channels inside and outside the nematic phase.

Fermi surface evolution of the 2D Hubbard model within a novel four-pole approximation

We present a novel solution of the 2D Hubbard model in the framework of the Composite Operator Method within a four-pole approximation. We adopt a basis of four fields: the two Hubbard operators plus two fields describing the Hubbard transitions dressed by nearest-neighbor spin fluctuations. We include these nonlocal operators because spin fluctuations play a dominant role in strongly correlated electronic systems with respect to other types of nonlocal charge, pair and double-occupancy fluctuations. The approximate solution performs very well once compared with advanced (semi-) numerical methods from the weak-to the strong-coupling regime, being by far less computational-resource demanding. We adopt this solution to study the single-particle properties of the model in the strong coupling regime, where the effects of strong short-range magnetic correlations are more relevant and could be responsible for anomalous features. In particular, we will focus on the characterization of the Fermi surface and of its evolution with doping.

Change of Fermi surface states related with two different Tc -raising mechanisms in iron pnictide superconductors

Evolution of Fermi surface (FS) states of NdFeAs$_{1-x}$P$_x$O$_{0.9}$F$_{0.1}$ single crystals with As/P substitution has been investigated. The critical temperature $T_{\rm c}$ and the power law exponent ($n$) of temperature-dependent resistivity ($\rho(T) = \rho_0 + AT^n$) show a clear correlation above $x=$0.2, suggesting that $T_{\rm c}$ is enhanced with increasing bosonic fluctuation in the same type of FS state. Around $x=$0.2, all the transport properties show anomalies, indicating that $x$$\sim$0.2 is the critical composition of drastic FS change. The angle resolved photoemission spectroscopy has more directly revealed the distinct change of FS around $x=$0.2, that one hole FS disappears at Brillouin zone center and the other FS with propeller like shape appears at zone corner with decreasing $x$. These results are indicative of the existence of two types of FS state with different nesting conditions that are related with two $T_{\rm c}$-rising mechanisms in this system.

Type-II Dirac cone and Dirac cone protected by nonsymmorphic symmetry in the carbon-lithium compound C4Li

In this work, we predict a novel band structure for Carbon-Lithium(C4Li) compound using the first-principles method. We show that it exhibits two Dirac points near the Fermi level; one located at W point originating from the nonsymmophic symmetry of the compound, and the other one behaves like a type-II Dirac cone with higher anisotropy along the {\Gamma} to X line. The obtained Fermi surface sheets of the hole-pocket and the electron-pocket near the type-II Dirac cone are separated from each other, and they would touch each other when the Fermi level is doped to cross the type-II Dirac cone. The evolution of Fermi surface with doping is also discussed. The bands crossing from T to W make a line-node at the intersection of kx={\pi} and ky={\pi} mirror planes. The C4Li is a novel material with both nonsymmorphic protected Dirac cone and type-II Dirac cone near the Fermi level which may exhibit exceptional topological property for electronic applications.

Evidence for pressure-induced node-pair annihilation in Cd3As2

As an intermediate state in the topological phase diagram, Dirac semimetals are of particular interest as a platform for studying topological phase transitions under external modulations. Despite a growing theoretical interest in this topic, it remains a substantial challenge to experimentally tune the system across topological phase transitions. Here, we investigate the Fermi surface evolution of Cd3As2 under high pressure through magnetotransport. A sudden change in Berry phase occurs at 1.3 GPa along with the unanticipated shrinkage of the Fermi surface, which occurs well below the structure transition point (~2.5 GPa). High pressure X-ray diffraction also reveals an anisotropic compression of the Cd3As2 lattice around a similar pressure. Corroborated by the first-principles calculations we show that an axial compression will shift the Dirac nodes towards the Brillouin zone center and eventually introduces a finite energy gap. The ability to tune the node position, a vital parameter of Dirac semimetals, can have dramatic impacts on the corresponding topological properties such as the Fermi arc surface states and the chiral anomaly. Our study demonstrates axial compression as an efficient approach for manipulating the band topology and exploring the critical phenomena near the topological phase transition in Cd3As2.

Fermi surface evolution of Na-doped PbTe studied through density functional theory calculations and Shubnikov–de Haas measurements

We present a combined experimental and theoretical study of the evolution of the low-temperature Fermi surface of lead telluride, PbTe, when holes are introduced through sodium substitution on the lead site. Our Shubnikov-de-Haas measurements for samples with carrier concentrations up to $9.4\times10^{19}$cm$^{-3}$ (0.62 Na atomic $\%$) show the qualitative features of the Fermi surface evolution (topology and effective mass) predicted by our density functional (DFT) calculations within the generalized gradient approximation (GGA): we obtain perfect ellipsoidal L-pockets at low and intermediate carrier concentrations, evolution away from ideal ellipsoidicity for the highest doping studied, and cyclotron effective masses increasing monotonically with doping level, implying deviations from perfect parabolicity throughout the whole band. Our measurements show, however, that standard DFT calculations underestimate the energy difference between the L-point and $\Sigma$-line valence band maxima, since our data are consistent with occupation of a single Fermi surface pocket over the entire doping range studied, whereas the calculations predict an occupation of the $\Sigma$-pockets at higher doping. Our results for low and intermediate compositions are consistent with a non-parabolic Kane-model dispersion, in which the L-pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy.

Anomalous Hall effect in a ferromagneticFe3Sn2single crystal with a geometrically frustrated Fe bilayer kagome lattice

The anomalous Hall effect is investigated for ferromagnetic Fe3Sn2 single crystal with geometrically frustrated kagome bilayer of Fe. The scaling behavior between anomalous Hall resistivity rho_{xy}^{A} and longitudinal resistivity rho_{xx} is quadratic and further analysis implies that the AHE in Fe3Sn2 single crystal should be dominated by the intrinsic Karplus-Luttinger mechanism rather than extrinsic skew-scattering or side-jump mechanisms. Moreover, there is a sudden jump of anomalous Hall conductivity sigma_{xy}^{A} appearing at about 100 K where the spin-reorientation transition from the c axis to the ab plane is completed. This change of sigma_{xy}^{A} might be related to the evolution of Fermi surface induced by the spin-reorientation transition.

Evolution of electron Fermi surface with doping in cobaltates

The notion of the electron Fermi surface is one of the characteristic concepts in the field of condensed matter physics, and it plays a crucial role in the understanding of the physical properties of doped Mott insulators. Based on the t-J model, we study the nature of the electron Fermi surface in the cobaltates, and qualitatively reproduce the essential feature of the evolution of the electron Fermi surface with doping. It is shown that the underlying hexagonal electron Fermi surface obeys Luttinger’s theorem. The theory also predicts a Fermi-arc phenomenon at the low-doped regime, where the region of the hexagonal electron Fermi surface along the -K direction is suppressed by the electron self-energy, and then six disconnected Fermi arcs located at the region of the hexagonal electron Fermi surface along the -M direction emerge. However, this Fermi-arc phenomenon at the low-doped regime weakens with the increase of doping.

Scrutinizing Hall Effect in Mn_{1-x}Fe_{x}Si: Fermi Surface Evolution and Hidden Quantum Criticality.

Separating between the ordinary Hall effect and anomalous Hall effect in the paramagnetic phase of Mn_{1-x}Fe_{x}Si reveals an ordinary Hall effect sign inversion associated with the hidden quantum critical (QC) point x^{*}∼0.11. The effective hole doping at intermediate Fe content leads to verifiable predictions in the field of fermiology, magnetic interactions, and QC phenomena in Mn_{1-x}Fe_{x}Si. The change of electron and hole concentrations is considered as a "driving force" for tuning the QC regime in Mn_{1-x}Fe_{x}Si via modifying the Ruderman-Kittel-Kasuya-Yosida exchange interaction within the Heisenberg model of magnetism.

Fermi arcs and pseudogap emerging from dimensional crossover at the Fermi surface in La2−xSrxCuO4

The doping mechanism and realistic Fermi surface (FS) evolution of La2−xSrxCuO4 (LSCO) are modelled within an extensive ab initio framework including advanced band-unfolding techniques. We show that ordinary Kohn-Sham DFT+U can reproduce the observed metal-insulator transition, when not restricted to the paramagnetic solution space. Arcs are self-doped by orbital charge transfer within the Cu-O planes, while the introduced Sr charge is strongly localized. Arc protection and the inadequacy of the rigid-band picture are consequences of a rapid change in orbital symmetry at the Fermi energy: the material undergoes a dimensional crossover along the Fermi surface, between the nodal (2D) and antinodal (3D) regions. In LSCO, this crossover accounts for FS arcs, the antinodal pseudogap, and insulating behavior in c-axis conductivity, all ubiquitous phenomena in high-Tc cuprates. Ligand Coulomb integrals involving out-of-plane sites are principally responsible for the most striking effects observed by ARPES in LSCO.