Fermi Surface Topology Research Articles

Pressure evolution of electronic and structural properties in transition metal dichalcogenide 1T-Co1.06Te2

Two-dimensional transition metal dichalcogenides (TMDs) are important materials for promising electronic devices because they usually exhibit excellent and highly tunable electronic properties. Here, we report on the pressure-driven electronic phase transition in a TMD 1T-Co1.06Te2. High-pressure transport measurements reveal a sign reversal of the Hall coefficients at a critical point of P C ∼ 32 GPa, evidencing a transition from hole band(s) dominated transport into one that is dominated by electron band(s). Synchrotron x-ray diffraction experiments demonstrate that no structural phase transition occurs below 46.3 GPa, indicating an electronic origin of the transition. Moreover, a kink anomaly of the lattice constant ratio c/a is also observed at P = P C. These results might indicate a Lifshitz transition which refers to a change of Fermi surface topology in absence of structural transition.

Anisotropic giant magnetoresistance and Fermi surface topology in the layered compound YbBi2

Magnetoresistance in novel materials has been attracting ever-increasing attention since its mechanism is still the subject of intense debate and the physics behind these emergent phenomena remains a wild space to be explored. Here, we grow ${\mathrm{YbBi}}_{2}$ single crystals and study their anisotropic giant magnetoresistance and Fermi surface topology via de Haas--van Alphen oscillation and Hall resistivity measurements, electronic band structure calculations, and so on. A detailed analysis of the angle-dependent quantum oscillations reveals the presence of nontrivial topological electronic states and several cylindrical Fermi surface sheets extended along the $b$ axis. Hall resistivity data suggest that multiple charge carriers participate in the transport, and electron and hole densities are nearly balanced. These findings are further confirmed by theoretical calculations. After checking several possible mechanisms, the giant magnetoresistance ($\ensuremath{\sim}1.2\ifmmode\times\else\texttimes\fi{}{10}^{3}%$ at 14 T and 2 K) in ${\mathrm{YbBi}}_{2}$ is ascribed to the carrier compensation instead of topological protection and open orbits. Additionally, we also find that Fermi surface anisotropy serves as a key element for the angular magnetoresistance in this compound. Our studies show that ${\mathrm{YbBi}}_{2}$ can be not only a topologically nontrivial material, but also a prototype system to check familiar magnetoresistance mechanisms.

Stabilizing superconductivity of ternary metal pentahydride hbox {CaCH}_{{5}} via electronic topological transitions under high pressure from first principles evolutionary algorithm

We explored the phase stability of ternary pentahydride hbox {CaCH}_{{5}} based on the first principles evolutionary algorithm. Here, we successfully search for a candidate structure up to 500 GPa. As a consequence, the possible stable structure of hbox {CaCH}_{{5}} is found be to a monoclinic structure with space group Pm at a pressure of 50 GPa. Moreover, the orthorhombic structure with a space group of Cmcm is found to be thermodynamically stable above 316 GPa. With this, the Kohn-Sham equation plays a crucial role in determining the structural stability and the electronic structure. Therefore, its structural stability is discussed in term of electronic band structure, Fermi surface topology, and dynamic stability. With these results, we propose that the superconducting transition temperature (hbox {T}_{{c}}) of Cmcm structure is estimated to be 50 K at 450 GPa. This could be implied that the proposed Cmcm structure may be emerging as a new class of superconductive ternary metal pentahydride. Our findings pave the way for further studies on an experimental observation that can be synthesized at high pressure.

Fermi surface of a system with strong valence fluctuations: Evidence for a noninteger count of valence electrons in EuIr2Si2

We present de Haas-van Alphen (dHvA) measurements on an Eu-based valence-fluctuating system. ${\mathrm{EuIr}}_{2}{\mathrm{Si}}_{2}$ exhibits a temperature-dependent, noninteger europium valence with ${\mathrm{Eu}}^{2.8+}$ at low temperatures. The comparison of experimental results from our magnetic-torque experiments in fields up to 32 T and density functional theory band-structure calculations with localized $4f$ electrons shows that the best agreement is reached for a Fermi surface based on a valence of ${\mathrm{Eu}}^{2.8+}$. The calculated quantum-oscillation frequencies for ${\mathrm{Eu}}^{3+}$ instead cannot explain all the experimentally observed frequencies. The effective masses, derived from the temperature dependence of the dHvA oscillation amplitudes, show not only a significant enhancement with masses up to $11\phantom{\rule{3.33333pt}{0ex}}{m}_{e}$ (${m}_{e}$ being the free electron mass), but also a magnetic-field dependence of the heaviest mass. We attribute the formation of these heavy masses to strong correlation effects resulting from valence fluctuations of $4f$ electrons being strongly hybridized with conduction electrons. The increase of the heavy masses with magnetic field likely results from the onset of the expected field-induced valence crossover that enhances these valence fluctuations but does not alter the Fermi-surface topology in the field range studied.

Fermi-Surface Origin of Skyrmion Lattices in Centrosymmetric Rare-Earth Intermetallics.

We show from first principles that barrel-shaped structures within the Fermi surface of the centrosymmetric intermetallic compounds GdRu_{2}Si_{2} and Gd_{2}PdSi_{3} give rise to Fermi surface nesting, which determines the strength and sign of quasi-two-dimensional Ruderman-Kittel-Kasuya-Yosida pairwise exchange interactions between the Gd moments. This is the principal mechanism leading to their helical single-q spin-spiral ground states, providing transition temperatures and magnetic periods in good agreement with experiment. Using atomistic spin-dynamic simulations, we draw a direct line between the subtleties of the three-dimensional Fermi surface topology and the stabilization of a square skyrmion lattice in GdRu_{2}Si_{2} at applied magnetic fields as observed in experiment.

Protonation-induced discrete superconducting phases in bulk FeSe single crystals

The superconducting transition temperature, $T_{\rm{c}}$, of FeSe can be significantly enhanced several-fold by applying pressure, electron doping, intercalating spacing layer, and reducing dimensionality. Various ordered electronic phases, such as nematicity and spin density waves, have also been observed accompanying high-$T_{\rm{c}}$ superconductivity. Investigation on the evolution of the electronic structure with $T_{\rm{c}}$ is essential to understanding electronic behavior and high-$T_{\rm{c}}$ superconductivity in FeSe and its derived superconductors. In this report, we have found a series of discrete superconducting phases, with a maximum $T_{\rm{c}}$ up to 44 K, in H$^+$-intercalated FeSe single crystals using an ionic liquid gating method. Accompanied with the increase of $T_{\rm{c}}$, suppression of the nematic phase and evolution from non-Fermi-liquid to Fermi-liquid behavior was observed. An abrupt change in the Fermi surface topology was proposed to explain the discrete superconducting phases. A band structure that favors the high-$T_{\rm{c}}$ superconducting phase was also revealed.

A unified theory of ferromagnetic quantum phase transitions in heavy fermion metals

Motivated by the recent discovery of a continuous ferromagnetic quantum phase transition in CeRh$_6$Ge$_4$ and its distinction from other U-based heavy fermion metals such as UGe$_2$, we develop a unified explanation of their different ground state properties based on an anisotropic ferromagnetic Kondo-Heisenberg model. We employ an improved large-$N$ Schwinger boson approach and predict a full phase diagram containing both a continuous ferromagnetic quantum phase transition for large magnetic anisotropy and first-order transitions for relatively small anisotropy. Our calculations reveal three different ferromagnetic phases including a half-metallic spin selective Kondo insulator with a constant magnetization. The Fermi surface topologies are found to change abruptly between different phases, consistent with that observed in UGe$_2$. At finite temperatures, we predict the development of Kondo hybridization well above the ferromagnetic long-range order and its relocalization near the phase transition, in good agreement with band measurements in CeRh$_6$Ge$_4$. Our results highlight the importance of magnetic anisotropy and provide a unified theory for understanding the ferromagnetic quantum phase transitions in heavy fermion metals.

Electronic structure and open-orbit Fermi surface topology in isostructural semimetals NbAs2 and W2As3 with extremely large magnetoresistance

In transition-metal dipnictides TmPn2 (Tm = Ta and Nb; Pn = P, As, and Sb), the origin of extremely large magnetoresistance (XMR) is yet to be studied by the direct visualization of the experimental band structures. Here, using angle-resolved photoemission spectroscopy, we map out the three-dimensional electronic structure of NbAs2. The open-orbit topology contributes to a non-negligible part of the Fermi surfaces (FSs), like that of the isostructural compound MoAs2, where the open FS is proposed to likely explain the origin of XMR. We further demonstrate the observation of open characters in the overall FSs of W2As3, which is also a XMR semimetal with the same space group of C12/m1 as the TmPn2 family and MoAs2. Our results suggest that the open-orbit FS topology may be a shared feature between XMR materials with the space group of C12/m1 and, thus, could possibly play a role in determining the corresponding XMR effect together with the electron–hole compensation.

Magnetization-driven Lifshitz transition and charge-spin coupling in the kagome metal YMn6Sn6

The Fermi surface (FS) is essential for understanding the properties of metals. It can change under both conventional symmetry-breaking phase transitions and Lifshitz transitions (LTs), where the FS, but not the crystal symmetry, changes abruptly. Magnetic phase transitions involving uniformly rotating spin textures are conventional in nature, requiring strong spin-orbit coupling (SOC) to influence the FS topology and generate measurable properties. LTs driven by a continuously varying magnetization are rarely discussed. Here we present two such manifestations in the magnetotransport of the kagome magnet YMn6Sn6: one caused by changes in the magnetic structure and another by a magnetization-driven LT. The former yields a 10% magnetoresistance enhancement without a strong SOC, while the latter a 45% reduction in the resistivity. These phenomena offer a unique view into the interplay of magnetism and electronic topology, and for understanding the rare-earth counterparts, such as TbMn6Sn6, recently shown to harbor correlated topological physics.

Anisotropic large magnetoresistance and Fermi surface topology of terbium monoantimonide

Rare-earth monopnictides have received a great deal of attention for their exotic magnetic and electronic properties. Here, we grow high-quality TbSb single crystals, and perform their magnetization, specific heat and transport measurements, and band structure calculations. In this compound, an antiferromagnetic phase transition emerges at ∼14.5 K (TN), below which metamagnetic behaviors can be observed. Specific heat data suggest that Γ4 triplet state dominates the ground magnetic properties, and thus gives rise to weak magnetic anisotropy. Analogous to other isostructural counterparts, TbSb shows extreme magnetoresistance and triangular temperature-field phase diagram. Hall resistivity measurements reveal that carrier concentrations and mobilities change their values in different magnetic states. These findings are supported by the theoretical calculations from which the effect of magnetic orderings on Fermi surface topology can be determined. Nevertheless, the magnetoresistance below and above TN in TbSb shares similar angle dependences, and follows the fashions as observed in those nonmagnetic sister compounds because of its weak anisotropy in magnetization. Our studies uncover the spin ordering effects on angular magnetoresistance and electronic band structures of TbSb, and could be employed to understand the related issues in other systems with similar magnetic behaviors.

Fermi surface topology and anisotropic superconducting gap in electron-doped hydride compounds at high pressure

The recent theoretical discovery of the predicted high-temperature superconductivity (superconducting transition temperature ${T}_{\mathrm{c}}\ensuremath{\sim}351\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 300 GPa) in clathrate ${\mathrm{Li}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$ is an important advance toward room-temperature superconductors. Here we use first-principle approaches to identify a new ternary hydride of clathrate structure of $Fd\overline{3}m\text{\ensuremath{-}}{\mathrm{Rb}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$ by Rb-doped $\mathrm{Mg}{\mathrm{H}}_{16}$ at 300 GPa. We first verified the thermal and dynamic stability for the $Fd\overline{3}m\text{\ensuremath{-}}{\mathrm{Rb}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$ through the formation enthalpy and phonon calculations, respectively. Next, we showcased that $Fd\overline{3}m\text{\ensuremath{-}}{\mathrm{Rb}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$ has two Fermi surface (FS) sheets, which are both dominated by the H2 $s$ orbital. Using the Eliashberg formalism, the ${T}_{\mathrm{c}}$ of $Fd\overline{3}m\text{\ensuremath{-}}{\mathrm{Rb}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$ was calculated to be 130 K at 300 GPa. Furthermore, the calculations of the electronic, phonon, and superconducting properties reveal that $Fd\overline{3}m\text{\ensuremath{-}}{\mathrm{Li}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$ has four FS sheets with mixed H1 $s$ and H2 $s$ orbital character and reaches the ${T}_{\mathrm{c}}$ of 352 K at 300 GPa. Compared with $Fd\overline{3}m\text{\ensuremath{-}}{\mathrm{Rb}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$, $Fd\overline{3}m\text{\ensuremath{-}}{\mathrm{Li}}_{2}\mathrm{Mg}{\mathrm{H}}_{16}$ produces high-frequency phonon softening, leading to the enhancement of ${T}_{\mathrm{c}}$. Moreover, it is demonstrated that Rb substitution for Li produces the reduction of contribution of H orbitals to the FS sheets and the decrease in FS sheets in number, which tend to decrease the electron-phonon coupling strength and ${T}_{\mathrm{c}}$. Our observed FS sheets and their associated superconducting gap will provide key insights into the understanding of the superconductivity mechanism in these and related systems.

Giant magnetoresistance, Fermi-surface topology, Shoenberg effect, and vanishing quantum oscillations in the type-II Dirac semimetal candidates MoSi2 and WSi2

We performed comprehensive theoretical and experimental studies of the electronic structure and the Fermi surface topology of two novel quantum materials, $\mathrm{Mo}{\mathrm{Si}}_{2}$ and $\mathrm{W}{\mathrm{Si}}_{2}$. The theoretical predictions of the electronic structure in the vicinity of the Fermi level was verified experimentally by thorough analysis of the observed quantum oscillations in both electrical resistivity and magnetostriction. We established that the Fermi surface sheets in $\mathrm{Mo}{\mathrm{Si}}_{2}$ and $\mathrm{W}{\mathrm{Si}}_{2}$ consist of 3D dumbbell-shaped holelike pockets and rosette-shaped electronlike pockets, with nearly equal volumes. Based on this finding, both materials were characterized as almost perfectly compensated semimetals. In conjunction, the magnetoresistance attains giant values of ${10}^{4}$ and ${10}^{5}%$ for $\mathrm{W}{\mathrm{Si}}_{2}$ and $\mathrm{Mo}{\mathrm{Si}}_{2}$, respectively. In turn, the anisotropic magnetoresistance achieves $\ensuremath{-}95%$ and $\ensuremath{-}98%$ at $T=2$ K and in $B=14$ T for $\mathrm{W}{\mathrm{Si}}_{2}$ and $\mathrm{Mo}{\mathrm{Si}}_{2}$, respectively. Furthermore, for both compounds we observed the Shoenberg effect in their Shubnikov-de Haas oscillations that persisted at as high temperature as $T=25$ K in $\mathrm{Mo}{\mathrm{Si}}_{2}$ and $T=12$ K in $\mathrm{W}{\mathrm{Si}}_{2}$. In addition, we found for $\mathrm{Mo}{\mathrm{Si}}_{2}$ a rarely observed spin-zero phenomenon. Remarkably, the electronic structure calculations revealed type-II Dirac cones located near 480 and 710 meV above the Fermi level in $\mathrm{Mo}{\mathrm{Si}}_{2}$ and $\mathrm{W}{\mathrm{Si}}_{2}$, respectively.

Exceptional hexagonal warping effect in multi-Weyl semimetals

Hexagonal warping (HW) in three-dimensional topological insulators is, by now, well-known. We show that non-Hermitian (NH) loss/gain can generate an exceptional HW effect in double Weyl-semimetals (DWSM). This unique feature of DWSMs has distinctive effects on Fermi surface topology. Importantly, in the presence of such a $k^3$ spin orbit coupling mimicking term, the symmetry associated with the DWSMs is changed, leading to four exceptional points, among which two are degenerate. Introducing a driving field removes this degeneracy. The combined action of the NH warping and driving parameters leads to notable effects, including merging and tuning of exceptional points. We analyze the topological nature of the generated exceptional contours by evaluating several topological invariants, such as winding number, vorticity, and NH Berry curvature. We hope that our theoretical results would initiate possible experiments exploring NH HW effects.

A Quantized Surprise from Fermi Surface Topology

A Quantized Surprise from Fermi Surface Topology

Stripes versus superconductivity in the doped Hubbard model on the honeycomb lattice

We study the ground state of the doped Hubbard model on the honeycomb lattice in the small doping and strongly interacting region. The nature of the ground state by doping holes into the anti-ferromagnetic Mott insulating states on the honeycomb lattice remains a long-standing unsolved issue, even though tremendous efforts have been spent to investigate this challenging problem. An accurate determination of the ground state in this system is crucial to understand the interplay between the topology of Fermi surface and strong correlation effect. In this work, we employ two complementary, state-of-the-art, many-body computational methods -- constrained path (CP) auxiliary-field quantum Monte Carlo (AFQMC) with self-consistent constraint and density matrix renormalization group (DMRG) methods. Systematic and detailed cross-validations are performed between these two methods for narrow systems where DMRG can produce reliable results. AFQMC are then utilized to study wider systems to investigate the thermodynamic limit properties. The ground state is found to be a half-filled stripe state in the small doping and strongly interacting region. The pairing correlation shows $d$-wave symmetry locally, but decays exponentially with the distance between two pairs.

Roles of optical phonons and logarithmic profile of electron-phonon coupling integration in superconducting Sc0.5Y0.5H6 superhydride under pressures

The stable structure of symmetrically Sc/Y-substituted hexahydride Sc0.5Y0.5H6 under high pressures is theoretically reported herein to superconduct with maximum Tc of 127 K, as supported by analyses of the electronic band structure, Fermi surface topologies, phonon dispersion, and the Eliashberg spectral function. We also expatiate on an alternative approach in describing the nature of Tc under pressures, i.e. the bandwidth function as an approximation of the spectral function (α2F) for Sc0.5Y0.5H6. Being a special case, the bandwidth function is derived from an analytic solution for the α2F function given constraints arising from the characteristics of the function itself. The consistency between the DFT-obtained α2F function and that of the proposed model suggests the dominant role played by the cutoff frequency for the optical modes in affecting Tc.

Revealing sign-reversal s+− -wave pairing by quasiparticle interference in the heavy-fermion superconductor CeCu2Si2

Recent observations of two nodeless gaps in superconducting CeCu$_2$Si$_2$ have raised intensive debates as to its exact gap structure of either sign-reversal ($s^{+-}$) or sign-preserving ($s^{++}$) pairing. Here we investigate the quasiparticle interference (QPI) using realistic Fermi surface topology for both weak and strong interband impurity scatterings. Our calculations of the QPI and integrated antisymmetrized local density of states reveal qualitative distinctions between $s^{+-}$ and $s^{++}$ pairing states, which include the intragap impurity resonance and a significant energy-dependence difference between two gap energies. Our predictions provide a guide for phase-sensitive QPI measurements to uncover decisively the true pairing symmetry in the heavy-fermion superconductor CeCu$_2$Si$_2$.

Observing the de Haas–van Alphen Effect to Unveil the Electronic Structure of UTe2 Superconductor

The topology of Fermi surfaces with heavy effective masses in strongly correlated topological superconductor UTe2 was investigated using quantum oscillation measurements and theoretical calculations.

Identifying the Dirac point composition in Bi1−xSbx alloys using the temperature dependence of quantum oscillations

The thermal chiral anomaly is a new mechanism for thermal transport that occurs in Weyl semimetals (WSMs). It is attributed to the generation and annihilation of energy at Weyl points of opposite chirality. The effect was observed in the Bi1−xSbx alloy system, at x = 11% and 15%, which are topological insulators at zero field and driven into an ideal WSM phase by an external field. Given that the experimental uncertainty on x is of the order of 1%, any systematic study of the effect over a wider range of x requires precise knowledge of the transition composition xc at which the electronic bands at the L-point in these alloys have Dirac-like dispersions. At x > xc, the L-point bands are inverted and become topologically non-trivial. In the presence of a magnetic field along the trigonal direction, these alloys become WSMs. This paper describes how the temperature dependence of the frequency of the Shubnikov–de Haas oscillations F(x,T) at temperatures of the order of the cyclotron energy can be used to find xc and characterize the topology of the electronic Fermi surface. Semimetallic Bi1−xSbx alloys with topologically trivial bands have dF(x,T)/dT ≥ 0; those with Dirac/Weyl fermions display dF(x,T)/dT < 0.

Structural Metastability and Fermi Surface Topology of SrAl2Si2.

SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, α phase or a superconducting, BaZn2P2-type, β phase. We explore possible transformations by employing pressure- and temperature-dependent free-energy calculations, vibrational spectral calculations, and room-temperature synchrotron powder X-ray diffraction (PXRD) measurements up to 14 GPa using a diamond anvil cell. Our theoretical and empirical analyses together with all reported baric and thermal events on both phases allow us to construct a preliminary P-T diagram of transformations. Our calculations show a relatively low critical pressure for the α-to-β transition (4.9 GPa at 0 K, 5.0 GPa at 300 K, and 5.3 GPa at 900 K); nevertheless, our nonequilibrium analysis indicates that the low-pressure low-temperature α phase is separated from a metastable β phase by a relatively high activation barrier. This analysis is supported by our PXRD data at ambient temperature and P ≤ 14 GPa, which shows an absence of the β phase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in the Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl2Si2 dimorphs or total chemical substitution of Ca by Sr in the isomorphous CaAl2Si2 α phase; empirically, the manifestation of such a topology modification is evident upon comparison of the evolution of the (magneto)transport properties of members of SrAl2Si2 dimorphs and α isomorphs.