Fermi Surface Topology Research Articles

Synthesis, formation mechanism, and intrinsic physical properties of several As/P-containing MAX phases

• Molten-salt sintering is applicable to P-based MAX phases • Solid-state sintering is suitable to As-based MAX phases • Route 1: NbP+Nb+C → Nb 3 P 2 C is most favorable to obtain pure Nb 3 P 2 C • Molten-salt sintering do well on all routes, especially for Nb 2 PC+NbP→Nb 3 P 2 C • Various intrinsic physical properties of 321 phases are firstly reported 321 phases are an atypical series of MAX phases, in which A = As/P, with superior elastic properties, featuring in the MA-triangular-prism bilayers in the crystal structure. Until now, besides Nb 3 As 2 C, the pure phases of the other 321 compounds have not been realized, hampering the study of their intrinsic properties. Here, molten-salt sintering (MSS) and solid-state synthesis (SSS) were applied to synthesize As/P-containing 321 phases and 211 phases. Analyzing the phase composition of the end-product via multiple-phase Rietveld refinement, we found that MSS can effectively improve the purity of P-containing MAX phases, with the phase content up to 99% in Nb 3 P 2 C and 75.4(5)% in Nb 2 PC. In contrast, MSS performed poorly on As-containing MAX phases, only 8.9(4)% for Nb 3 As 2 C and 64(2)% for Nb 2 AsC, as opposed to the pure phases obtained by SSS. The experimental analyses combined with first-principles calculations reveal that the dominant formation route of Nb 3 P 2 C is through NbP + Nb + C → Nb 3 P 2 C. Moreover, we found that the benefits of MSS on P-containing MAX phases are on the facilitation of three considered chemical reaction routes, especially on Nb 2 PC + NbP → Nb 3 P 2 C. Furthermore, the intrinsic physical properties and Fermi surface topology of two 321 phases consisting of electron, hole, and open orbits are revealed theoretically and experimentally, in which the electron carriers are dominant in electrical transport. The feasible synthesis methods and the formation mechanism are instructive to obtain high-purity As/P-containing MAX phases and explore new MAX phases. Meanwhile, the intrinsic physical properties will give great support for future applications on 321 phases.

Explaining the pseudogap through damping and antidamping on the Fermi surface by imaginary spin scattering

The mechanism of the pseudogap observed in hole-doped cuprates remains one of the central puzzles in condensed matter physics. We analyze this phenomenon via a Feynman-diagrammatic inspection of the Hubbard model. Our approach captures the pivotal interplay between Mott localization and Fermi surface topology beyond weak-coupling spin fluctuations, which would open a spectral gap near hot spots. We show that strong coupling and particle-hole asymmetry trigger a very different mechanism: a large imaginary part of the spin-fermion vertex promotes damping of antinodal fermions and, at the same time, protects the nodal Fermi arcs (antidamping). Our analysis naturally explains puzzling features of the pseudogap observed in experiments, such as Fermi arcs being cut off at the antiferromagnetic zone boundary and the subordinate role of hot spots.

Transport signatures of Van Hove singularities in mesoscopic twisted bilayer graphene

Magic-angle twisted bilayer graphene exhibits quasi-flat low-energy bands with Van Hove singularities close to the Fermi level. These singularities play an important role in the exotic phenomena observed in this material, such as superconductivity and magnetism, by amplifying electronic correlation effects. In this work, we study the correspondence of four-terminal conductance and the Fermi surface topology as a function of the twist angle, pressure, and energy in mesoscopic, ballistic samples of small-angle twisted bilayer graphene. We establish a correspondence between features in the wide-junction conductance and the presence of van Hove singularities in the density of states. Moreover, we identify additional transport features, such as a large, pressure-tunable minimal conductance, conductance peaks coinciding with non-singular band crossings, and unusually large conductance oscillations as a function of the system size. Our results suggest that twisted bilayer graphene close the magic angle is a unique system featuring simultaneously large conductance due to the quasi-flat bands, strong quantum nonlinearity due to the Van Hove singularities and high sensitivity to external parameters, which could be utilized in high-frequency device applications and sensitive detectors.

Local electron correlation effects on the fermiology of the weak itinerant ferromagnet ZrZn2

The Fermi surface topology plays an important role in the macroscopic properties of metals. It can be particularly sensitive to electron correlation, which appears to be especially significant for the weak itinerant ferromagnet ZrZn2. Here, we look at the differences in the predicted Fermi surface sheets of this metallic compound in its paramagnetic phase for both density functional theory (DFT) and the combination of DFT with dynamical mean field theory (DFT + DMFT). The theoretical spectral functions evaluated at the Fermi level were used along with calculations of the electron–positron momentum density (also known as the two-photon momentum density) in k-space to provide insights into the origin of certain features of the Fermi surface topology. We compare this two photon momentum density to that extracted from the positron annihilation experimental data (2004 Phys. Rev. Lett. 92 107003). The DFT + DMFT densities are in better agreement with the experiment than the DFT, particularly with regard to the flat bands around the L and W high symmetry points. The experimental neck around L, which relates to a van Hove singularity, is present in DFT + DMFT but not in the DFT. We find that these flat bands, and as such the Fermi surface topology, are sensitive to the many body electron correlation description, and show that the positron annihilation technique is able to probe this. This description is significant for the observed behavior such as the Lifshiftz transition around the quantum critical point.

Ab initio electronic structure of metallized NiS2 in the noncollinear magnetic phase

We investigate the electronic structure of the archetypical Mott insulator ${\mathrm{NiS}}_{2}$ by means of density functional theory calculations in which we explicitly account for the noncollinear antiferromagnetic order, as recently established in the isoelectronic analog $\mathrm{Ni}{(\mathrm{S},\mathrm{Se})}_{2}$. For metallic ${\mathrm{NiS}}_{2}$ under high pressures, our calculations predict a Fermi surface topology and volume which are in excellent agreement with recent quantum oscillation studies. However, we find that density functional theory wrongly predicts a metallic ground state even at ambient pressures, similar to previous nonmagnetic or collinear antiferromagnetic models. By including a Hubbard interaction $U$ and an on-site exchange interaction $J$, the metallic phase is suppressed, but even such an extended model fails to describe the nature of the metal-to-insulating phase transition and describes the insulating phase itself incorrectly. These results highlight the importance of more sophisticated computational approaches even deep in the insulating phase, far away from the Mott insulating phase transition.

The carriers doping effect on electronic and optical behaviors of newly layered Sr1- xHfxFBiS2 alloying materials for light-modulator devices

BiS2-layered superconductors have captivated the colossal attention of the scientific community owing to their structural analogies to other unconventional superconducting materials, termed cuprates and Fe-based superconducting materials. Herein, we inspect how the physical characteristics will be altered in the SrFBiS2 mother compound upon the compositional doping of the hafnium element. The electronic structure characteristics of Sr1−xHfxFBiS2 (x = 0 to 1) are simulated with the virtue of the full-potential linearized augmented plane-wave technique. Accordingly, the evaluated band gap of the parent compound (SrFBiS2) is roughly about 0.88 eV, which vanishes under the substitutional impurity impact of tetravalent (Hf+4). However, a metallic character occurs when Hf is substituted in the SrFBiS2 mother compound, although the bottom conduction band is relocated downward to the Fermi level with the emergence of a viable superconducting state. The band structures and Fermi surface (FS) topology vary obviously with the augmentation of Hf concentration and FS nesting is acquired at the wave vector (π, π, 0). Moreover, the optical characteristics of the explored compounds are probed, while the optical anisotropy in the optical absorption spectra is well marked in the mother material and diminishes through the substitutional Hf impurity effect. The metallic character with Drude-model arises when the incident photon energies span from zero photon energy to infrared spectrum when Hf substitutes Sr in the SrFBiS2 parent compound. The spin orbit coupling (SOC) significantly influences the band structures, Fermi surface topologies, and optical features of these doped systems based on the heavy elements, like Hf or Bi. Hence, the induced nesting FS outcomes may turn Sr1−xHfxFBiS2 into striking materials for newly layered superconductors under the doping effect of hafnium element at low compositional content. Eventually, it is inferred that the SrFBiS2 parent compound would be a potential contender for new superconductivity and may be useful for optical communications and laser devices.

Superatomic-Charge-Density-Wave in Cluster-Assembled Au6Te12Se8 Superconductors.

Superatomic crystals are a class of hierarchical materials composed of atomically precise clusters assembled via van der Waals or covalent-like interactions. Au6Te12Se8, an all-inorganic superatomic superconductor exhibiting superatomic-charge-density-wave (S-CDW), provides the first platform to study the response of its collective quantum phenomenon to the external pressure in superatomic crystals. We reveal a competition between S-CDW and superconductivity in an ultra-narrow pressure range. Distinct from conventional CDW ordering, S-CDW shows the lowest threshold (0.1 GPa) toward external pressure that is 1-2 orders of magnitude lower than other atomic compounds. Prominently, a second superconducting phase emerges above 7.3 GPa with a threefold enhancement in the transition temperature (Tc) to 8.5 K, indicating a switch of the conduction channel from the a- to b-axis. In situ synchrotron diffractions and theoretical calculations reveal a pressure-mediated mesoscopic slip of the superatoms and a 2D-3D transition of the Fermi surface topology, which well explains the observed dimensional crossover of conductivity and re-entrant superconductivity.

Fermiology of a topological line-nodal compound CaSb2 and its implication to superconductivity: Angle-resolved photoemission study

We performed angle-resolved photoemission spectroscopy with micro-focused beam on a topological line-nodal compound CaSb2 which undergoes a superconducting transition at the onset Tc~1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the G point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the Y(C) point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar Ts's. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in CaSb2.

Solvable periodic Anderson model with infinite-range Hatsugai-Kohmoto interaction: Ground-states and beyond

In this paper we introduce a solvable two-orbital/band model with infinite-range Hatsugai-Kohmoto interaction, which serves as a modified periodic Anderson model. Its solvability results from strict locality in momentum space, and is valid for arbitrary lattice geometry and electron filling. Case study on a one-dimension ($1D$) chain shows that the ground-states have Luttinger theorem-violating non-Fermi-liquid-like metallic state, hybridization-driven insulator and interaction-driven featureless Mott insulator. The involved quantum phase transition between metallic and insulating states belongs to the universality of Lifshitz transition, i.e. change of topology of Fermi surface or band structure. Further investigation on $2D$ square lattice indicates its similarity with the $1D$ case, thus the findings in the latter may be generic for all spatial dimensions. We hope the present model or its modification may be useful for understanding novel quantum states in $f$-electron compounds, particularly the topological Kondo insulator SmB$_{6}$ and YbB$_{12}$.

To 95-th birthday of Professor E.I. Rashba (looking back ones again)

The theory of spin-orbit interaction, developed by E.I. Rashba more than 30 years ago, stimulated the rapid development of a new discipline – spintronics – the physics of processes and devices based on the control of spins. The paper summarizes achievements of Prof. Rashba in the early stage of his scientific researches, particularly those, which were performed in Ukraine. Among them, prediction of electric dipole spin resonance (EDSR), phase transitions in spin-orbit coupled systems driven by change of the Fermi surface topology, giant oscillator strength of impurity excitons, and coexistence of free and self-trapped excitons. Solid state physics is the basis of contemporary electronics and optoelectronics. Various electronic, optical, acoustical and other effects and processes in solid define performances of modern solid state devices. Multitude of groups and thousands researchers are involved in discovering, study and using relevant new phenomena. Among them, Professor Emmanuel Rashba with his outstanding results in physics of crystals is seen (rises) as a profound personality. His contribution in almost all branches of solid state physics cannot be exaggerated, some of his results have found important applications. Prof. E.I. Rashba is known as one of the leading theorists in Ukraine, in Soviet Union, and he continued the successful career in United States. Although many years have already passed, scientific community in Ukraine remembers Prof. E.I. Rashba and thankfully appreciates his impact to formation of condensed matter researches in our country. This short text is devoted to Prof. E.I. Rashba and is written on the occasion of his birthday.

Superconductivity Induced by Lifshitz Transition in Pristine SnS2 under High Pressure.

The importance of electronic structure evolutions and reconstitutions is widely acknowledged for strongly correlated systems. The precise effect of pressurized Fermi surface topology on metallization and superconductivity is a much-debated topic. In this work, an evolution from insulating to metallic behavior, followed by a superconducting transition, is systematically investigated in SnS2 under high pressure. In-situ X-ray diffraction measurements show the stability of the trigonal structure under compression. Interestingly, a Lifshitz transition, which has an important bearing on the metallization and superconductivity, is identified by the first-principles calculations between 35 and 40 GPa. Our findings provide a unique playground for exploring the relationship of electronic structure, metallization, and superconductivity under high pressure without crystal structural collapse.

Ultrafast carrier and lattice dynamics in the Dirac semimetal NiTe2

The nonequilibrium fermionic quasiparticle responding to bosonic modes is critical to understand the electromagnetic response of different materials. Using ultrafast optical spectroscopy, this work reveals that sudden variation in the Fermi surface topology induces exotic carrier and phonon dynamics via electron-phonon interaction in the type-II Dirac semimetal NiTe${}_{2}$. This provides the feasibility to manipulate the nonequilibrium carrier and lattice responses in a variety of materials with similar electronic structure, and benefits the development of future ultrafast optoelectronic devices.

Spanning Fermi arcs in a two-dimensional magnet

The discovery of topological states of matter has led to a revolution in materials research. When external or intrinsic parameters break symmetries, global properties of topological materials change drastically. A paramount example is the emergence of Weyl nodes under broken inversion symmetry. While a rich variety of non-trivial quantum phases could in principle also originate from broken time-reversal symmetry, realizing systems that combine magnetism with complex topological properties is remarkably elusive. Here, we demonstrate that giant open Fermi arcs are created at the surface of ultrathin hybrid magnets where the Fermi-surface topology is substantially modified by hybridization with a heavy-metal substrate. The interplay between magnetism and topology allows us to control the shape and the location of the Fermi arcs by tuning the magnetization direction. The hybridization points in the Fermi surface can be attributed to a non-trivial mixed topology and induce hot-spots in the Berry curvature, dominating spin and charge transport as well as magneto-electric coupling effects.

Identifying topological superconductivity in two-dimensional transition-metal dichalcogenides

We study the superconducting pairing instabilities and gap functions for prototypical two-dimensional (2D) transition-metal dichalcogenides (TMDCs) WS$_2$, MoTe$_2$, and MoS$_2$ in the 2H phase under both hole and electron doping at 10 K. Our first-principles quantum many-body Green's function approach allows us to treat the full $d$ and $p$ manifold of orbitals with strong spin-orbit coupling, yielding pairing predictions with material specific detail. The resulting gap functions exhibit a variety of mixed-parity superconducting states, including $s$, $p$, $d$, $f$, $d\pm id$, and $p\pm ip$ pairing modes. In particular, we predict 3% and 4% hole-doped WS$_2$ to be a chiral $p\pm ip$ topological superconductor. For 1% hole-doped MoS$_2$, we find a competition between three doubly degenerate chiral and non-chiral instabilities. Overall, the relative pairing strengths are found to follow the Fermi surface topology, due to nesting between the Fermi surface sheets. Finally, we discuss our predictions in relation to available experimental data and classify the topology of the predicted superconducting pairing symmetries.

Anomalous Hall effect at the Lifshitz transition in ZrTe5

Zirconium pentatelluride ZrTe5 is a topological semimetal. The presence of a temperature induced Lifshitz transition, in which the Fermi level goes from the conduction band to the valence band with increasing temperature, provides unique opportunities to study the interplay between Fermi-surface topology, dynamics of Dirac fermions, and Berry curvature in one system. Here we present a combined experimental and theoretical study and show that a low energy model can be used to understand the complicated Hall response and large anomalous Hall effect observed in ZrTe5 over a wide range of temperature and magnetic field. We found that the anomalous Hall contribution dominates the Hall response in a narrow temperature window around the Lifshitz transition, away from which the orbital contribution dominates. Moreover, our results indicate that a topological phase transition coexists with the Lifshitz transition. Our model provides a unifying framework to understand the Hall effect in semimetals with large Zeeman splitting and non-trivial topology.

Ground states of atomic Fermi gases in a two-dimensional optical lattice with and without population imbalance

We study the ground state phase diagram of population balanced and imbalanced ultracold atomic Fermi gases with a short range attractive interaction throughout the crossover from BCS to Bose-Einstein condensation (BEC), in a two-dimensional optical lattice (2DOL) comprised of two lattice and one continuum dimensions. We find that the mixing of lattice and continuum dimensions, together with population imbalance, has an extraordinary effect on pairing and the superfluidity of atomic Fermi gases. In the balanced case, the superfluid ground state prevails the majority of the phase space. However, for relatively small lattice hopping integral $t$ and large lattice constant $d$, a pair density wave (PDW) emerges unexpectedly at intermediate coupling strength, and the nature of the in-plane and overall pairing changes from particle-like to hole-like in the BCS and unitary regimes, associated with an abnormal increase in the Fermi volume with the pairing strength. In the imbalanced case, the stable polarized superfluid phase shrinks to only a small portion of the entire phase space spanned by $t$, $d$, imbalance $p$ and interaction strength $U$, mainly in the bosonic regime of low $p$, moderately strong pairing, and relatively large $t$ and small $d$. Due to the Pauli exclusion between paired and excessive fermions within the confined momentum space, a PDW phase emerges and the overall pairing evolves from particle-like into hole-like, as the pairing strength grows stronger in the BEC regime. In both cases, the ground state property is largely governed by the Fermi surface topology. These findings are very different from the cases of pure 3D continuum, 3D lattice or 1DOL.

Magnus spin Hall and spin Nernst effects in gapped two-dimensional Rashba systems

We study the Magnus transport in a gapped two-dimensional electron gas with Rashba spin-orbit coupling using semiclassical Boltzmann transport formalism. Apart from its signature in the charge transport coefficients, the inclusion of Magnus velocity in the spin current operator enables us to study Magnus spin transport in the system. In particular, we study the roles of mass gap and Fermi surface topology on the behavior of Magnus Hall and Nernst conductivities and their spin counterparts. We find that the Magnus spin Hall conductivity vanishes in the limit of zero gap, unlike the universal spin Hall conductivity ${\ensuremath{\sigma}}_{s}=e/(8\ensuremath{\pi})$. The Magnus spin currents with spin polarization perpendicular to the applied bias (electrical/thermal) are finite while with polarization along the bias vanishes. Each Magnus conductivity displays a plateau as Fermi energy sweeps through the gap and has peaks (whose magnitudes decrease with the gap) when the Fermi energy is at the gap edges.

Surface and electronic structure at atomic length scales of the nonsymmorphic antiferromagnet Eu5In2Sb6

We performed scanning tunneling microscopy and spectroscopy (STM/STS) measurements to investigate the Zintl phase ${\mathrm{Eu}}_{5}{\mathrm{In}}_{2}{\mathrm{Sb}}_{6}$, a nonsymmorphic antiferromagnet. The theoretical prediction of a nontrivial Fermi surface topology stabilized by the nonsymmorphic symmetry motivated our research. On the cleaved (010) plane, we obtained striped patterns that can be correlated to the stacking of the [${\mathrm{In}}_{2}{\mathrm{Sb}}_{6}{]}^{10\ensuremath{-}}$ double chains along the crystallographic $c$ axis. The attempted cleavage along the $a$ axis revealed a more complex pattern. We combined the STS measurement on nonreconstructed (010) and (081) surfaces with DFT calculations to further elucidate the electronic structure of ${\mathrm{Eu}}_{5}{\mathrm{In}}_{2}{\mathrm{Sb}}_{6}$. From our investigations so far, direct experimental evidence of the predicted topological surface states remains elusive.

Proposed Fermi-surface reservoir engineering and application to realizing unconventional Fermi superfluids in a driven-dissipative nonequilibrium Fermi gas

We develop a theory to describe the dynamics of a driven-dissipative many-body Fermi system, to pursue our proposal to realize exotic quantum states based on reservoir engineering. Our idea is to design the shape of a Fermi surface so as to have multiple Fermi edges, by properly attaching multiple reservoirs with different chemical potentials to a fermionic system. These emerged edges give rise to additional scattering channels that can destabilize the system into unconventional states, which is exemplified in this work by considering a driven-dissipative attractively interacting Fermi gas. By formulating a quantum kinetic equation using the Nambu-Keldysh Green's function technique, we explore nonequilibrium steady states in this system and assess their stability. We find that, in addition to the BCS-type isotropic pairing state, a Fulde-Ferrell-type anisotropic superfluid state being accompanied by Cooper pairs with non-zero center-of-mass momentum exists as a stable solution, even in the absence of a magnetic Zeeman field. Our result implies a great potential of realizing quantum matter beyond the equilibrium paradigm, by engineering the shape and topology of Fermi surfaces in both electronic and atomic systems.

Thermal transport in two-dimensional nematic superconductors

We study the thermal transport in a two-dimensional system with coexisting superconducting (SC) and nematic orders. We analyze the nature of the coexistence phase in a tight-binding square lattice where the nematic state is modelled as a $d$-wave Pomeranchuk-type instability and the feedback of the symmetry breaking nematic state on the SC order is accounted for by mixing of the $s, d$ paring interaction. The electronic thermal conductivity is computed within the framework of Boltzmann kinetic theory where the impurity scattering collision integral is treated in the Born and unitary limits. We present qualitative, analytical, and numerical results that show that the heat transport properties of SC states emerging from a nematic background are quite distinct and depend on the degree of anisotropy of the SC gap induced by nematicity. We describe the influence of the Fermi surface topology, the van Hove singularities, and the presence or absence of zero-energy excitations in the coexistence phase on the the low-temperature behavior of the thermal conductivity. Our main conclusion is that the interplay of nematic and SC orders has visible signatures in the thermal transport, which can be used to infer SC gap structure in the coexistence phase.