Open Fermi Surface Research Articles

Electronic structure in a transition metal dipnictide TaAs2

The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently, , a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the () surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of are trivial bulk bands. The absence of symmetry-protected surface state on the () surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of .

Fermi Arc Reconstruction in Synthetic Photonic Lattice.

The chiral surface states of Weyl semimetals have an open Fermi surface called a Fermi arc. At the interface between two Weyl semimetals, these Fermi arcs are predicted to hybridize and alter their connectivity. In this Letter, we numerically study a one-dimensional (1D) dielectric trilayer grating where the relative displacements between adjacent layers play the role of two synthetic momenta. The lattice emulates 3D crystals without time-reversal symmetry, including Weyl semimetal, nodal line semimetal, and Chern insulator. Besides showing the phase transition between Weyl semimetal and Chern insulator at telecom wavelength, this system allows us to observe the Fermi arc reconstruction between two Weyl semimetals, confirming the theoretical predictions.

Quantum Oscillation and Electronic Structure of Sn4As3 and Sn4P3

We report the magnetotransport properties and electronic structures of the single crystals of Sn4As3 and Sn4P3. The magnetization curves display strong de Haas–van Alphen oscillations at low temperatures and high magnetic fields. The oscillation frequencies and corresponding Fermi surfaces (FSs) are identified from the analysis of the oscillations. The in-plane magnetization of Sn4As3 shows the split of FSs induced by nonsymmetric spin–orbit interaction. The extracted cyclotron effective masses are very small and in the range 0.03–0.11 me. First-principles calculations reveal the electronic structures and topological properties. Large transverse magnetoresistance (MR) has also been observed in Sn4As3 and Sn4P3. The Hall measurements eliminate the possibility of electron–hole compensation. It is suggested that the large MR mainly originates from the open FSs.

Optical absorption in two-dimensional materials with tilted Dirac cones

The interband optical absorption of linearly polarized light by two-dimensional (2D) semimetals hosting tilted and anisotropic Dirac cones in the band structure is analysed theoretically. Supercritically tilted (type-II) Dirac cones are characterized by an absorption that is highly dependent on the incident photon polarization and frequency and is tunable by changing the Fermi level with a back-gate voltage. Type-II Dirac cones exhibit open Fermi surfaces and large regions of the Brillouin zone where the valence and conduction bands sit either above or below the Fermi level. As a consequence, unlike their subcritically tilted (type-I) counterparts, type-II Dirac cones have many states that are Pauli blocked even when the Fermi level is tuned to the level crossing point. We analyze the interplay of the tilt parameter with the Fermi velocity anisotropy, demonstrating that the optical response of a Dirac cone cannot be described by its tilt alone. As a special case of our general theory, we discuss the proposed 2D type-I semimetal 8-$Pmmn$ borophene. Guided by our in-depth analytics, we develop an optical recipe to fully characterize the tilt and Fermi velocity anisotropy of any 2D tilted Dirac cone solely from its absorption spectrum. We expect our paper to encourage Dirac cone engineering as a major route to create gate-tunable thin-film polarizers.

Mesoscopic finite-size effects of unconventional electron transport in PdCoO2

A wide range of unconventional transport phenomena have recently been observed in single-crystal delafossite metals. Here, we present a theoretical framework to elucidate electron transport using a combination of first-principles calculations and numerical modeling of the anisotropic Boltzmann transport equation. Using PdCoO$_2$ as a model system, we study different microscopic electron and phonon scattering mechanisms and establish the mean free path hierarchy of quasiparticles at different temperatures. We treat the anisotropic Fermi surface explicitly to numerically obtain experimentally-accessible transport observables, which bridge between the "diffusive", "ballistic", and "hydrodynamic" transport regime limits. We illustrate that distinction between the "quasi-ballistic", and "quasi-hydrodynamic" regimes is challenging and often needs to be quantitative in nature. From first-principles calculations, we populate the resulting transport regime plots, and demonstrate how the Fermi surface orientation adds complexity to the observed transport signatures in micro-scale devices. Our work provides key insights into microscopic interaction mechanisms on open hexagonal Fermi surfaces and establishes their connection to the macroscopic electron transport in finite-size channels.

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.

Type-II quadratic and cubic Weyl fermions

Type-II Weyl fermions characterized by tilted energy dispersions have attracted significant interest because such quasiparticles exhibit unique properties due to their open Fermi surfaces. Herein, we propose type-II quadratic (cubic) Weyl fermions by combining tilted energy dispersions with higher Chern numbers $\mathcal{C}=\ifmmode\pm\else\textpm\fi{}2$ ($\ifmmode\pm\else\textpm\fi{}3$), making the classification of chiral fermions more complete. Unlike type-II Weyl fermions with tilted energy dispersions along any direction, type-II quadratic (cubic) Weyl fermions can only be tilted along a rotational axis. Our analytical solutions of Landau levels and magneto-optical spectra show that type-II quadratic (cubic) Weyl fermions have unique absorption properties, such as different locations and widths of the absorption peaks compared with known chiral fermions, which can be used as a reference to detect these Weyl fermions in experiments. Using first-principles calculations, we identify the type-II quadratic Weyl fermions in cubic ${\mathrm{CaGe}}_{2}$, in which the electron and hole pockets touch at the nodes with $\mathcal{C}=+2$ and two visible Fermi arcs come out from the type-II quadratic Weyl points.

Pauli paramagnetism of triplet Cooper pairs in a nematic superconductor

We investigate the response of a doped topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ with spin-triplet nematic superconductivity to external magnetization. We calculate the Zeeman part of the magnetic susceptibility for nematic and chiral superconducting phases near ${T}_{c}$ in the Ginzburg-Landau formalism. The superconducting order parameter from the ${E}_{u}$ representation has nontrivial coupling with the transversal Zeeman field that results in a paramagnetic response to magnetization. The topology of a Fermi surface has a strong influence on magnetic susceptibility. The Lifshitz transition from a closed to open Fermi surface eventually leads to a phase transition from the nematic to chiral phase. At the transition point, magnetic susceptibility diverges. Also, we study the effects of the electron-electron interaction on the competition between nematic and chiral phases. We find that in a real system, electron-electron interaction can drive the nematic to chiral phase only in the vicinity of the phase transition. We compare our results with the existing experimental data.

Lifshitz transition in dirty doped topological insulator with nematic superconductivity

We study the effects of the Lifshitz transition from closed to open Fermi surface in dirty topological insulators with the nematic superconductivity near the critical temperature. We solve linearized Gor'kov equations and find that the nematic superconductor with an open Fermi surface has a lower critical temperature and more susceptible to the disorder than the superconductor with the closed Fermi surface. We propose that correspondence between the critical temperature and stability against the disorder is the general feature of the superconductivity. We investigate the effects of the Lifshitz transition on the competition between superconducting phases in a topological insulator. Open Fermi surface is beneficial for the nematic order parameter $\Delta_4$ in competition with orbital-triplet $\Delta_2$ and disfavors nematic state over the s-wave order parameter. We study Meissner currents in both clean and dirty limits. We found that transition from closed to open Fermi surface increases anisotropy of Meissner currents. Finite disorder suppresses superconducting density stronger than critical temperature. We compare our results with the existing experimental data.

Singular angular magnetoresistance and sharp resonant features in a high-mobility metal with open orbits, ReO3

The oxide ReO3 is one of the most conductive materials known. At 4 Kelvin, its resistivity is 6-10 times lower than that in high-purity gold and the electronic mean free path is 25 microns. It has an open Fermi surface comprised of intersecting cylinders. When an external magnetic field is applied, it exhibits a huge magnetoresistance with a highly singular angular dependence. Tilting the field in the longitudinal plane leads to a sharp decrease in the resistivity whereas tilting in the perpendicular direction sharply increases it. The authors analyze the experimental data using the Shockley-Chambers tube integral, showing that slight tilts lead to the obstruction of closed orbits on the Fermi surface which re-attach to form open orbits. The associated shadowing effect leads to a new way to measure the Fermi wavevector. However, the tube integral approach does not account for very narrow peaks observed at small tilt angles.

Large magnetoresistance of a compensated metal Cu2Sb correlated with its Fermi surface topology

We report electrical transport properties and electronic structure of a nonmagnetic metal ${\mathrm{Cu}}_{2}\mathrm{Sb}$ single crystal. ${\mathrm{Cu}}_{2}\mathrm{Sb}$ was found to be a compensated metal with high carrier density $\ensuremath{\sim}{10}^{22}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ and high carrier mobility $\ensuremath{\ge}{10}^{3}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$ for both electron and hole carriers. The current-in-plane magnetoresistance at 2 K and 9 T was 730%, while the current-perpendicular-to-plane magnetoresistance at 2 K and 9 T was 2700% without the saturation. Angle-resolved photoemission spectroscopy throughout the three-dimensional (3D) bulk Brillouin zone signified a quasi-two-dimensional (2D) electron pocket axially centered along the M-A line and a 3D hole pocket at the \ensuremath{\Gamma} point, in accordance with the electron-hole compensated nature. The presence of quasi-2D open Fermi surface, in line with the first-principles band-structure calculations, is likely responsible for the observed nonsaturating current-in-plane magnetoresistance. The present result lays the foundation for realizing large magnetoresistance via Fermiology engineering in compensated metals.

Asymmetrical plasmonic absorber and reflector based on tilted Weyl semimetals

We investigate the surface plasmon polariton dispersion and optical spectra of a thin film of tilted Weyl semimetal. Tilted Weyl semimetals possess tilted Weyl cones at the Weyl nodes and are categorized to type-I with closed Fermi surfaces and type-II with overtilted Weyl cones and open Fermi surfaces. We find that the surface plasmon polariton dispersion of this system is nonreciprocal even in the absence of the external magnetic field. Moreover, we demonstrate that the tilt parameter has a profound effect in controlling this nonreciprocity. We reveal that the thin film of type-II Weyl semimetal hosts the surface plasmon polariton modes with the negative group velocity. Furthermore, we show that the angular optical spectra of this structure are highly asymmetric and this angular asymmetry in the absorptivity and reflectivity depends profoundly on the tilt parameter of the tilted Weyl semimetal. These exciting features propose employing the tilted Weyl semimetals in optical sensing devices, optical data storage, and devices for quantum information processing.

Simulating Exceptional Non-Hermitian Metals with Single-Photon Interferometry.

We experimentally simulate in a photonic setting non-Hermitian (NH) metals characterized by the topological properties of their nodal band structures. Implementing nonunitary time evolution in reciprocal space followed by interferometric measurements, we probe the complex eigenenergies of the corresponding NH Bloch Hamiltonians, and study in detail the topology of their exceptional lines (ELs), the NH counterpart of nodal lines in Hermitian systems. We focus on two distinct types of NH metals: two-dimensional systems with symmetry-protected ELs, and three-dimensional systems possessing symmetry-independent topological ELs in the form of knots. While both types feature open Fermi surfaces, we experimentally observe their distinctions by analyzing the impact of symmetry-breaking perturbations on the topology of ELs.

Link between superconductivity and a Lifshitz transition in intercalated Bi2Se3

Topological superconductivity is an exotic phase of matter in which the fully gapped superconducting bulk hosts gapless Majorana surface states protected by topology. Intercalation of copper, strontium or niobium between the quintuple layers of the topological insulator Bi$_2$Se$_3$ increases the carrier density and leads to superconductivity that is suggested to be topological. Here we study the electronic structure of strontium-intercalated Bi$_2$Se$_3$ using angle resolved photoemission spectroscopy (ARPES) and Shubnikov-de Haas (SdH) oscillations. Despite the apparent low Hall number of $\sim2 \times 10 ^{19}$cm$^{-3}$, we show that the Fermi surface is shaped as an open cylinder with a larger carrier density of $\sim 10 ^{20}$cm$^{-3}$. We suggest that superconductivity in intercalated Bi$_2$Se$_3$ emerges with the appearance of a quasi-2D open Fermi surface.

Anisotropic scattering in the goniopolar metal NaSn2As2

Recent experimental discoveries in axis-dependent conduction polarity, or goniopolarity, have observed that the charge carriers can conduct like either electrons or holes depending on the crystallographic direction they travel along in layered compounds such as $\mathrm{Na}{\mathrm{Sn}}_{2}{\mathrm{As}}_{2}$. The original theoretical proposal is based on the opposite signs of the carrier effective mass, or the curvature of the Fermi surface, without examining the effect of electron lifetimes, thus leaving a crucial question to address. To elucidate this unusual transport behavior, we present an ab initio study of electron scattering in such systems. We study different microscopic scattering mechanisms in $\mathrm{Na}{\mathrm{Sn}}_{2}{\mathrm{As}}_{2}$, and we present the electron-phonon scattering time distribution on its Fermi surface in momentum space, the open concave shape of which is proposed to be the origin of the axis-dependent conduction polarity. Further, we obtain the overall anisotropic lifetime tensors in real space at different electron chemical potentials and temperatures, and we discuss how they contribute to the macroscopic thermopower. While we find that the contribution of the in-plane and cross-plane lifetimes exhibits a similar trend, the concave portion of the Fermi surface alters the electron motion significantly in the presence of a magnetic field, thus flipping the conduction polarity as measured via the Hall effect. Our calculations and analysis of $\mathrm{Na}{\mathrm{Sn}}_{2}{\mathrm{As}}_{2}$, in comparison with similar systems, also suggest the strong possibility of hydrodynamic electron flow in the system. Finally, our work has implications for anisotropic electron lifetimes in a broad class of goniopolar materials and provides key, general insights into electron scattering on open Fermi surfaces.

Superfluidity and pairing phenomena in ultracold atomic Fermi gases in one-dimensional optical lattices. II. Effects of population imbalance

In this paper, we study the effect of population imbalance and its interplay with pairing strength and lattice effect in atomic Fermi gases in a one-dimensional optical lattice. We compute various phase diagrams as the system undergoes BCS-BEC crossover, using the same pairing fluctuation theory as in Part I of this work [Phys. Rev. A 101, 053617 (2020)]. We find widespread pseudogap phenomena beyond the BCS regime and intermediate temperature superfluid states for relatively low population imbalances. The Fermi surface topology plays an important role in the behavior of ${T}_{\mathrm{c}}$. For large $d$ and/or small $t$, which yield an open Fermi surface, superfluidity can be readily destroyed by a small amount of population imbalance $p$. The superfluid phase, especially in the BEC regime, can exist only for a highly restricted volume of the parameter space. An open Fermi surface often leads to destruction of the superfluid ground state in the BEC regime. Due to the continuum-lattice mixing, population imbalance gives rise to a new mechanism for pair hopping, as assisted by excessive majority fermions, which may lead to significant enhancement of ${T}_{\mathrm{c}}$ on the BEC side of the Feshbach resonance, and also render ${T}_{\mathrm{c}}$ approaching a constant asymptote in the BEC limit, when it exists. Furthermore, we find that not all minority fermions will be paired up in BEC limit, unlike the 3D continuum case. These predictions can be tested in future experiments.

Novel Undamped Gapless Plasmon Mode in a Tilted Type-II Dirac Semimetal.

We predict the existence of a novel long-lived gapless plasmon mode in a type-II Dirac semimetal (DSM). This gapless mode arises from the out-of-phase oscillations of the density fluctuations in the electron and the hole pockets of a type-II DSM. It originates beyond a critical wave vector along the direction of the tilt axis, owing to the momentum separation of the electron and hole pockets. A similar out-of-phase plasmon mode arises in other multicomponent charged fluids as well, but generally, it is Landau damped and lies within the particle-hole continuum. In the case of a type-II DSM, the open Fermi surface prohibits low-energy finite momentum single-particle excitations, creating a "gap" in the particle-hole continuum. The gapless plasmon mode lies within this particle-hole continuum gap and, thus, it is protected from Landau damping.

Role of correlations in determining the Van Hove strain in Sr2RuO4

Uniaxial pressure applied along an Ru-Ru bond direction induces an elliptical distortion of the largest Fermi surface of Sr$_2$RuO$_4$, eventually causing a Fermi surface topological transition, also known as a Lifshitz transition, into an open Fermi surface. There are various anomalies in low-temperature properties associated with this transition, including maxima in the superconducting critical temperature and in resistivity. In the present paper, we report new measurements, employing new uniaxial stress apparatus and new measurements of the low-temperature elastic moduli, of the strain at which this Lifshitz transition occurs: a longitudinal strain $\varepsilon_{xx}$ of $(-0.44\pm0.06)\cdot10^{-2}$, which corresponds to a B$_{1g}$ strain $\varepsilon_{xx} - \varepsilon_{yy}$ of $(-0.66\pm0.09)\cdot10^{-2}$. This is considerably smaller than the strain corresponding to a Lifshitz transition in density functional theory calculations, even if the spin-orbit coupling is taken into account. Using dynamical mean-field theory we show that electronic correlations reduce the critical strain. It turns out that the orbital anisotropy of the local Coulomb interaction on the Ru site is furthermore important to bring this critical strain close to the experimental number, and thus well into the experimentally accessible range of strains.

Chiral-symmetry protected exceptional torus in correlated nodal-line semimetals

We analyze a diamond-lattice Hubbard model with the spatially modulated Hubbard interaction. Our dynamical mean-field analysis with special emphasis on non-Hermitian properties elucidates that the gapless nodal line changes into symmetry-protected exceptional torus (SPET) at the Fermi level enclosing the three-dimensional open Fermi surface, which is unique to non-Hermitian physics with chiral symmetry. Furthermore, we also elucidate the effects of the SPETs on the magnetic response; our results based on the random-phase approximation combined with the the dynamical mean-field theory shows that SPETs enhance the magnetic susceptibility at the weakly correlated sites, exemplifying effects of non-Hermitian degeneracies on responses to external fields.

Anisotropic upper critical field of pristine and proton-irradiated single crystals of the magnetically ordered superconductor RbEuFe4As4

We present a study of the upper critical field, H$_{c2}$, of pristine and proton-irradiated RbEuFe$_4$As$_4$ crystals in pulsed magnetic fields of up to 65 T. The data for H$_{c2}$ reveal pronounced downwards curvature, particularly for the in-plane field orientation, and a superconducting anisotropy that decreases with decreasing temperature. These features are indicative of Pauli paramagnetic limiting. For the interpretation of these data, we use a model of a clean single-band superconductor with an open Fermi surface in the shape of a warped cylinder, which includes strong paramagnetic limiting. Fits to the data reveal that the in-plane upper critical field is Pauli paramagnetic limited, while the out-of-plane upper critical field is orbitally limited and that the orbital and paramagnetic fields have opposite anisotropies. A consequence of this particular combination is the unusual inversion of the anisotropy, $H_{c2}^{ab} < H_{c2}^c$, of the irradiated sample at temperatures below 10 K. The fits also yield an in-plane Maki parameter, $\alpha_M^{110} \approx$ 2.6, exceeding the critical value for the formation of the Fulde-Ferrell-Larkin-Ovchinnikov state. Nevertheless, the current measurements did not reveal direct evidence for the occurrence of this state.