Two-dimensional Fermi Surface Research Articles

Giant lattice softening at a Lifshitz transition in Sr2RuO4.

The interplay of electronic and structural degrees of freedom in solids is a topic of intense research. More than 60 years ago, Lifshitz discussed a counterintuitive possibility: lattice softening driven by conduction electrons at topological Fermi surface transitions. The effect that he predicted, however, was small and has not been convincingly observed. Using a piezo-based uniaxial pressure cell to tune the ultraclean metal strontium ruthenate while measuring the stress-strain relationship, we reveal a huge softening of the Young's modulus at a Lifshitz transition of a two-dimensional Fermi surface and show that it is indeed driven entirely by the conduction electrons of the relevant energy band.

Method to Reveal and Investigate Almost 2D Fermi Surfaces in Layered Conductors: Universal Resistivity in a Parallel Magnetic Field

We suggested an original method to investigate the Fermi surfaces (FSs) in the quasi-two-dimensional conductors some time ago [A.G. Lebed and N.N. Bagmet, Phys. Rev. B 55, R8654 (1997)]. It was based on a consideration of a perpendicular conductivity in quasi-two-dimensional metals in parallel magnetic fields in the framework of the Boltzmann kinetic equation, where it was shown that the conductivity was independent on impurities. In this paper, we demonstrate that the above mentioned result is much more general than the kinetic equation and can be obtained even in a fully quantum mechanical case. We suggest to investigate this possible phenomenon in the quasi-two-dimensional organic, high-{{T}_{c}}, and some others superconductors in a metallic phase to judge if the Fermi liquid picture is valid for them or not. If the Fermi liquid picture is valid, then study of the perpendicular resistivity in the rotated parallel magnetic field allows to extract important information about the two-dimensional Fermi surfaces.

Anomalous Hall effect and two-dimensional Fermi surfaces in the charge-density-wave state of kagome metal RbV3Sb5

AV3Sb5 (A = Cs, K, Rb) is a recently discovered superconducting system (–2.5 K) in which the vanadium atoms adopt the kagome structure. Intriguingly, these systems enter a charge-density-wave (CDW) phase (–100 K), and further evidence shows that the time-reversal symmetry is broken in the CDW phase. Concurrently, the anomalous Hall effect (AHE) has been observed in KV3Sb5 and CsV3Sb5 inside the novel CDW phase. Here, we report a comprehensive study of a high-quality RbV3Sb5 single crystal with magnetotransport measurements. Our data demonstrate the emergence of the AHE in RbV3Sb5 when the CDW state develops. The magnitude of the anomalous Hall resistivity in the low temperature limit is comparable to the reported values in KV3Sb5 and CsV3Sb5. The magnetoresistance channel further reveals a rich spectrum of quantum oscillation frequencies, many of which have not been reported before. In particular, a large quantum oscillation frequency (2235 T), which occupies ∼56% of the Brillouin zone area, was recorded. For the quantum oscillation frequencies with sufficient signal-to-noise ratio, we further perform field angle-dependent measurements, and our data indicate two-dimensional Fermi surfaces in RbV3Sb5. Our results provide indispensable information for understanding the AHE and band structure in kagome metal AV3Sb5.

Synthesis of TSF Donors Substituted with the meso-Dimethylethylenedithio Group: Structures and Conducting Properties of (meso-DM-BETS)2X (X− = PF6− and AsF6−)

Abstract Synthesis and X-ray structure analysis of TSF-based donors substituted with the meso-dimethylethylenedithio group (meso-DM-EDT-TSF and meso-DM-BETS) were successfully performed. The donor molecules in (meso-DM-BETS)2X (X− = PF6− and AsF6−) formed a dimerized face-to-face π-stack, and the packing pattern could be classified as β-type. A tight-binding band calculation suggested that the meso-DM-BETS salts have a two-dimensional Fermi surface with a wide bandwidth. Both salts exhibited metallic conductivities down to low temperatures with σr.t. = 45–46 S cm−1.

Transport evidence of the spin-polarized topological surface states of α-Sn grown on CdTe by molecular beam epitaxy

It is necessary but challenging to verify topological surface states of α-Sn by electrical transport. In this work, we demonstrate conclusive transport evidence on topological properties of an α-Sn film grown on a CdTe substrate by molecular beam epitaxy. A Berry phase determined from Shubnikov–de Haas oscillations is 0.98π. A two-dimensional (2D) Fermi surface is clearly demonstrated by angle-dependent oscillations. We believe the nontrivial topology originates from the 2D Dirac fermions of the topological surface states. In addition, both anisotropic magneto-resistance and planar Hall effect have negative amplitudes at higher fields, which we attribute to the spin-flip backscattering in the topological surface states. We also show that these topological surface states have a long relaxation time of ∼95 fs, making α-Sn a potential candidate for high-efficiency spintronics.

Spin-triplet superconductivity driven by finite-momentum spin fluctuations

A small number of superconductors are believed to exhibit intrinsic spin-triplet pairing, and they are often discussed in terms of a simple, $^{3}\mathrm{He}$-like picture where ferromagnetic spin fluctuations provide the ``glue.'' However, in some cases in which reliable inelastic neutron scattering measurements are available, spin excitations are found to be peaked at finite momentum $\mathbf{q}$ rather than $\mathbf{q}=\mathbit{0}$. Here we investigate some simple models that exhibit triplet pairing arising from antiferromagnetic spin fluctuations. We show that a strong peak at larger $\mathbf{q}$ in the magnetic susceptibility can drive such states and can give rise to pairing states with nodes in the ${k}_{z}$ plane even in the presence of a pure two-dimensional Fermi surface. In these situations, dominant pair scattering processes occur between Fermi surface segments with like signs of the superconducting order parameter, yet they are consistent with an overall odd-parity state. We examine the applicability of these scenarios to the putative triplet superconductor $\mathrm{U}{\mathrm{Te}}_{2}$.

Interaction between interface and massive states in multivalley topological heterostructures

Topological interface states (TISs) in multivalley systems are studied to unravel their valley sensitivity. For this purpose, multivalley IV--VI topological crystalline insulator (TCI) heterostructures are explored using magnetooptical Landau level spectroscopy up to 34 teslas. We characterize the TISs emerging from the distinct $L$ valleys in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}\mathrm{Se}$ multiquantum wells grown along the [111] direction. It is shown that the shape of the two-dimensional (2D) Fermi surfaces of TISs residing at the TCI-trivial insulator interfaces are strongly affected by the valley anisotropy of topologically trivial ${\mathrm{Pb}}_{1\ensuremath{-}y}{\mathrm{Eu}}_{y}\mathrm{Se}$ barriers. This phenomenon is shown to be due to the deep penetration of the TISs into the barriers. For the valleys tilted with respect to the confinement direction, a significant interaction between topological states and the conventional massive quantum well states is observed, evidenced by the resulting large anticrossings between Landau levels. These are theoretically well described by a $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model that considers tilt and anisotropy of the valleys in 2D. Therefore, in this paper, we provide a precise characterization of the TIS valley splitting as well as an accurate determination of the anisotropy of their Dirac cone dispersion.

Spatially inhomogeneous magnetic superconductors

We consider a problem of superconductivity coexistence with the spin-density-wave order in disordered multiband metals. It is assumed that random variations of the disorder potential on short length scales render the interactions between electrons to develop spatial correlations. As a consequence, both superconducting and magnetic order parameters become spatially inhomogeneous and are described by the universal phenomenological quantities, whereas all the microscopic details are encoded in the correlation function of the coupling strength fluctuations. We consider a minimal model with two nested two-dimensional Fermi surfaces and disorder potentials which include both intra- and inter-band scattering. The model is analyzed using the quasiclassical approach to show that short-scale pairing-potential disorder leads to a broadening of the coexistence region.

Presence versus absence of two-dimensional Fermi surface anomalies

We theoretically consider Fermi surface anomalies manifesting in the temperature-dependent quasiparticle properties of two-dimensional (2D) interacting electron systems, comparing and contrasting with the corresponding three-dimensional (3D) Fermi liquid situation. In particular, employing microscopic many-body perturbative techniques, we obtain analytically the leading-order and the next-to-leading-order interaction corrections to the renormalized effective mass for three distinct physical interaction models: electron-phonon, electron-paramagnon, and electron-electron Coulomb coupling. We find that the 2D renormalized effective mass does not develop any Fermi surface anomaly due to electron-phonon interaction, manifesting $\mathcal{O}({T}^{2})$ temperature correction and thus remaining consistent with the Sommerfeld expansion of the noninteracting Fermi function, in contrast to the corresponding 3D situation where the temperature correction to the renormalized effective mass has the anomalous ${T}^{2}logT$ behavior. In contrast, both electron-paramagnon and electron-electron interactions lead to the anomalous $\mathcal{O}(T)$ corrections to the 2D effective mass renormalization in contrast to ${T}^{2}logT$ behavior in the corresponding 3D interacting systems. We provide detailed analytical results, and comment on the conditions under which a ${T}^{2}logT$ term could possibly arise in the 2D quasiparticle effective mass from electron-phonon interactions. We also compare results for the temperature-dependent specific heat in the interacting 2D and 3D Fermi systems, using the close connection between the effective mass and specific heat.

Two-dimensional Dirac dispersion in the layered compound BaCdSb2

We report a comprehensive study of the electronic structure of layered compound $\mathrm{Ba}\mathrm{Cd}{\mathrm{Sb}}_{2}$ by using electrical transport measurements, first-principles calculations, and angle-resolved photoemission spectroscopy (ARPES). The samples show semiconductorlike temperature dependence of resistivity and low carrier density. The Shubnikov-de Haas (SdH) oscillations with a single frequency reveal a small two-dimensional (2D) cylinderlike Fermi surface (FS), a light cyclotron effective mass, and trivial Berry's phase. Quantum limit can be achieved under a moderate magnetic field. The calculated bands without any renormalization are well consistent with the ARPES results, showing the Dirac band crossing with 2D character near the Fermi energy and a gap of about 34 meV at the Dirac point induced by spin-orbit interaction. The SdH oscillations can be identified to the Dirac band by the similar cross sectional areas of FSs. Our findings indicate $\mathrm{Ba}\mathrm{Cd}{\mathrm{Sb}}_{2}$ is a promising material for researching the quantum phenomena of the 2D Dirac fermions.

Successive destruction of charge density wave states by pressure in LaAgSb2

We comprehensively studied the magnetotransport properties of LaAgSb$_2$ under high pressure up to 4 GPa, which showed unique successive charge density wave (CDW) transitions at $T_{CDW1}\sim 210$ K and $T_{CDW2}\sim 190$ K at ambient pressure. With the application of pressure, both $T_{CDW1}$ and $T_{CDW2}$ were suppressed and disappeared at the critical pressures of $P_{CDW1}=3.0$--3.4 GPa and $P_{CDW2}=1.5$--1.9 GPa, respectively. At $P_{CDW1}$, the Hall conductivity showed a step-like increase, which is consistently understood by the emergence of two-dimensional hollow Fermi surface at $P_{CDW1}$. We also observed a significant negative magnetoresistance effect when the magnetic field and current were applied parallel to the $c$ axis. Shubnikov--de Haas (SdH) oscillation measurements under pressure directly showed the changes in the Fermi surface across the CDW phase boundaries. In $P<P_{CDW2}$, three major oscillation components, $\alpha$, $\beta$, and $\gamma$, were identified, whose frequencies were increased by application of pressure. The increment rate of these frequencies was considerably larger than that expected from the shrinkage of lattice constant, indicating the unignorable band modification under pressure. In the normal metallic phase above $P>P_{CDW1}$, we observed a single frequency of $\sim 48$ T with a cyclotron effective mass of 0.066 $m_0$, whose cross section in the reciprocal space corresponded to only 0.22\% of the first Brillouin zone. Besides, we observed another oscillation component with frequency of $\sim 9.2$ T, which is significantly enhanced in the limited pressure range of $P_{CDW2}<P<P_{CDW1}$. The amplitude of this oscillation was anomalously suppressed in the high-field and low-temperature region, which cannot be explained by the conventional Lifshitz--Kosevich formula.

Particle number fluctuations, Rényi entropy, and symmetry-resolved entanglement entropy in a two-dimensional Fermi gas from multidimensional bosonization

In this paper, we revisit the computation of particle number fluctuations and the R\'{e}nyi entanglement entropy of a two-dimensional Fermi gas using multi-dimensional bosonization. In particular, we compute these quantities for a circular Fermi surface and a circular entangling surface. Both quantities display a logarithmic violation of the area law, and the R\'{e}nyi entropy agrees with the Widom conjecture. Lastly, we compute the symmetry-resolved entanglement entropy for the two-dimensional circular Fermi surface and find that, while the total entanglement entropy scales as $R\log R$, the symmetry-resolved entanglement scales as $\sqrt{R\log R}$, where $R$ is the radius of the subregion of our interest.

Calculations of quantum oscillations in cuprate superconductors considering the pseudogap

The observations of quantum oscillations frequencies in overdoped cuprates were in agreement with a charge density contained in a cylindrical Fermi surface but the measured frequencies of underdoped compounds were much smaller than expected. This was attributed to a topological transition into small pockets of Fermi surface associated with the existence of charge density waves. On the other hand, spectroscopic measurements suggested that the large two-dimensional Fermi surface changes continuously into a set of four disconnected arcs. Here we take into account the effect of the pseudogap that limits the available k-space area where the Landau levels are developed on the Luttinger theorem and obtain the correct total carrier densities. The calculations show how the disconnected arcs evolve into a closed Fermi surface reconciling the experiments.

Hard antinodal gap revealed by quantum oscillations in the pseudogap regime of underdoped high-Tc superconductors

An understanding of the missing antinodal electronic excitations in the pseudogap state is essential for uncovering the physics of the underdoped cuprate high temperature superconductors. The majority of high temperature experiments performed thus far, however, have been unable to discern whether the antinodal states are rendered unobservable due to their damping, or whether they vanish due to their gapping. Here we distinguish between these two scenarios by using quantum oscillations to examine whether the small Fermi surface pocket, found to occupy only 2% of the Brillouin zone in the underdoped cuprates, exists in isolation against a majority of completely gapped density of states spanning the antinodes, or whether it is thermodynamically coupled to a background of ungapped antinodal states. We find that quantum oscillations associated with the small Fermi surface pocket exhibit a signature sawtooth waveform characteristic of an isolated two-dimensional Fermi surface pocket. This finding reveals that the antinodal states are destroyed by a hard gap that extends over the majority of the Brillouin zone, placing strong constraints on a drastic underlying origin of quasiparticle disappearance over almost the entire Brillouin zone in the pseudogap regime.

Identifying possible pairing states in Sr2RuO4 by tunneling spectroscopy

We examine the tunneling spectroscopy of three-dimensional normal-metal/Sr$_2$RuO$_4$ junctions as an experimental means to identify pairing symmetry in Sr$_2$RuO$_4$. In particular, we consider three different possible pairing states in Sr$_2$RuO$_4$: spin-singlet chiral $d$-wave, spin-triplet helical $p$-wave, and spin-nematic $f$-wave ones, all of which are consistent with recent nuclear-magnetic-resonance experiments [A. Pustogow et al., Nature 574, 72 (2019)]. The Blonder-Tinkham-Klapwijk theory is employed to calculate the tunneling conductance, and the cylindrical two-dimensional Fermi surface of Sr$_2$RuO$_4$ is properly taken into account as an anisotropic effective mass and a cutoff in the momentum integration. It is pointed out that the chiral $d$-wave pairing state is inconsistent with previous tunneling conductance experiments along the $c$-axis. We also find that the remaining candidates, the spin-triplet helical $p$-wave pairing state and the spin-nematic $f$-wave ones, can be distinguished from each other by the in-plane tunneling spectroscopy along the $a$- and $b$-axes.

Magnetoresistance and Shubnikov–de Haas oscillations in layered Nb3SiTe6 thin flakes

We present the magnetoresistance (MR) and Hall resistivity of layered ternary ${\mathrm{Nb}}_{3}\mathrm{Si}{\mathrm{Te}}_{6}$ thin flakes with magnetic field up to 33 T. The flakes show strong angle-dependent MR properties and unsaturated quasilinear dependence accompanied with Shubnikov--de Haas (SdH) oscillations. Hall resistivity study demonstrates that the hole carriers dominate the transport properties in the whole temperature range. The angle-dependent SdH oscillations reveal a two-dimensional Fermi surface of ${\mathrm{Nb}}_{3}\mathrm{Si}{\mathrm{Te}}_{6}$. Furthermore, by analysis of SdH oscillations, we observed a nontrivial $\ensuremath{\pi}$-Berry phase in the SdH oscillations which suggests the nontrivial topological nature of ${\mathrm{Nb}}_{3}\mathrm{Si}{\mathrm{Te}}_{6}$.

Converting topological insulators into topological metals within the tetradymite family

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula $T_2$$Ch_2$$Pn$, obtained as a modification to the well-known topological insulator binaries Bi$_2$(Se,Te)$_3$ by replacing one chalcogen ($Ch$) with a pnictogen ($Pn$) and Bi with the tetravalent transition metals $T$ $=$ Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to Bi$_2$Se$_3$, where the surface Dirac bands are at the $\Gamma-$point, for (Zr,Hf)$_2$Te$_2$(P,As) there are Dirac cones of strong topological character around both the $\bar {\Gamma}$- and $\bar {M}$-points which are above and below the Fermi energy, respectively. For Ti$_2$Te$_2$P the surface state is predicted to exist only around the $\bar {M}$-point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus, this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).

Interplay between short-range correlated disorder and Coulomb interaction in nodal-line semimetals

In nodal-line semimetals, Coulomb interactions and short-range correlated disorder are both marginal perturbations to the clean non-interacting Hamiltonian. We analyze their interplay using a weak-coupling renormalization group approach. In the clean case, the Coulomb interaction has been found to be marginally irrelevant, leading to Fermi liquid behavior. We extend the analysis to incorporate the effects of disorder. The nodal line structure gives rise to kinematical constraints similar to that for a two-dimensional Fermi surface, which plays a crucial role in the one-loop renor- malization of the disorder couplings. For a two-fold degenerate nodal loop (Weyl loop), we show that disorder flows to strong coupling along a unique fixed trajectory in the space of symmetry inequiv- alent disorder couplings. Along this fixed trajectory, all symmetry inequivalent disorder strengths become equal. For a four-fold degenerate nodal loop (Dirac loop), disorder also flows to strong coupling, however the strengths of symmetry inequivalent disorder couplings remain different. We show that feedback from disorder reverses the sign of the beta function for the Coulomb interaction, causing the Coulomb interaction to flow to strong coupling as well. However, the Coulomb interac- tion flows to strong coupling asymptotically more slowly than disorder. Extrapolating our results to strong coupling, we conjecture that at low energies nodal line semimetals should be described by a noninteracting nonlinear sigma model. We discuss the relation of our results with possible many-body localization at zero temperatures in such materials.

Quasi-two-dimensional Fermi surfaces with localized f electrons in the layered heavy-fermion compound CePt2In7

We report measurements of the de Haas-van Alphen effect in the layered heavy-fermion compound CePt$_2$In$_7$ in high magnetic fields up to 35 T. Above an angle-dependent threshold field, we observed several de Haas-van Alphen frequencies originating from almost ideally two-dimensional Fermi surfaces. The frequencies are similar to those previously observed to develop only above a much higher field of 45 T, where a clear anomaly was detected and proposed to originate from a change in the electronic structure [M. M. Altarawneh et al., Phys. Rev. B 83, 081103 (2011)]. Our experimental results are compared with band structure calculations performed for both CePt$_2$In$_7$ and LaPt$_2$In$_7$, and the comparison suggests localized $f$ electrons in CePt$_2$In$_7$. This conclusion is further supported by comparing experimentally observed Fermi surfaces in CePt$_2$In$_7$ and PrPt$_2$In$_7$, which are found to be almost identical. The measured effective masses in CePt$_2$In$_7$ are only moderately enhanced above the bare electron mass $m_0$, from 2$m_0$ to 6$m_0$.

Resistivity plateau and large magnetoresistance in the charge density wave system TaTe4

Due to the time reversal symmetry, the metallic surface of the topological insulator (TI) survives and results in the resistivity plateau at low temperature, which is the transport signature of the metallic surface state. Such a universal character has been observed in many materials, such as Bi2Te2Se, SmB6, and so on. Recently, a similar behavior has also been found in the possible topological semimetal LaSb/Bi and the metallic compounds Nb/TaAs2. Herein, we have mainly explored the resistivity plateau and the magnetoresistance (MR) of TaTe4 single crystal with the charge density wave (CDW) transition temperature TCDW = 475 K. There are some interesting observations: (i) The large MR (MR is about 1200% in a magnetic field 16 T at T = 2 K) is observed at low temperature; (ii) A field-induced universal TI resistivity with a plateau at roughly T = 10 K and high quantum mobility of carriers in the plateau region are present; (iii) Quantum oscillations with the angle dependence of a two-dimensional Fermi surface are shown. Owing to the CDW character that existed in TaTe4, it represents an ideal system to investigate the relationship between the CDW and the topological physics.