Fermi-surface structure and field ionization.
Field ionization at metal surfaces as rearrangement collision, considering anisotropic electron tunneling probabilities produced by Fermi surface
Fonash, S. J.↗
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Random magnetic field and the Dirac Fermi surface
In this paper, we study a single two-dimensional Dirac fermion at finite density, subject to a quenched random magnetic field. At low energies and sufficiently weak disorder, the theory maps onto an infinite collection of 1D chiral fermions (associated to each point on the Fermi surface) coupled by a random vector potential. This low-energy theory exhibits an exactly solvable random fixed line, along which we directly compute various disorder-averaged observables without the need for the usual replica, supersymmetry, or Keldysh techniques. We find the longitudinal dc conductivity in the collisionless $\hbar$ω/k B T→∞ limit to be nonuniversal and to vary continuously along the fixed line.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Observation of a two-dimensional Fermi surface and Dirac dispersion in YbMnSb 2
Here we present the crystal structure, electronic structure, and transport properties of the material ${\mathrm{YbMnSb}}_{2}$, a candidate system for the investigation of Dirac physics in the presence of magnetic order. Our measurements reveal that this system is a low-carrier-density semimetal with a two-dimensional Fermi surface arising from a Dirac dispersion, consistent with the predictions of density-functional-theory calculations of the antiferromagnetic system. Finally, the low temperature resistivity is very large, suggesting that scattering in this system is highly efficient at dissipating momentum despite its Dirac-like nature.
36 MATERIALS SCIENCE↗
Quantum oscillations from the reconstructed Fermi surface in electron-doped cuprate superconductors
We have studied the electronic structure of electron-doped cuprate superconductors via measurements of high-field Shubnikov–de Haas oscillations in thin films. In optimally doped Pr 2-x Ce x CuO 4±δ and La 2-x Ce x CuO 4±δ , quantum oscillations indicate the presence of a small Fermi surface, demonstrating that electronic reconstruction is a general feature of the electron-doped cuprates, despite the location of the superconducting dome at very different doping levels. Negative high-field magnetoresistance is correlated with an anomalous low-temperature change in scattering that modifies the amplitude of quantum oscillations. This behavior is consistent with effects attributed to spin fluctuations.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Large magnetoresistance and Fermi surface study of Sb 2 Se 2 Te single crystal
Here, we have studied the magnetotransport properties of a Sb 2 Se 2 Te single crystal. Magnetoresistance (MR) is maximum when the magnetic field is perpendicular to the sample surface and reaches a value of 1100% at B = 31 T with no sign of saturation. MR shows Shubnikov de Haas (SdH) oscillations above B = 15 T. The frequency spectrum of SdH oscillations consists of three distinct peaks at α = 32 T, β = 80 T, and γ = 117 T indicating the presence of three Fermi surface pockets. Among these frequencies, β is the prominent peak in the frequency spectrum of SdH oscillations measured at different tilt angles of the sample with respect to the magnetic field. From the angle dependence β and Berry phase calculations, we have confirmed the trivial topology of the β-pocket. The cyclotron masses of charge carriers, obtained by using the Lifshitz–Kosevich formula, are found to be m$*\atop{β}$= 0.16m o and m$*\atop{γ}$= 0.63m o for the β and γ bands, respectively. The Large MR of Sb 2 Se 2 Te is suitable for utilization in electronic instruments such as computer hard discs, high field magnetic sensors, and memory devices.
36 MATERIALS SCIENCE↗
Fermi surface, possible unconventional fermions, and unusually robust resistive critical fields in the chiral-structured superconductor AuBe
The noncentrosymmetric superconductor (NCS) AuBe is investigated using a variety of thermodynamic and resistive probes in magnetic fields of up to 65 T and temperatures down to 0.3 K. Despite the polycrystalline nature of the samples, the observation of a complex series of de Haas–van Alphen (dHvA) oscillations has allowed the calculated band structure for AuBe to be validated. This permits a variety of BCS parameters describing the superconductivity to be estimated, despite the complexity of the measured Fermi surface. In addition, AuBe displays a nonstandard field dependence of the phase of dHvA oscillations associated with a band thought to host unconventional fermions in this chiral lattice. Overall, this result demonstrates the power of the dHvA effect to establish the properties of a single band despite the presence of other electronic bands with a larger density of states, even in polycrystalline samples. In common with several other NCSs, we find that the resistive upper critical field exceeds that measured by heat capacity and magnetization by a considerable factor. We suggest that our data exclude mechanisms for such an effect associated with disorder, implying that topologically protected superconducting surface states may be involved.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Reconstruction of the Bi 2 Sr 2 CaCu 2 O 8 + δ Fermi surface
The effects of structural supermodulation with the period $λ ≈ 26$ Å along the $b$ axis of Bi 2 Sr 2 CaCu 2 O 8 + δ have been observed in photoemission studies from the early days as the presence of “diffraction replicas” of the intrinsic electronic structure. Although predicted to affect the electronic structure of the Cu-O plane, the influence of supermodulation potential on Cu-O electrons has never been observed in photoemission. In the present study, we clearly see the effects on the Bi 2 Sr 2 CaCu 2 O 8 + δ electronic structure; we observe a hybridization of the intrinsic bands with the supermodulation replica bands in the form of avoided crossings and a corresponding reconstruction of the Fermi surface. We estimate the hybridization gap, $2Δ_h ~ 25$ meV, in the slightly underdoped samples. The hybridization weakens with doping and the anticrossing can no longer be resolved in strongly overdoped samples. In contrast, the “shadow” replica, shifted by ($π , π$) , is found not to hybridize with the original bands within our detection limits.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Observation of Reentrant Correlated Insulators and Interaction-Driven Fermi-Surface Reconstructions at One Magnetic Flux Quantum per Moiré Unit Cell in Magic-Angle Twisted Bilayer Graphene
The discovery of flat bands with nontrivial band topology in magic-angle twisted bilayer graphene (MATBG) has provided a unique platform to study strongly correlated phenomena including superconductivity, correlated insulators, Chern insulators, and magnetism. A fundamental feature of the MATBG, so far unexplored, is its high magnetic field Hofstadter spectrum. Here, we report on a detailed magnetotransport study of a MATBG device in external magnetic fields of up to B = 3 1 T , corresponding to one magnetic flux quantum per moiré unit cell Φ 0 . At Φ 0 , we observe reentrant correlated insulators at a flat band filling factors of ν = + 2 and of ν = + 3 , and interaction-driven Fermi-surface reconstructions at other fillings, which are identified by new sets of Landau levels originating from these. These experimental observations are supplemented by theoretical work that predicts a new set of eight well-isolated flat bands at Φ 0 , of comparable band width, but with different topology than in zero field. Overall, our magnetotransport data reveal a qualitatively new Hofstadter spectrum in MATBG, which arises due to the strong electronic correlations in the reentrant flat bands.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Evolution of nontrivial Fermi surface features in the band structures of the homologous members Pb 5 Bi 6 Se 14 and Pb 5 Bi 12 Se 23
High-quality single crystals of (PbSe) 5 (Bi 2 Se 3 ) 3m were grown and analyzed by nuclear magnetic resonance (NMR) spectroscopy. We report on 77 Se and 207 Pb NMR shifts and nuclear spin-lattice relaxation measurements in the naturally formed heterostructure homology (PbSe) 5 (Bi 2 Se 3 ) 3m with m = 1 (Pb 5 Bi 6 Se 14 ) and m = 2 (Pb 5 Bi 12 Se 23 ). A distinct site-specific contribution has been detected for both nuclei as a function of temperature, which reveals an electronic changeover from a semiconducting (Pb 5 Bi 6 Se 14 ) to a semimetalliclike (Pb 5 Bi 12 Se 23 ) system with nontrivial band structure features near the Fermi level. The temperature dependences of the relaxation rates are dominated by significant changes in the topology of energy dispersions accompanied with band edges and crossings in the region of the Fermi surface. These results, which interrogate nuclear spin interactions from selected atomic sites, clearly expose the effects of the added Bi 2 Se 3 layer on the crystal and electronic structure of (Pb 5 Bi 12 Se 23 ). These findings provide direct microscopic insight into the unconventional and dual nature of the electronic structure of these homologous thermoelectric and topologically nontrivial compounds.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Fermi surface of the flat-band intermetallics A P d 3 ( A = Pb , Sn )
The intermetallic phases APd 3 (A = Pb, Sn) were recently predicted to host an unconventional combination of unique electronic structure features, namely, flat bands near the Fermi energy coexisting with topologically protected surface states at the Gamma point. These features each could independently produce alternative electronic states, including electronically or magnetically ordered states coexisting with unconventional edge dominated transport and a significantly large thermopower coexisting with topological characteristics. To explore these expectations, we report the synthesis, structural/chemical characterization, electrical and thermal transport properties, magnetic torque (up to 45 T), and Fermi surface mapping for single crystals produced using the Czochralski technique. X-ray diffraction and scanning transmission electron microscope measurements establish the absence of defects, while small measured values of the thermopower indicate that the Fermi level is located away from the flat-band region. The electronic properties are further clarified by the topography of the Fermi surfaces, measured through the de Haas–van Alphen effect. We find that the Fermi levels are placed at higher energy values than the original ones resulting from the density functional theory calculations, 54 meV higher for PbPd 3 and 68 meV higher for SnPd 3 . Furthermore, the molten flux method was used to synthesize PbPd 3 , yielding nearly identical Fermi surfaces between the specimens grown using different synthesis techniques, indicating the robustness of the Fermi level position. According to the density functional theory calculations, the flat band is mainly formed by the 4d bands of Pd. Thus, we propose monovalent doping on the Pb/Sn site as a viable approach to accessing the flat band while maintaining the unique band structure features of these compounds.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Fermi surface reconstruction due to orthorhombic distortion in the Dirac semimetal YbMnSb 2
A Dirac semimetal with magnetic atoms as constituents delivers an interesting platform to investigate the interplay of Fermi surface (FS) topology, electron correlation, and magnetism. One such family of semimetals is YbMn 𝑃𝑛 2 (𝑃𝑛 = Sb,Bi), which is being actively studied due to the intertwined spin and charge degrees of freedom. Here, in this study, we investigate the relationship between the magnetic/crystal structures and FS topology of YbMnSb 2 using single crystal x-ray diffraction, neutron scattering, magnetic susceptibility, magnetotransport measurements, and complimentary density functional theory (DFT) calculations. Contrary to previous reports, the x-ray and neutron diffraction reveal that YbMnSb 2 crystallizes in an orthorhombic 𝑃𝑛𝑚𝑎 structure with a notable antiphase displacement of the magnetic Mn ions that increases in magnitude upon cooling. As a result of this orthorhombicity, first-principles DFT calculation reveals a reduced Brillouin zone and more anisotropic FS of YbMnSb 2 compared to YbMnBi 2 . Moreover, the hole-type carrier density drops by two orders of magnitude as YbMnSb 2 orders antiferromagnetically, indicating band folding in a magnetic ordered state. In addition, the Landau level fan diagram yields a nontrivial nature of the Shubnikov–de Haas quantum oscillation frequency arising from the Dirac-like Fermi pocket. These results imply that YbMnSb 2 is an ideal platform to explore the interplay of subtle lattice distortion, magnetic order, and topological transport arising from relativistic quasiparticles.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Evolution of the Fermi Surface of 1T-VSe 2 across a Structural Phase Transition
Periodic lattice distortion, known as the charge density wave, is generally attributed to electron–phonon coupling. This correlation is expected to induce a pseudogap at the Fermi level in order to gain the required energy for stable lattice distortion. The transition metal dichalcogenide 1T-VSe 2 also undergoes such a transition at 110 K. Here, we present detailed angle-resolved photoemission spectroscopy experiments to investigate the electronic structure in 1T-VSe 2 across the structural transition. Previously reported warping of the electronic structure and the energy shift of a secondary peak near the Fermi level as the origin of the charge density wave phase are shown to be temperature independent and hence cannot be attributed to the structural transition. Our work reveals new states that were not resolved in previous studies. Earlier results can be explained by the different dispersion natures of these states and temperature-induced broadening. Only the overall size of the Fermi surface is found to change across the structural transition. These observations, quite different from the charge density wave scenario commonly considered for 1T-VSe 2 and other transition metal dichalcogenides, bring fresh perspectives toward correctly describing structural transitions. Therefore, these new results can be applied to material families in which the origin of the structural transition has not been resolved.
36 MATERIALS SCIENCE↗
Fermi Surface of Metallic V 2 O 3 from Angle-Resolved Photoemission: Mid-level Filling of e g π Bands
Using angle resolved photoemission spectroscopy, we report the first band dispersions and distinct features of the bulk Fermi surface (FS) in the paramagnetic metallic phase of the prototypical metal-insulator transition material V$_{2}O_{3}$. Along the c axis we observe both an electron pocket and a triangular holelike FS topology, showing that both V 3d $a_{1g}$ and e$_{g}^{π}$ states contribute to the FS. Here, these results challenge the existing correlation-enhanced crystal field splitting theoretical explanation for the transition mechanism and pave the way for the solution of this mystery.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Spin excitations and the Fermi surface of superconducting FeS
Here, high-temperature superconductivity occurs near antiferromagnetic instabilities and the nematic state. Debate remains on the origin of nematic order in FeSe and its relation with superconductivity. Here, we use transport, neutron scattering and Fermi surface measurements to demonstrate that hydrothermo grown superconducting FeS, an isostructure of FeSe, is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Only stripe-type spin excitations are observed up to 100 meV. No direct coupling between spin excitations and superconductivity in FeS is found, suggesting that FeS is less correlated and the nematic order in FeSe is due to competing checkerboard and stripe spin fluctuations.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Indirect measurements of Fermi surface parameters of some chevrel phase materials
A series of measurements of normal state and superconducting properties were made in zero and in high magnetic fields. When these results are combined with a complete set of theoretical expressions, a number of Fermi surface parameters are found.
Woollam, J. A.↗
Multiband Fermi surface in $1T–$$\mathrm{VSe_2}$ and its implication for the charge density wave phase
Angle-resolved photoemission spectroscopy experiments reveal a surprisingly richer surface electronic structure in $1T–$$\mathrm{VSe_2}$ than previously predicted or probed. Earlier claims supporting a charge density wave phase in this material are reexamined in terms of these findings and are found to be untenable. Here, the Fermi surface is found to be gapless, while band warping effects, currently attributed to three-dimensional lattice distortion, result from the simultaneous dispersion of the closely lying multiple bands. Based on these findings, a charge density wave scenario in $1T–$$\mathrm{VSe_2}$ is unlikely. On the other side, the presence of multiple states crossing the Fermi level should constitute relevant constraints for any viable microscopic model of the structural phase transition of $\mathrm{VSe_2}$.
36 MATERIALS SCIENCE↗
Mixed-valence insulators with neutral Fermi surfaces
Samarium hexaboride is a classic three-dimensional mixed valence system with a high-temperature metallic phase that evolves into a paramagnetic charge insulator below 40 K. A number of recent experiments have suggested the possibility that the low-temperature insulating bulk hosts electrically neutral gapless fermionic excitations. Here we show that a possible ground state of strongly correlated mixed valence insulators—a composite exciton Fermi liquid—hosts a three dimensional Fermi surface of a neutral fermion, that we name the “composite exciton.” We introduce the mechanism responsible for the formation of such excitons, discuss the phenomenology of the composite exciton Fermi liquids and make comparison to experiments in SmB 6 .
74 ATOMIC AND MOLECULAR PHYSICS↗
Large Fermi surface expansion through anisotropic mixing of conduction and $\textit{f}$ electrons in the semimetallic Kondo lattice CeBi
Using angle-resolved photoemission spectroscopy (ARPES) and resonant ARPES, we report evidence of strong anisotropic conduction-$\textit{f}$ electron mixing ($\textit{c-f}$ mixing) in CeBi by observing a largely expanded Ce 5$d$ pocket at low temperature, with no change in the Bi-6$\textit{p}$ bands. We note the Fermi surface (FS) expansion is accompanied by a pronounced spectral weight transfer from the local 4$f^0$ peak of Ce (corresponding to Ce 3+ ) to the itinerant conduction bands near the Fermi level. Careful analysis suggests that the observed large FS change (with a volume expansion of the electron pocket up to 40%) can most naturally be explained by a small valence change (~1%) of Ce, which coexists with a very weak Kondo screening. Our work therefore provides evidence for a FS change driven by real charge fluctuations deep in the Kondo limit, which are made possible by the low carrier density. Our results highlight the large tunability of the FS in low carrier density Kondo systems.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗