Fermi Pockets Research Articles

Dominant Charge Density Order in TaTe_{4}.

Electronic orders such as charge density wave (CDW) and superconductivity raise exotic physics and phenomena as evidenced in recently discovered kagome superconductors and transition metal chalcogenides. In most materials, CDW induces a weak, perturbative effect, manifested as shadow bands, minigaps, resistivity kinks, etc. Here we demonstrate a unique example-transition metal tetratellurides TaTe_{4}, in which the CDW order dominates the electronic structure and transport properties. Using angle-resolved photoemission spectroscopy, we found that the band structure of CDW TaTe_{4} is characterized by small, bulk electron pockets. Density functional theory analyses reveal their CDW origin from the folding of the original, large Fermi pockets. Importantly, the CDW induced pockets result in prominent frequencies in the quantum oscillation of the magnetoresistance. Satisfactory agreements are reached between results from photoemission spectroscopy, density functional theory, and quantum oscillation, concerning the shape, size, location, and angle dependence of the CDW pockets. Our results underline transition metal tetratellurides as an outstanding example for exploring the interplay between CDW, pressure induced superconductivity, and potential topological states under strong field.

The de Haas–van Alphen quantum oscillations in the kagome metal RbTi3Bi5

Abstract The kagome system has attracted great interest in condensed matter physics due to its unique structure that can host various exotic states such as superconductivity (SC), charge density waves (CDWs) and nontrivial topological states. The topological semimetal RbTi3Bi5 consisting of a Ti kagome layer shares a similar crystal structure to the topological correlated materials AV3Sb5 (A = K, Rb, Cs) but without the absence of CDW and SC. Systematic de Haas–van Alphen oscillation measurements are performed on single crystals of RbTi3Bi5 to pursue nontrivial topological physics and exotic states. Combining this with theoretical calculations, the detailed Fermi surface topology and band structure are investigated. A two-dimensional Fermi pocket β is revealed with a light effective mass, consistent with the semimetal predictions. The Landau fan diagram of RbTi3Bi5 reveals a zero Berry phase for the β oscillation in contrast to that of CsTi3Bi5. These results suggest that kagome RbTi3Bi5 is a good candidate for exploring nontrivial topological exotic states and topological correlated physics.

Anomalous normal-state gap in an electron-doped cuprate.

In the underdoped n-type cuprate Nd2-xCexCuO4, long-range antiferromagnetic order reconstructs the Fermi surface, resulting in a putative antiferromagnetic metal with small Fermi pockets. Using angle-resolved photoemission spectroscopy, we observe an anomalous energy gap, an order of magnitude smaller than the antiferromagnetic gap, in a wide portion of the underdoped regime and smoothly connecting to the superconducting gap at optimal doping. After considering all the known ordering tendencies in tandem with the phase diagram, we hypothesize that the normal-state gap in the underdoped n-type cuprates originates from Cooper pairing. The high temperature scale of the normal-state gap raises the prospect of engineering higher transition temperatures in the n-type cuprates comparable to those of the p-type cuprates.

Large magnetoresistance and nontrivial Fermi surface topology in quasi-one-dimensional Ta2PdSe6

We report the experimental investigation of quasi-one-dimensional (quasi-1D) Ta2PdSe6 single crystals by magnetotransport measurements under high magnetic fields up to 31 T. The crystals show large unsaturated magnetoresistance up to (1.8 × 105%) under 31 T at 1.6 K, accompanied by remarkable resistivity oscillations. The analysis of quantum oscillations reveals the existence of five Fermi pockets, consistent with theoretical calculations. Furthermore, our investigation suggested that band structure is modified by the electron correlations in Ta2PdSe6, which may provide a platform for exploring the interplay between electron effects and topological properties in quasi-1D materials.

Spin-orbit coupled Hubbard skyrmions

When the material phases exhibit topological quantum numbers, they host defects protected by the nontrivial topology. Magnetic skyrmions are such “quantized” objects, and although many of them are metals, they had been most likely treated in theories as purely classical spin states. Here, we show that the electrons described by the Hubbard model with strong spin-orbit couplings can exhibit various nano- or flake-size skyrmions in their ground state. Importantly, the conducting electrons forming Fermi pockets themselves carry textured magnetic moments in both real and momentum space and on top of that possess Chern numbers in their energy bands. This quantum and conducting skyrmion is related to small skyrmions observed in atomic-layered compounds. We clarify how the effective magnetic interactions and magnetic anisotropies are tied to the spin-orbit coupling, and how they influence the stability of skyrmions beyond the phenomenology. Published by the American Physical Society 2024

Comprehensive investigation of quantum oscillations in semimetal using an ac composite magnetoelectric technique with ultrahigh sensitivity

Quantum oscillation (QO), a physical phenomenon that reflects the characteristics of the Fermi surface and transport fermions, has been extensively observed in metals and semimetals through various approaches, like magnetostriction, magnetization, resistivity, and thermoelectric power. However, only limited oscillation frequencies can be revealed by the aforementioned methods, particularly in semimetals with intricate Fermi pockets and associated magnetic breakdown phenomena. In this paper, we present the application of an ac composite magnetoelectric (ME) technique to measure the QOs of a topological nodal-line semimetal, ZrSiS, which possesses six fundamental QO frequencies. By employing the ME technique with a maximum magnetic field of 13 T and a minimum temperature of 2 K, we are able to capture all the fundamental frequencies and most of the permissible magnetic breakdown frequencies. Remarkably, a series of magnetic breakdown frequencies around 8000 T were revealed even in a magnetic field as low as 7.5 T. These findings highlight the ME technique as an ultrahigh-sensitive tool for studying Dirac Fermions and other topological semimetals with complex Fermi surfaces.

Frustrated superconductivity and intrinsic reduction of <i>T</i><sub><i>c</i></sub> in trilayer nickelate

<p>Identifying the key factors controlling the magnitude of <i>T</i><sub><i>c</i></sub> is of critical importance in the pursuit of high-temperature superconductivity. In cuprates, <i>T</i><sub><i>c</i></sub> reaches its maximal value in trilayer structure, leading to the belief that interlayer coupling may help promote the pairing. In contrast, for the recently discovered nickelate superconductors under high pressure, the maximum <i>T</i><sub><i>c</i></sub> is reduced from about 80 K in the bilayer La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> to 30 K in the trilayer La<sub>4</sub>Ni<sub>3</sub>O<sub>10</sub>. Motivated by this opposite trend, we propose an interlayer pairing scenario for the superconductivity of La<sub>4</sub>Ni<sub>3</sub>O<sub>10</sub>. Our theory reveals intrinsic frustration in the spin-singlet pairing that the inner layer tends to form with both of the two outer layers respectively, leading to strong superconducting fluctuations between layers. This explains the reduction of its maximum <i>T</i><sub><i>c</i></sub> compared to that of the bilayer La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>. Our findings support a fundamental distinction between multilayer nickelate and cuprate superconductors, and ascribe it to their different (interlayer versus intralayer) pairing mechanisms. Furthermore, our theory predicts extended <i>s</i><sup><i>±</i></sup>-wave gap structures in La<sub>4</sub>Ni<sub>3</sub>O<sub>10</sub>, with varying signs and possible nodes on different Fermi pockets. We also find an intrinsic Josephson coupling with potentially interesting consequences that may be examined in future experiments. Our work reveals the possibility of rich novel physics in multilayer superconductors with interlayer pairing.</p>

Extremely large magnetoresistance with coexistence of a nontrivial Berry phase in Nb0.5Ta0.5P: an experimental and theoretical study

We synthesized Nb0.5Ta0.5P, exhibiting XMR at low temperatures with charge carrier compensation up to 50 K. SdH oscillations reveal multiple Fermi pockets and non-zero Berry phase. SOC effects were analyzed via band structure calculations.

Magnetic stability, Fermi surface topology, and spin-correlated dielectric response in monolayer 1T-CrTe2.

We have carried out density-functional theory (DFT) calculations to study the magnetic stability of both ferromagnetic (FM) and anti-ferromagnetic (AFM) states in monolayer 1T-CrTe2. Our results show that the AFM order is lower in energy and thus is the ground state. By tuning the lattice parameters, the AFM order can transition to the FM order, in good agreement with experimental observation. We observe a commensurate spin density wave (SDW) alongside the previously predicted charge density wave (CDW), and attribute the AFM order to the SDW. The SDW order leads to distinct hole and electron Fermi pockets and a pronounced optical anisotropy, suggesting quasi-one-dimensional behavior in this material.

Temperature-dependent Fermi surface probed by Shubnikov–de Haas oscillations in topological semimetal candidates DyBi and HoBi

Rare earth-based monopnictides are among the most intensively studied groups of materials in which extremely large magnetoresistance has been observed. This study explores magnetotransport properties of two representatives of this group, DyBi and HoBi. The extreme magnetoresistance is discovered in DyBi and confirmed in HoBi. At T=2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T=2$$\\end{document} K and in B=14\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B=14$$\\end{document} T for both compounds, magnetoresistance reaches the order of magnitude of 104%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^4\\%$$\\end{document}. For both materials, standard Kohler’s rule is obeyed only in the temperature range from 50 to 300 K. At lower temperatures, extended Kohler’s rule has to be invoked because carrier concentrations and mobilities strongly change with temperature and magnetic field. This is further proven by the observation of a quite rare temperature-dependence of oscillation frequencies in Shubnikov–de Haas effect. Rate of this dependence clearly changes at Néel temperature, reminiscent of a novel magnetic band splitting. Multi-frequency character of the observed Shubnikov–de Haas oscillations points to the coexistence of electron- and hole-type Fermi pockets in both studied materials. Overall, our results highlight correlation of temperature dependence of the Fermi surface with the magnetotransport properties of DyBi and HoBi.

Multiple channel transport properties of topological surface states in SmB6 nanoribbons

SmB6, a topological Kondo insulator, possesses topologically protected surface states, in which carrier mobility is supposed to be high but is significantly varied in orders of magnitude dependent upon characterization methods. Herein, we performed magnetoresistance measurements on a Hall-bar device constructed on a highly uniform SmB6 nanoribbon with (001) surface grown by chemical vapor deposition. Unusual non-linear Hall resistance was observed at low temperature and explained with a two-carrier model. The two types of carriers were attributed to two types of Fermi pockets on the Brillouin zone of the SmB6 (001) surface. The Fermi pocket on Γ point provided high hole mobility up to 1.4×103 cm2/V s but low hole density at a temperature of 2.2 K. On the other hand, the Fermi pockets on X points supplied low electron mobility down to 1.8 cm2/V s but high electron density. The identification of the two types of pockets laid the foundation for the understanding of the transport via the topological surface states of the SmB6 (001) surface.

Anisotropic and High‐Mobility Electronic Transport in a Quasi 2D Antiferromagnet NdSb2

AbstractAdvancements in low‐dimensional functional device technology heavily rely on the discovery of suitable materials which have interesting physical properties as well as can be exfoliated down to the 2D limit. Exfoliable high‐mobility magnets are one such class of materials that, not due to lack of effort, has been limited to only a handful of options. So far, most of the attention has been focused on the van der Waals (vdW) systems. However, even within the non‐vdW, layered materials, it is possible to find all these desirable features. Using chemical reasoning, it is found that NdSb2 is an ideal example. Even with a relatively small interlayer distance, this material can be exfoliated down to few layers. NdSb2 has an antiferromagnetic ground state with a quasi 2D spin arrangement. The bulk crystals show a very large, non‐saturating magnetoresistance along with highly anisotropic electronic transport properties. It is confirmed that this anisotropy originates from the 2D Fermi pockets which also imply a rather quasi 2D confinement of the charge carrier density. Both electron and hole‐type carriers show very high mobilities. The possible non‐collinear spin arrangement also results in an anomalous Hall effect.

Magnetic breakdown and spin-zero effect in quantum oscillations in kagome metal CsV3Sb5

In the recently discovered kagome metal CsV3Sb5, an intriguing proposal invoking a doped Chern insulator state suggests the presence of small Chern Fermi pockets hosting spontaneous orbital-currents and large orbital magnetic moments. While the net thermodynamic magnetization is nearly insensitive to these moments, due to their antiferromagnetic alignment, their presence can be revealed by the Zeeman effect, which shifts electron energies in magnetic fields with a proportionality given by the effective g−factor. Here, we determine the g-factor using the spin-zero effect in magnetic quantum oscillations. A large g-factor enhancement is visible only in magnetic breakdown orbits between conventional and concentrated Berry curvature Fermi pockets that host large orbital moments. Such Berry-curvature-generated large orbital moments are almost always concealed by other effects. In this system, however, magnetic breakdown orbits due to the proximity to a conventional Fermi-surface section allow them to be visibly manifested in magnetic quantum oscillations. Our results provide a remarkable example of the interplay between electronic correlations and more conventional electronic bands in quantum materials.

Spin and valley-polarized multiple Fermi surfaces of α -RuCl3/bilayer graphene heterostructure

We report the transport properties of α-RuCl3/bilayer graphene heterostructures, where carrier doping is induced by a work function difference, resulting in distinct electron and hole populations in α-RuCl3 and bilayer graphene, respectively. Through a comprehensive analysis of multi-channel transport signatures, including Hall measurements and quantum oscillation, we unveil significant band modifications within the system. In particular, we observe the emergence of spin and valley-polarized multiple hole-type Fermi pockets, originating from the spin-selective band hybridization between α-RuCl3 and bilayer graphene, breaking the spin degree of freedom. Unlike the α-RuCl3/monolayer graphene system, the presence of different hybridization strengths between α-RuCl3 and the top and bottom graphene layers leads to an asymmetric behavior of the two layers, confirmed by effective mass experiments, resulting in the manifestation of valley-polarized Fermi pockets. These compelling findings establish α-RuCl3 proximitized to bilayer graphene as an outstanding platform for engineering its unique low-energy band structure.

Potential Lifshitz transition at optimal substitution in nematic pnictide Ba1-xSrxNi2As2.

BaNi2As2 is a structural analog of the pnictide superconductor BaFe2As2, which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature (Tc). To understand the mechanisms responsible for enhancement of Tc, we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba1-xSrxNi2As2 series, which agree well with our density functional theory (DFT) calculations throughout the substitution range. Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and reasonably nested electron and hole Fermi pockets near optimal substitution where Tc is maximum. These nested pockets host Ni dxz/dyz orbital compositions, which we associate with the enhancement of nematic fluctuations, revealing unexpected connections to the iron-pnictide superconductors. This gives credence to a scenario in which nematic fluctuations drive an enhanced Tc.

Electron-Correlation-Induced Charge Density Wave in FeGe

As the first magnetic kagome material to exhibit the charge density wave (CDW) order, FeGe has attracted much attention in recent research. Similar to AV3Sb5 (A = K, Cs, Rb), FeGe exhibits the CDW pattern with an in-plane 2×2 structure and the existence of van Hove singularities near the Fermi level. However, sharply different from AV3Sb5 which has phonon instability at M point, all the theoretically calculated phonon frequencies in FeGe remain positive. Based on first-principles calculations, we surprisingly find that the maximum of nesting function is at K point instead of M point. Two Fermi pockets with Fe-dxz and Fe-d x 2–y 2 /dxy orbital characters have large contribution to the Fermi nesting, which evolve significantly with kz , indicating the highly three-dimensional (3D) feature of FeGe in contrast to AV3Sb5. Considering the effect of local Coulomb interaction, we reveal that the instability at K point is significantly suppressed due to the sublattice interference mechanism. Meanwhile, the wave functions nested by vector M have many ingredients located at the same Fe site, thus the instability at M point is enhanced. This indicates that the electron correlation, rather than electron-phonon interaction, plays a key role in the CDW transition at M point.

Small Fermi Pockets Intertwined with Charge Stripes and Pair Density Wave Order in a Kagome Superconductor

Small Fermi Pockets Intertwined with Charge Stripes and Pair Density Wave Order in a Kagome Superconductor

Unveiling phase diagram of the lightly doped high-Tc cuprate superconductors with disorder removed

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO2 planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba2Ca5Cu6O12(F,O)2 with inner CuO2 layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.

Semimetallic spin-density wave state in iron pnictides

We examine the existence of semimetallic spin-density wave (SDW) states in iron pnictides. In the experimentally observed metallic SDW state, the symmetry-protected Dirac cones are located away from the Fermi surface giving rise to tiny pockets and there are also additional Fermi pockets such as one around Γ. We find that the location of a pair of Dirac points with respect to the Fermi surface exhibits significant sensitivity to the orbital splitting between the d xz and d yz orbitals. Besides, in the presence of orbital splitting, the Fermi pockets not associated with the Dirac cones, can be suppressed so that a semimetallic SDW state can be realized. We explain these findings in terms of difference in the slopes and orbital contents of the bands which form the Dirac cone, and obtain the necessary conditions dependent on these two and other parameters for the coexisting Dirac semimetallic and SDW states. Additionally, the topologically protected edge states are studied in the ribbon geometry when the same are oriented either along x or y axes.

Quasi-one-dimensional bulk thermoelectrics

In a recent Science Bulletin article, Dong et al. reported a quasi-one-dimensional TlCu3Te2 bulk material with an application-level room temperature ZT of 1.3 benefited from the pancake-like Fermi pocket with quasi-one-dimensional dispersion at narrow band-gap edge and the large, anharmonic quasi-one-dimensional lattice of heavy Tl+ and weakly bonded Cu+ ions. In a recent Science Bulletin article, Dong et al. reported a quasi-one-dimensional TlCu3Te2 bulk material with an application-level room temperature ZT of 1.3 benefited from the pancake-like Fermi pocket with quasi-one-dimensional dispersion at narrow band-gap edge and the large, anharmonic quasi-one-dimensional lattice of heavy Tl+ and weakly bonded Cu+ ions.