Lifshitz Transition Research Articles

Transport properties and electronic phase transitions in two-dimensional tellurium at high pressure

Utilizing in situ Raman spectroscopy, resistivity, and Hall-effect measurements, we conducted an extensive investigation on the continuous electronic phase transitions and transport properties of two-dimensional (2D) tellurium (Te) under high pressure at room and low temperature (80–300 K). The distinguishable decrease in the A1 Raman mode's full width at half maximum in the trigonal phase (Te-I) indicated an electronic phase transition at 2.2 GPa. The following Hall-effect experiments located the Lifshitz transition and the semiconductor-semimetal transition at 0.9 and 1.9 GPa, respectively, and the semiconductor-semimetal transition was also confirmed by resistivity variation through temperature. The charge carrier types of the Te changed from hole to electron during the phase transition from Te-I to Te-II (triclinic phase) at low temperature, while the transport parameters remained almost unchanged during the phase transition from Te-II to Te-III (monoclinic phase). The results offered complete and thorough electronic phase transitions and transport characteristics of 2D Te, hence great advancing the potential application of Te in electronic devices.

Revealing the temperature-driven Lifshitz transition in p-type Mg3Sb2-based thermoelectric materials

The manipulation of native atomic defects and their thermal excitations plays vital roles in the thermoelectric performance of Mg3Sb2-based materials. While native defects manipulation has been intensively studied in p-type Mg3Sb2, there exists interesting unsolved issue regarding the abnormal semiconducting electrical behavior in most of samples. In this work, high quality Mg3Sb2 and Mg3Bi2 (00l) films are fabricated by molecular beam epitaxy technique, while variable temperature angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy measurements are utilized for resolving the aforementioned issue. The thermal excitation of Mg interstitials (the electron donor) results in an obvious downshift of valence bands with rising temperature in both the p-type Mg3Sb2 and Mg3Bi2. Meanwhile, the interesting temperature-driven Lifshitz transition is discovered in the p-type Mg3Sb2, as indicated by the change of Fermi surface topology. Above the Lifshitz transition temperature, the Fermi level of p-type Mg3Sb2 will enter the bandgap, which leads to the abnormal semiconducting electrical behavior. This work discloses the excitation of native defects and temperature-driven Lifshitz transition, which are the main causes for the anomalies in electrical transport of p-type Mg3Sb2-based materials, and also provides valuable insights for further improving their thermoelectric performance.

Pressure-induced Lifshitz transition in the type-II Weyl semimetal WP2

We report evidences of a pressure-induced Lifshitz transition in the type-II Weyl semimetal WP2 via resistance, synchrotron X-ray diffraction, and Raman scattering measurements with the aid of density functional theory calculations up to 50 GPa. The resistance at 2 K and 300 K and residual resistance ratio rise precipitously at ca. 20 GPa. The pristine orthorhombic structure is robust against pressures up to 50 GPa. There occurs a discontinuity in the unit cell volume, lattice parameter, lattice parameter ratio and axial compressibility at ～20 GPa. Two Raman-active modes soften abruptly accompanied with appearance of a new Raman-active mode at ～15 GPa. Density functional theory calculations identify drastic changes in the Fermi surface topology at～30 GPa. Our findings stimulate further pursuit of pressure-induced superconductivity via a Lifshitz transition in WP2.

Nonlocal Acoustic Moiré Hyperbolic Metasurfaces.

The discovery of the topological transition in twisted bilayer (tBL) materials has attracted considerable attention in nano-optics. In the analogue of acoustics, however, no such topological transition has been found due to the inherent nondirectional scalar property of acoustic pressure. In this work, by using a theory-based nonlocal anisotropic design, the in-plane acoustic pressure is transformed into a spatially distributed vector field using twisted multilayer metasurfaces. So-called "acoustic magic angle"-related acoustic phenomena occur, such as nonlocal polariton hybridization and the topological Lifshitz transition. The dispersion becomes flat at the acoustic magic angle, enabling polarized excitations to propagate in a single direction. Moreover, the acoustic topological transition (from hyperbolic to elliptic dispersion) is experimentally observed for the first time as the twist angle continuously changes. This unique characteristic facilitates low-loss tunable polariton hybridization at the subwavelength scale. A twisted trilayer acoustic metasurface is also experimentally demonstrated, and more possibilities for manipulating acoustic waves are found. These discoveries not only enrich the concepts of moiré physics and topological acoustics but also provide a complete framework of theory and methodologies for explaining the phenomena that are observed.

Kagome Quantum Oscillations in Graphene Superlattices.

Electronic spectra of solids subjected to a magnetic field are often discussed in terms of Landau levels and Hofstadter-butterfly-style Brown-Zak minibands manifested by magneto-oscillations in two-dimensional electron systems. Here, we present the semiclassical precursors of these quantum magneto-oscillations which appear in graphene superlattices at low magnetic field near the Lifshitz transitions and persist at elevated temperatures. These oscillations originate from Aharonov-Bohm interference of electron waves following open trajectories that belong to a kagome-shaped network of paths characteristic for Lifshitz transitions in the moire superlattice minibands of twistronic graphenes.

A theoretical study of Lifshitz transition for 2H-TaS2.

Lifshitz transition was proposed to explain a change of the topology structure in a Fermi surface induced by continuous lattice deformation without symmetry breaking since 1960. It is well known that the anomalies of the kinetic coefficients (the coefficient of heat conduction and electrical conductivity, viscosity, sound absorption, etc.) are usually closely connected with the Lifshitz transition behavior. 2H-TaS2 is a typical representative to study its anomalies of temperature dependence of heat capacity, resistivity, Hall effect, and magnetic susceptibility. Its geometrical structure of the charge density wave (CDW) phase and layer number dependence of carrier-sign alternation upon cooling in the Hall measurements have not been well understood. The geometrical structure (T-Ts) of the CDW phase was predicted through first principles calculations for bulk and mono-layer 2H-TaS2. Driven by electron-lattice coupling, Ta atoms contract to form a partially gapped CDW phase. The CDW phase has a larger average interlayer separation of S-S atoms in the adjacent two layers compared with the metal phase, which results in a weaker chemical bonding among S-S atoms in the adjacent two layers and then a narrower bandwidth of the energy band. The narrower bandwidth of the energy band leads to a larger density of states (DOS) in the out-of-plane direction above the Fermi level for the CDW phase. As the Fermi level continually drops from the DOS region with a negative slope to that with a positive slope on cooling, the reversal of the p → n type carrier and the pocket-vanishing-type Lifshitz transition occur in the bulk 2H-TaS2. However, the Fermi level slightly drops by 6 meV and happens to be at the positions of pseudo band gaps, so the reduction of in-plane DOS and total DOS is responsible for the always p-type carrier in the mono-layer samples. Our CDW vector of the k-space separation between two saddle points is QSP ≈ 0.62 GK and can provide a theoretical support for the "saddle-point" CDW mechanism proposed by Rice and Scott. Our theoretical explanation gives a new understanding of both Lifshitz transition for symmetry breaking and reversal for the p-n carrier sign in the Hall measurements in various two-dimensional transition metal disulfides.

Theoretical study of CDW phases for bulk NbX2 (X = S and Se).

In most two-dimensional transition metal chalcogenides, the superconducting phase coexists with the charge density wave (CDW) phase. There exists at least one case, i.e. bulk 2H-NbS2, that does not conform to this picture. Scientists have shown great interest in trying to experimentally find the CDW phase of bulk NbS2 since 1975. Is there any theoretically more stable thermodynamic state than its higher-temperature metal phase, especially in the case of charge injection? Theoretically more stable CDW bulk configurations (TC for 2H-NbS2 and TTs for 2H-NbSe2) with partial pseudo energy gaps were predicted through the harmonic phonon softening theory and first-principles calculations. The ratios of larger to smaller pseudo gaps around K-H segment in the Brillouin zone for CDW phases are basically equal to those of superconductivity phases for bulk 2H-NbX2 (X = S and Se). The CDW phase should coexist with its superconductor state below the critical temperature rather than the metal phase for bulk 2H-NbS2. The presence of CDW phase should be more easily observed experimentally when the injected charge reaches 0.5e/Nb18S36 for bulk 2H-NbS2. Our calculations of density of state (DOS) indicated that, during Nb atoms contracting to form the CDW phases with symmetry breaking in the in-plane direction, dominant conductive carriers are always of p-type for bulk 2H-NbS2 while the alternation of carrier type from p-type to n-type occurs for bulk 2H-NbSe2. The Fermi level continuously drops and then the M-L segment of the out-of-plane energy band emerges from the Fermi surface, which corresponds to the reversal of p-n type sign. Lifshitz transition of pocket-vanishing types occurs in the out-of-plane direction without symmetry breaking during the geometrical structural phase transition for bulk 2H-NbSe2. Our calculations have theoretically addressed the long-standing coexistence issue of CDW and superconducting phases.

Perekhody Lifshitsa i uglovye diagrammy provodimosti v metallakh so slozhnymi poverkhnostyami Fermi

We consider the Lifshitz topological transitions and the corresponding changes in the galvano-magnetic properties of a metal from the point of view of the general classification of open electron trajectories arising on Fermi surfaces of arbitrary complexity in the presence of magnetic field. The construction of such a classification is the content of the Novikov problem and is based on the division of non-closed electron trajectories into topologically regular and chaotic trajectories. The description of stable topologically regular trajectories gives a basis for a complete classification of non-closed trajectories on arbitrary Fermi surfaces and is connected with special topological structures on these surfaces. Using this description, we describe here the distinctive features of possible changes in the picture of electron trajectories during the Lifshitz transitions, as well as changes in the conductivity behavior in the presence of a strong magnetic field. As it turns out, the use of such an approach makes it possible to describe not only the changes associated with stable electron trajectories, but also the most general changes of the conductivity diagram in strong magnetic fields.

Spintronic Quantum Phase Transition in a Graphene/Pb0.24Sn0.76Te Heterostructure with Giant Rashba Spin‐Orbit Coupling

AbstractMechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a 2D electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. The spintronic properties of this heterostructure with non‐local spin valve devices are studied. This study observes spin‐momentum locking at lower temperatures that transitions to regular spin channel transport only at ≈40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin‐momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition‐like behavior in the Landé g‐factor and spin relaxation time.

Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5

During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators – an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to strong coupling is not clear. In this work, using angle-resolved photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray diffraction (XRD), we investigate the broken symmetry state across the semimetal-to-semiconductor transition in a leading bulk excitonic insulator candidate system Ta2Ni(Se,S)5. A broken symmetry phase is found to be continuously suppressed from the semimetal side to the semiconductor side, contradicting the anticipated maximal excitonic instability around the Lifshitz transition. Bolstered by first-principles and model calculations, we find strong interband electron-phonon coupling to play a crucial role in the enhanced symmetry breaking on the semimetal side of the phase diagram. Our results not only provide insight into the longstanding debate of the nature of intertwined orders in Ta2NiSe5, but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems.

Pressure-Induced Superconductivity and Topological Quantum Phase Transitions in the Topological Semimetal ZrTe2.

Topological transition metal dichalcogenides (TMDCs) have attracted much attention due to their potential applications in spintronics and quantum computations. In this work, the structural and electronic properties of topological TMDCs candidate ZrTe2 are systematically investigated under high pressure. A pressure-induced Lifshitz transition is evidenced by the change of charge carrier type as well as the Fermi surface. Superconductivity is observed at around 8.3GPa without structural phase transition. A typical dome-shape phase diagram is obtained with the maximum Tc of 5.6K for ZrTe2 . Furthermore, the theoretical calculations suggest the presence of multiple pressure-induced topological quantum phase transitions, which coexists with emergence of superconductivity. The results demonstrate that ZrTe2 with nontrivial topology of electronic states displays new ground states upon compression.

Charge transfer induced Lifshitz transition and magnetic symmetry breaking in ultrathin CrSBr crystals

Charge transfer induced Lifshitz transition and magnetic symmetry breaking in ultrathin CrSBr crystals

Unveiling the double-peak structure of quantum oscillations in the specific heat

Quantum oscillation phenomenon is an essential tool to understand the electronic structure of quantum matter. Here we report a systematic study of quantum oscillations in the electronic specific heat Cel in natural graphite. We show that the crossing of a single spin Landau level and the Fermi energy give rise to a double-peak structure, in striking contrast to the single peak expected from Lifshitz-Kosevich theory. Intriguingly, the double-peak structure is predicted by the kernel term for Cel/T in the free electron theory. The Cel/T represents a spectroscopic tuning fork of width 4.8kBT which can be tuned at will to resonance. Using a coincidence method, the double-peak structure can be used to accurately determine the Landé g-factors of quantum materials. More generally, the tuning fork can be used to reveal any peak in fermionic density of states tuned by magnetic field, such as Lifshitz transition in heavy-fermion compounds.

Lifshitz Transitions and Angular Conductivity Diagrams in Metals with Complex Fermi Surfaces

Lifshitz Transitions and Angular Conductivity Diagrams in Metals with Complex Fermi Surfaces

Topological Hall effect and the transition of the anomalous Hall effect mechanism in hexagonal alloys Mn3.1− xFexGe0.9 (x = 1.6, 1.8, 2.0)

Hexagonal Mn3Ge, with both kagome lattice and triangular antiferromagnetism, has gained significant attention due to its large anomalous Hall effect (AHE), resulting from the non-vanishing Berry phase. In this study, we present the magnetic and anomalous transport properties of a series hexagonal D019 type Fe-doped Mn3Ge alloys with the composition of Mn3.1−xFexGe0.9 (x = 1.6, 1.8, 2.0). The ferromagnetic interactions gradually increase with increasing Fe content. The longitudinal resistivity of all alloys exhibits a typical metallic behavior, increasing with temperature from 5 to 390 K. The residual resistivity decreases from 120.4 to 67.8 μΩ·cm as x increases from 1.6 to 2.0. A temperature-driven Lifshitz transition and a spin reorientation have been observed in the x = 1.6 alloy. Topological Hall effect accompanied by the spin reorientation is demonstrated. The maximum value of the topological Hall resistivity ρxyT is approximately 0.16 μΩ·cm. The relationship of ρxyA∝ ρxx in x = 1.6 alloy suggests that the extrinsic skew scattering predominantly contributes to the AHE mechanism. In the case of x = 1.8 and 2.0, both intrinsic and extrinsic factors contribute to the AHE. The anomalous Hall conductivity of our polycrystalline samples at room temperature is comparable to that of single crystal Mn3Ge, which is advantageous for practical applications. This study reveals the effectiveness of chemical engineering in tailoring nontrivial spin textures and the AHE.

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.

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.

Magnetic field driven Lifshitz transition and one-dimensional Weyl nodes in three-dimensional pentatellurides

Magnetic field driven Lifshitz transition and one-dimensional Weyl nodes in three-dimensional pentatellurides

Transport properties and doping evolution of the Fermi surface in cuprates

Measured transport properties of three representative cuprates are reproduced within the paradigm of two electron subsystems, itinerant and localized. The localized subsystem evolves continuously from the Cu 3d^9 hole at half-filling and corresponds to the (pseudo)gapped parts of the Fermi surface. The itinerant subsystem is observed as a pure Fermi liquid (FL) with material-independent universal mobility across the doping/temperature phase diagram. The localized subsystem affects the itinerant one in our transport calculations solely by truncating the textbook FL integrals to the observed (doping- and temperature-dependent) Fermi arcs. With this extremely simple picture, we obtain the measured evolution of the resistivity and Hall coefficients in all three cases considered, including LSCO which undergoes a Lifshitz transition in the relevant doping range, a complication which turns out to be superficial. Our results imply that prior to evoking polaronic, quantum critical point, quantum dissipation, or even more exotic scenarios for the evolution of transport properties in cuprates, Fermi-surface properties must be addressed in realistic detail.

Gate-Tunable Lifshitz Transition of Fermi Arcs and Its Transport Signatures

One hallmark of Weyl semimetals is the emergence of Fermi arcs (FAs) in surface Brillouin zones, where FAs connect the projected Weyl nodes of opposite chiralities. Unclosed FAs can give rise to various exotic effects that have attracted tremendous research interest. Configurations of FAs are usually thought to be determined fully by the band topology of the bulk states, which seems impossible to manipulate. Here, we show that FAs can be simply modified by a surface gate voltage. Because the penetration length of the surface states depends on the in-plane momentum, a surface gate voltage induces an effective energy dispersion. As a result, a continuous deformation of the surface band can be implemented by tuning the surface gate voltage. In particular, as the saddle point of the surface band meets the Fermi energy, the topological Lifshitz transition takes place for the FAs, during which the Weyl nodes switch their partners connected by the FAs. Accordingly, the magnetic Weyl orbits composed of the FAs on opposite surfaces and chiral Landau bands inside the bulk change their configurations. We show that such an effect can be probed by the transport measurements in a magnetic field, in which the switch-on and switch-off conductances by the surface gate voltage signal the Lifshitz transition. Our work opens a new route for manipulating the FAs by surface gates and exploring novel transport phenomena associated with the topological Lifshitz transition.