Semimetal Phase Research Articles

Observation of an acoustic topological Euler insulator with meronic waves

Topological band theory has conventionally been concerned with the topology of bands around a single gap. Only recently non-Abelian topologies that thrive on involving multiple gaps were studied, unveiling a new horizon in topological physics beyond the conventional paradigm. Here, we report on the first experimental realization of a topological Euler insulator phase with unique meronic characterization in an acoustic metamaterial. We demonstrate that this topological phase has several nontrivial features: First, the system cannot be described by conventional topological band theory, but has a nontrivial Euler class that captures the unconventional geometry of the Bloch bands in the Brillouin zone. Second, we uncover in theory and probe in experiments a meronic configuration of the bulk Bloch states for the first time. Third, using a detailed symmetry analysis, we show that the topological Euler insulator evolves from a non-Abelian topological semimetal phase via. the annihilation of Dirac points in pairs in one of the band gaps. With these nontrivial properties, we establish concretely an unconventional bulk-edge correspondence which is confirmed by directly measuring the edge states via. pump-probe techniques. Our work thus unveils a nontrivial topological Euler insulator phase with a unique meronic pattern and paves the way as a platform for non-Abelian topological phenomena.

Non-Hermitian Strongly Interacting Dirac Fermions.

Exotic quantum phases and phase transition in the strongly interacting Dirac systems have attracted tremendous interests. On the other hand, non-Hermitian physics, usually associated with dissipation arising from the coupling to environment, emerges as a frontier of modern physics in recent years. In this Letter, we investigate the interplay between non-Hermitian physics and strong correlation in Dirac-fermion systems. We generalize the projector quantum Monte-Carlo (PQMC) algorithm to the non-Hermitian interacting fermionic systems. Employing PQMC simulation, we decipher the ground-state phase diagram of the honeycomb Hubbard model with spin resolved non-Hermitian asymmetric hopping processes. The antiferromagnetic (AFM) ordering induced by Hubbard interaction is enhanced by the non-Hermitian asymmetric hopping. Combining PQMC simulation and renormalization group analysis, we reveal that the quantum phase transition between Dirac semi-metal and AFM phases belongs to Hermitian chiral XY universality class, implying that a Hermitian Gross-Neveu transition is emergent at the quantum critical point although the model is non-Hermitian.

Investigating the impact of growth temperature on the direct integration of pure phase 2H-MoTe2 with Si(1 1 1) using molecular beam epitaxy

Group-VI transition metal dichalcogenides (TMDCs) have garnered significant attention due to their novel properties in the two-dimensional realm. Among these TMDCs, MoTe2 is a particularly intriguing material with a small difference between Gibbs free energies of its semiconducting and semimetallic phases, rendering it suitable for a wide array of applications. In this study, we report the molecular beam epitaxy (MBE) growth of 2H-MoTe2 directly on a silicon (111) substrate, which holds great significance due to its compatibility with the existing integrated circuit industry. By employing a growth strategy involving a Te-rich silicon surface, we achieved the growth of high-quality, phase-pure 2H-MoTe2 films. The effect of growth temperature on the film quality was investigated, revealing that the growth at 450 °C led to highly crystalline and layered growth. The thickness, roughness and stoichiometry of the grown films were characterized using spectroscopic ellipsometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy confirmed the presence of the desired 2H-phase for the grown films. The direct integration of MoTe2 with silicon opens up exciting possibilities for its utilization in nanoelectronics and optoelectronics devices.

Universal Features of Entanglement Entropy in the Honeycomb Hubbard Model.

The entanglement entropy is a unique probe to reveal universal features of strongly interacting many-body systems. In two or more dimensions these features are subtle, and detecting them numerically requires extreme precision, a notoriously difficult task. This is especially challenging in models of interacting fermions, where many such universal features have yet to be observed. In this Letter we tackle this challenge by introducing a new method to compute the Rényi entanglement entropy in auxiliary-field quantum MonteCarlo simulations, where we treat the entangling region itself as a stochastic variable. We demonstrate the efficiency of this method by extracting, for the first time, universal subleading logarithmic terms in a two-dimensional model of interacting fermions, focusing on the half-filled honeycomb Hubbard model at T=0. We detect the universal corner contribution due to gapless fermions throughout the Dirac semi-metal phase and at the Gross-Neveu-Yukawa critical point, where the latter shows a pronounced enhancement depending on the type of entangling cut. Finally, we observe the universal Goldstone mode contribution in the antiferromagnetic Mott insulating phase.

Magnetotransport of Sm2Ir2O7 across the pressure-induced quantum-critical phase boundary

Rare-earth pyrochlore iridates host two interlocking magnetic sublattices of corner-sharing tetrahedra and can harbour a unique combination of frustrated moments, exotic excitations and highly correlated electrons. They are also the first systems predicted to display both topological Weyl semimetal and axion insulator phases. We have measured the transport and magnetotransport properties of single-crystal Sm2Ir2O7 up to and beyond the pressure-induced quantum critical point for all-in-all-out (AIAO) Ir order at pc = 63 kbar previously identified by resonant X-ray scattering and close to which Weyl semimetallic behavior has been previously predicted. Our findings overturn the accepted expectation that the suppression of AIAO order should lead to metallic conduction persisting down to zero temperature. Instead, the resistivity-minimum temperature, which tracks the decrease in the AIAO ordering temperature for pressures up to 30 kbar, begins to increase under further application of pressure, pointing to the presence of a second as-yet unidentified mechanism leading to non-metallic behavior. The magnetotransport does track the suppression of Ir magnetism, however, with a strong hysteresis observed only within the AIAO phase boundary, similar to that found for Ho2Ir2O7 and attributed to plastic deformation of Ir domains. Around pc we find the emergence of a new type of electronic phase, characterized by a negative magnetoresistance with small hysteresis at the lowest temperatures, and hysteresis-free positive magnetoresistance above approximately 5 K. The temperature dependence of our low-temperature transport data are found to be best described by a model consistent with a Weyl semimetal across the entire pressure range.

Topological semimetal phase in non-Hermitian Su–Schrieffer–Heeger model

We explore the non-Hermitian Su–Schrieffer–Heeger model with long-range hopping and off-diagonal disorders. In the non-Hermitian clean limit, we find that the phase diagram holds topological semimetal phase with exceptional points except the normal insulator phase and the topological insulator phase. Interestingly, it is found that the topological semimetal phase is induced by long-range nonreciprocal term when the long-range hopping is not equal to the intercell hopping. Especially, we show the existence of topological semimetal phase with exceptional points and determine the transition point analytically and numerically under the Hermitian clean limit when the long-range hopping is equal to the intercell hopping. Furthermore, we also investigate the effects of the disorders on topological semimetal phase, and show that the disorders can enhance the region of topological semimetal phase in contrast to the case of non-Hermitian clean limit, indicating that it is beneficial to topological semimetal phase whether there is one disorder or two disorders in the system, that is, the topological semimetal phase is stable against the disorders in this one-dimensional non-Hermitian system. Our work provides an alternative avenue for studying topological semimetal phase in non-Hermitian lattice systems.

Scanning tunneling microscopy study of epitaxial Fe3GeTe2 monolayers on Bi2Te3

Introducing magnetism to the surface state of topological insulators, such as Bi2Te3, can lead to a variety of interesting phenomena. We use scanning tunneling microscopy (STM) to study a single quintuple layer (QL) of the van der Waals magnet Fe3GeTe2 (FGT) that is grown on Bi2Te3 via molecular beam epitaxy. STM topographic images show that the FGT grows as free-standing islands on Bi2Te3 and outwards from Bi2Te3 steps. Atomic resolution imaging shows triangular lattices of 390 ± 10 pm for FGT and 430 ± 10 pm for Bi2Te3, consistent with the respective bulk crystals. A moiré pattern is observed on FGT regions with a periodicity of 4.3 ± 0.4 nm that can be attributed solely to this lattice mismatch and thus indicates zero rotational misalignment. While most of the surface is covered by a single QL of the FGT, there are small double QL regions, as well as regions with distinct chemical terminations due to an incomplete QL. The most common partial QL surface termination is the FeGe layer, in which the top two atomic layers are missing. This termination has a distinctive electronic structure and a 3 x 3R30∘ reconstruction overlaid on the moiré pattern in STM images. Magnetic circular dichroism measurements confirm these thin FGT films are ferromagnetic with T C ∼190 K.

Exploring the effect of chromium doping on electronic properties, half-metallic and ferromagnetism on AlSb DMS compound through first-principles calculations

With the aim of improving and investigating magnetocaloric insights for spintronic applications, we investigated the electronic structure of Cr-doped AlSb compounds via first-principles calculations using the KKR-CPA-DFT method. Here, we investigated the stability of alloy materials as a function of magnetic contamination. A stable semimetallic ferromagnetic phase can be obtained by introducing the Cr element into AlSb, which is inherent in nonmagnetic semiconductors. The total magnetic moment of the doped material changes from 0.1167 μB to 0.716 μB for 4% and 25% Cr, respectively. Curie temperature changed to 740 K at 25% Cr concentration.
 KEY WORDS: Magnetic materials, Semiconductor, Alloys, Impurities
 Bull. Chem. Soc. Ethiop. 2024, 38(2), 297-304. 
 DOI: https://dx.doi.org/10.4314/bcse.v38i2.2

Multiplicative topological semimetals

Exhaustive study of topological semimetal (TSM) phases of matter in equilibriated electonic systems and myriad extensions has built upon the foundations laid by earlier introduction and study of Weyl semimetals, with broad applications in topologically protected quantum computing, spintronics, and optical devices. We extend recent introduction of multiplicative topological phases to find previously overlooked TSM phases of electronic systems in equilibrium, with minimal symmetry protection. We show these multiplicative TSM (MTSM) phases exhibit rich and distinctive bulk-boundary correspondence and response signatures that greatly expand understanding of consequences of topology in condensed matter settings, such as the limits on Fermi arc connectivity and structure, and transport signatures such as the chiral anomaly. In this paper, we therefore lay the foundation for extensive future study of MTSMs. Published by the American Physical Society 2024

Strain-tunable Dirac semimetal phase transition and emergent superconductivity in a borophane

A two-dimensional (2D) Dirac semimetal with concomitant superconductivity has been long sought but rarely reported. It is believed that light-element materials have the potential to realize this goal owing to their intrinsic lightweight and metallicity. Here, based on the recently synthesized β12 hydrogenated borophene, we investigate its counterpart named β12-B5H3. Our first-principles calculations suggest it has good stability. β12-B5H3 is a scarce Dirac semimetal demonstrating a strain-tunable phase transition from three Dirac cones to a single Dirac cone. Additionally, β12-B5H3 is also a superior phonon-mediated superconductor with a superconducting critical temperature of 32.4 K and can be further boosted to 42 K under external strain. The concurrence of Dirac fermions and superconductivity, supplemented with dual tunabilities, reveals β12-B5H3 is an attractive platform to study either quantum phase transition in 2D Dirac semimetal or the superconductivity or the exotic physics brought about by their interplay.

Intrinsic insulating transport characteristics in low-carrier density EuCd2As2 films

Searching for an ideal magnetic Weyl semimetal hosting only a single pair of Weyl points has been a focal point for systematic clarification of its unique magnetotransport derived from the interplay between topology and magnetization. Among the candidates, triangular-lattice antiferromagnet EuCd2As2 has been attracting special attention due to the prediction of the ideal Weyl semimetal phase in the ferromagnetic state; however, transport properties of low-carrier density samples have remained elusive. Here, we report molecular beam epitaxy growth of EuCd2As2 films, achieving low-hole density in the range of 1015–1016 cm−3 at low temperature. Transport measurements of such low-carrier density films reveal an insulating behavior with an activation gap of about 200 meV, which persists even in the field-induced ferromagnetic state. Our work provides an important experimental clue that EuCd2As2 is intrinsically insulating, contrary to the previous prediction.

Layer-dependent ultrafast carrier dynamics of PdSe2 investigated by photoemission electron microscopy.

For atomically thin two-dimensional materials, variations in layer thickness can result in significant changes in the electronic energy band structure and physicochemical properties, thereby influencing the carrier dynamics and device performance. In this work, we employ time- and energy-resolved photoemission electron microscopy to reveal the ultrafast carrier dynamics of PdSe2 with different layer thicknesses. We find that for few-layer PdSe2 with a semiconductor phase, an ultrafast hot carrier cooling on a timescale of approximately 0.3 ps and an ultrafast defect trapping on a timescale of approximately 1.3 ps are unveiled, followed by a slower decay of approximately tens of picoseconds. However, for bulk PdSe2 with a semimetal phase, only an ultrafast hot carrier cooling and a slower decay of approximately tens of picoseconds are observed, while the contribution of defect trapping is suppressed with the increase of layer number. Theoretical calculations of the electronic energy band structure further confirm the transition from a semiconductor to a semimetal. Our work demonstrates that TR- and ER-PEEM with ultrahigh spatiotemporal resolution and wide-field imaging capability has great advantages in revealing the intricate details of ultrafast carrier dynamics of nanomaterials.

Topological magnon bands, quantum correlation and spin conductivity in layered ferrimagnets

In this paper, we analyze the influence of topological magnon phases on quantum correlation and spin transport on layered ferrimagnets which is a fascinating topic in condensed matter physics. In the study of topological phases manipulated by an external magnetic field in layered ferrimagnets for the field applied along the easy axis, the system can transfer between a nodal-loop semimetal phase and a Weyl semimetal phase in perpendicularly and laterally magnetized geometries, respectively. The anisotropic nature of these materials means that the interactions between spins depend on their relative orientations in different directions. We analyze the effect of topological magnon bands induced by the coupling parameters on quantum correlation. The influence of these coupling parameters is also investigated on spin conductivity longitudinal and transversal as well.

Control of valley optical conductivity and topological phases in buckled hexagonal lattice by orientation of in-plane magnetic field

We investigate the optical conductivity, along with longitudinal and transverse conductivities, in buckled hexagonal lattice such as silicene subjected to both an in-plane magnetic field and a perpendicular electric field. In this model, we neglect the effect of the spin-orbit interaction, which is of a smaller order compared to the strong staggered potential and the next-nearest hoping energy. The orientations of the in-plane magnetic field and the perpendicular electric field give rise to a non-uniform, tunable gap. The Chern number for each valley degree of freedom deviates from being constant but remains steady when summed over the entire Brillouin zone. The longitudinal and transverse currents, in the case of a specific valley, can be selected by adjusting the direction of the electric field in the semimetal phase. Furthermore, the defining characteristics of topological phases induces the rapid change in longitudinal conductivity when varying the angle of orientation of the in-plane magnetic field under monochromatic light, and perfect valley filtering in transverse conductivity. The transverse current associated with a specific valley can be selected when the angle of orientation satisfies the specific conditions. This investigation paves the way for materials design with valley-locked current, using a specific orientation of the in-plane magnetic field.

High-pressure induced Weyl semimetal phase in 2D Tellurium

Relativistic Weyl fermion quasiparticles in Weyl semimetal bring the electron’s chirality degree of freedom into the electrical transport and give rise to exotic phenomena. A topological phase transition from a topological trivial phase to a topological non-trivial phase offers a route to control electronic devices through its topological properties. Here, we report the Weyl semimetal phase in hydrothermally grown two-dimensional Tellurium (2D Te) induced by high hydrostatic pressure (up to 2.47 GPa). The unique chiral crystal structure gives rise to chiral fermions with different topological chiral charges ({{C}}=-{{1}},+{{1}},{{and}}-{{2}}). The highly tunable chemical potential in 2D Te provides comprehensive information for understanding the pressure-dependent electron band structure. The pressure-induced insulator-to-metal transition, two-carrier transport, and the non-trivial π Berry phase shift in quantum oscillations are observed in the 2D Te Weyl semimetal phase. Our work demonstrates the pressure-induced bandgap closing in the inversion asymmetric narrow bandgap semiconductor 2D Te.

Spontaneous Hall effect in the magnetic Weyl semimetallic Eu2Ir2O7 (111) thin films

Here, we have carried out a low-temperature magnetization and magnetotransport study on epitaxial antiferromagnetic Eu2Ir2O7 (111) thin films with an all-in-all-out/all-out-all-in spin structure to delve into the possibility of realizing spontaneous Hall effect that can provide strong experimental evidence for the existence of the Weyl semimetallic phase. The temperature-dependent resistivity on Eu2Ir2O7 (111) thin films indicates a semimetallic transition below 90 K and the existence of a correlated metallic state in the high-temperature regime (90–300 K). Magnetoresistance measurements suggest that at a low temperature (below 10 K), the all-in-all-out (AIAO) spin structure is stable enough against the applied magnetic field, whereas above 10 K, application of magnetic field causes field-induced modification of the spin structure and plastic domain deformation. Hall resistivity shows a spontaneous Hall effect (SHE) in the low-temperature semimetallic phase of Eu2Ir2O7 (111) thin films with minimal magnetization (AIAO/all-out-all-in spin structure). The observed SHE provides strong evidence for the emergent Weyl semimetallic phase in the (111) thin film geometry of antiferromagnetic Eu2Ir2O7.

A Possible Quantum Spin Liquid Phase in the Kitaev–Hubbard Model

The quantum spin liquid (QSL) state of Kitaev-like materials, such as iridium oxides A 2IrO3 and α-RuCl3, has been explored in depth. The half-filled Kitaev–Hubbard model with bond-dependent hopping terms is used to describe the Kitaev-like materials, which is calculated using the state-of-the-art fermionic projected entangled pair states method. We find a QSL phase near the Mott insulator transition, which has a strong first-order transition to the semi-metal phase with the decrease of Hubbard U. We suggest that a promising approach to finding QSL states is by finding iridium oxides that are near the Mott insulator transition.

Phase diagram of three dimensional disordered nodal-line semimetals: weak localization to Anderson localization

Nodal-line semimetals are new members of the topological materials family whose experimental characterization has seen recent progress using both ARPES and quantum oscillation measurements. Here, we theoretically study the presence of a disorder-induced phase transition in a cubic lattice nodal-line semimetal using numerical diagonalization and spectral calculations. In contrast to the 3D nodal-point semimetals, we found that nodal-line semimetals do not display a stable disordered semimetal phase, as an infinitely weak disorder can lead to a diffusive metal phase. The absence of a semimetal phase is also reflected in the quadratic relationship of the electronic specific heat at low temperatures. Furthermore, we illustrate that a localization transition occurs under the influence of strong disorder, shifting the material from a weakly localized diffusive metal state to an Anderson insulator. This transition is substantiated by calculating the adjacent gap ratio and the typical density of states.

Real higher-order Weyl photonic crystal

Higher-order Weyl semimetals are a family of recently predicted topological phases simultaneously showcasing unconventional properties derived from Weyl points, such as chiral anomaly, and multidimensional topological phenomena originating from higher-order topology. The higher-order Weyl semimetal phases, with their higher-order topology arising from quantized dipole or quadrupole bulk polarizations, have been demonstrated in phononics and circuits. Here, we experimentally discover a class of higher-order Weyl semimetal phase in a three-dimensional photonic crystal (PhC), exhibiting the concurrence of the surface and hinge Fermi arcs from the nonzero Chern number and the nontrivial generalized real Chern number, respectively, coined a real higher-order Weyl PhC. Notably, the projected two-dimensional subsystem with kz = 0 is a real Chern insulator, belonging to the Stiefel-Whitney class with real Bloch wavefunctions, which is distinguished fundamentally from the Chern class with complex Bloch wavefunctions. Our work offers an ideal photonic platform for exploring potential applications and material properties associated with the higher-order Weyl points and the Stiefel-Whitney class of topological phases.

Berry curvature induced spontaneous and topological-like Hall effect in magnetic Weyl semimetallic Nd2Ir2O7 (111) thin films

We report a study of longitudinal resistivity, anisotropic magnetoresistance (MR), and Hall effect on epitaxial Nd2Ir2O7 (111) thin films grown by the solid phase epitaxy technique, in which spin–orbit coupling, electronic correlation, magnetic frustration, and f-d exchange interaction are present. Temperature-dependent longitudinal resistivity (ρxx) data indicate semimetallic charge transport in the low-temperature region. Field-cooled longitudinal resistivity measurements detect negligible domain-wall conductance compared to the polycrystal or single-crystal samples. Angle-dependent MR measurement shows that the magnetic structure of Ir4+ 5d moments can be finely tuned by applying a magnetic field along different crystallographic directions. MR measurements show a field-induced modification of the Nd3+ 4f spin structure from all-in-all-out/all-out-all-in (AIAO/AOAI) (4-in-0-out) to 1-in-3-out for the applied field (H) along the [111] direction, resulting in field-induced plastic deformation of the Ir4+ domain distribution. In contrast, the application of field (H) along the [001] and [011] directions could not modify any domain distribution. A large spontaneous Hall effect (SHE) signal has been observed on Nd2Ir2O7 (111) thin film with AIAO/AOAI antiferromagnetic ordering for the application of field (H) along [001], [1̄1̄0], and [111] directions. The appearance of a large spontaneous Hall signal for the applied field along [001] and [1̄1̄0] directions rules out domain switching as the origin of the Hall effect and confirms the presence of the Weyl semimetallic phase in Nd2Ir2O7 (111) thin films. In addition to SHE, a large topological-like Hall signal is also observed, possibly due to the presence of multiple Weyl nodes in the electronic band structure.