Unconventional superconducting pairing in a B20 multifold Weyl fermion semimetal
Not provided.
Materials Science↗
Engineering topics
Lin, Hsin
Publications and source records attributed to Lin, Hsin.
An antiferromagnetic diode effect in even-layered MnBi 2 Te 4
In a p–n junction, the separation of positive and negative charges leads to diode transport, in which charge flows in only one direction. Non-centrosymmetric polar conductors are intrinsic diodes that could be of use in the development of nonlinear applications. Such systems have recently been extended to non-centrosymmetric superconductors, and the superconducting diode effect has been observed. Here, in this paper, we report an antiferromagnetic diode effect in a centrosymmetric crystal without directional charge separation. We observed large second-harmonic transport in a nonlinear electronic device enabled by the compensated antiferromagnetic state of even-layered MnBi 2 Te 4 . We show that this antiferromagnetic diode effect can be used to create in-plane field-effect transistors and microwave-energy-harvesting devices. We also show that electrical sum-frequency generation can be used as a tool to detect nonlinear responses in quantum materials.
electronic properties and materials↗
Realization of a two-dimensional Weyl semimetal and topological Fermi strings
A two-dimensional (2D) Weyl semimetal, akin to a spinful variant of graphene, represents a topological matter characterized by Weyl fermion-like quasiparticles in low dimensions. The spinful linear band structure in two dimensions gives rise to distinctive topological properties, accompanied by the emergence of Fermi string edge states. We report the experimental realization of a 2D Weyl semimetal, bismuthene monolayer grown on SnS(Se) substrates. Using spin and angle-resolved photoemission and scanning tunneling spectroscopies, we directly observe spin-polarized Weyl cones, Weyl nodes, and Fermi strings, providing consistent evidence of their inherent topological characteristics. Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials.
36 MATERIALS SCIENCE↗
Large anomalous Hall effect and negative magnetoresistance in half-topological semimetals
Proposed mechanisms for large intrinsic anomalous Hall effect (AHE) in magnetic topological semimetals include diverging Berry curvatures of Weyl nodes, anticrossing nodal rings or points of non-trivial bands. Here we demonstrate that a half-topological semimetal (HTS) state near a topological critical point can provide an alternative mechanism for a large AHE via systematic studies on an antiferromagnetic (AFM) half-Heusler compound TbPdBi. We not only observe a large AHE with tanΘ H ≈ 2 in its field-driven ferromagnetic (FM) phase, but also find a distinct Hall resistivity peak in its canted AFM phase. Moreover, we observe a large negative magnetoresistance with a value of ~98%. Our in-depth theoretical modelling indicates that these exotic transport properties originate from the HTS state which exhibits Berry curvature cancellation between the trivial spin-up and nontrivial spin-down bands. Our study offers alternative strategies for improved materials design for spintronics and other applications.
36 MATERIALS SCIENCE↗
Doubling the superconducting transition temperature of ultraclean wafer-scale aluminum nanofilms
Here, we studied the role of reduced dimensionality and disorder in the superconducting properties of wafer-scale aluminum (Al) nanofilms. This new generation of ultrathin films were grown using molecular beam epitaxy and depict normal-state sheet resistance at least 20 times lower than the quantum resistance ℎ/(4𝑒 2 ). Defying general expectations, the superconducting transition temperature of our films increases with decreasing Al film thickness, reaching 2.4 K for a 3.5-nm-thick Al film grown on GaAs: twice that of bulk Al (1.2 K). Surface phonon softening is shown to impact superconductivity in pure ultrathin films, offering a route for materials engineering in two dimensions.
2-dimensional systems↗
Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure
Quantum geometry in condensed matter physics has two components: the real part quantum metric and the imaginary part Berry curvature. Whereas the effects of Berry curvature have been observed through phenomena such as the quantum Hall effect in 2D electron gases and the anomalous Hall effect (AHE) in ferromagnets, quantum metric has rarely been explored. Here, in this paper, we report a nonlinear Hall effect induced by quantum metric dipole by interfacing even-layered MnBi 2 Te 4 with black phosphorus. The quantum metric nonlinear Hall effect switches direction upon reversing the AFM spins and exhibits distinct scaling that is independent of the scattering time. Our results open the door to discovering quantum metric responses predicted theoretically and pave the way for applications that bridge nonlinear electronics with AFM spintronics.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Observation of Fermi arcs and Weyl nodes in a noncentrosymmetric magnetic Weyl semimetal
Weyl semimetal (WSM), a novel state of quantum matter, hosts Weyl fermions as emergent quasiparticles resulting from the breaking of either inversion or time-reversal symmetry. Magnetic WSMs that arise from broken time-reversal symmetry provide an exceptional platform to understand the interplay between magnetic order and Weyl physics, but only a few WSMs have been realized. Here, in this study, we identify CeAlSi as a new noncentrosymmetric magnetic WSM via angle-resolved photoemission spectroscopy (ARPES) and first-principles, density-functional theory based calculations. Our surface-sensitive vacuum ultraviolet ARPES data confirm the presence of surface Fermi arcs as the conclusive evidence for the existence of the Weyl semimetallic state in CeAlSi. We also observe bulk Weyl cones at the Fermi arc terminations in CeAlSi using bulk-sensitive soft-x-ray ARPES measurements. These results implicate CeAlSi as a unique platform for investigating exotic quantum phenomena resulting from the interaction of topology and magnetism.
36 MATERIALS SCIENCE↗
Stripe helical magnetism and two regimes of anomalous Hall effect in NdAlGe
Here, we report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate spin density wave (T inc =6.8 K) whose spins are predominantly pointing along the out-of-plane direction and have a small helical spin canting of 3°. The spin density wave has a long wavelength of ≈35 nm and transitions to a commensurate ferrimagnetic state below T com =5.1K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are, respectively, governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our paper suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAlX, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.
36 MATERIALS SCIENCE↗
Axion optical induction of antiferromagnetic order
Using circularly polarized light to control quantum matter is a highly intriguing topic in physics, chemistry and biology. Previous studies have demonstrated helicity-dependent optical control of chirality and magnetization, with important implications in asymmetric synthesis in chemistry; homochirality in biomolecules; and ferromagnetic spintronics. We report the surprising observation of helicity-dependent optical control of fully compensated antiferromagnetic order in two-dimensional even-layered MnBi 2 Te 4 , a topological axion insulator with neither chirality nor magnetization. Furthermore, to understand this control, we study an antiferromagnetic circular dichroism, which appears only in reflection but is absent in transmission. We show that the optical control and circular dichroism both arise from the optical axion electrodynamics. Our axion induction provides the possibility to optically control a family of PT-symmetric antiferromagnets (P, inversion; T, time-reversal) such as Cr 2 O 3 , even-layered CrI3 and possibly the pseudo-gap state in cuprates. In MnBi 2 Te 4 , this further opens the door for optical writing of a dissipationless circuit formed by topological edge states.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Tuning the many-body interactions in a helical Luttinger liquid
In one-dimensional (1D) systems, electronic interactions lead to a breakdown of Fermi liquid theory and the formation of a Tomonaga-Luttinger Liquid (TLL). The strength of its many-body correlations can be quantified by a single dimensionless parameter, the Luttinger parameter K, characterising the competition between the electrons’ kinetic and electrostatic energies. Recently, signatures of a TLL have been reported for the topological edge states of quantum spin Hall (QSH) insulators, strictly 1D electronic structures with linear (Dirac) dispersion and spin-momentum locking. Here we show that the many-body interactions in such helical Luttinger Liquid can be effectively controlled by the edge state’s dielectric environment. This is reflected in a tunability of the Luttinger parameter K, distinct on different edges of the crystal, and extracted to high accuracy from the statistics of tunnelling spectra at tens of tunnelling points. The interplay of topology and many-body correlations in 1D helical systems has been suggested as a potential avenue towards realising non-Abelian parafermions.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Evidence for electronic signature of a magnetic transition in the topological magnet HoSbTe
Topological insulators with intrinsic magnetic order are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. To study these systems and their physics, people have proposed a variety of magnetic topological insulator systems, including HoSbTe, an antiferromagnetic weak topological insulator candidate. Here, in this work, we use scanning tunneling microscopy to probe the electronic structure of HoSbTe with antiferromagnetic and ferromagnetic orders that are tuned by applying an external magnetic field. Although around the Fermi energy we find minor differences between the quasiparticle interferences under the ferromagnetic and antiferromagnetic orders, deep inside the valance region, a quasiparticle interference signal emerges with ferromagnetism. This observation is consistent with our first-principles calculations indicating the magnetism-driven transition of the electronic states in this spin-orbit coupled topological magnet.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Evidence of a room-temperature quantum spin Hall edge state in a higher-order topological insulator
Room-temperature realization of macroscopic quantum phases is one of the major pursuits in fundamental physics. The quantum spin Hall phase is a topological quantum phase that features a two-dimensional insulating bulk and a helical edge state. Here we use vector magnetic field and variable temperature based scanning tunnelling microscopy to provide micro-spectroscopic evidence for a room-temperature quantum spin Hall edge state on the surface of the higher-order topological insulator Bi 4 Br 4 . Here, we find that the atomically resolved lattice exhibits a large insulating gap of over 200 meV, and an atomically sharp monolayer step edge hosts an in-gap gapless state, suggesting topological bulk–boundary correspondence. An external magnetic field can gap the edge state, consistent with the time-reversal symmetry protection inherent in the underlying band topology. We further identify the geometrical hybridization of such edge states, which not only supports the Z 2 topology of the quantum spin Hall state but also visualizes the building blocks of the higher-order topological insulator phase. Our results further encourage the exploration of high-temperature transport quantization of the putative topological phase reported here.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Observation of a smoothly tunable Dirac point in Ge ( Bi x Sb 1 - x ) 2 Te 4
State-of-the-art topological devices require the use of topologically protected surface states to drive electronic transport. Here, we examine a tunable topological system, Ge(Bi x Sb 1-x ) 2 Te 4 , for a range of $\mathcal{x}$ values from 0 to 1, using a combination of Fourier transform scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. Our results show that the Dirac point shifts linearly with $\mathcal{x}$, crossing the Fermi energy near $\mathcal{x}$=0.7. This observation of a smoothly tunable, isolated Dirac point crossing through the topological transport regime and having strong linear dependence with substitution can be critical for future topological spintronics applications.
36 MATERIALS SCIENCE↗
Weyl semimetal in the rare-earth hexaboride family supporting a pseudonodal surface and a giant anomalous Hall effect
Rare-earth hexaborides offer a rich tapestry for exploring the interplay of topological orders, magnetism, and electron-correlation effects associated with protected quantum phenomena. Using first-principles modeling, we identify in cerium hexaboride (CeB 6 ) a topological state that harbors a pseudonodal surface (PNS) along with double-Weyl fermions. An analysis of the electronic states in the vicinity of the Fermi level reveals the presence of M z -mirror-symmetry-protected band crossings that involve itinerant (d) and localized (f) states of Ce. These band crossings are found to remain nearly gapless as one goes away from the mirror plane, driving the formation of PNS states. We also demonstrate the presence of coexisting double-Weyl fermions with chiral charges of ±2 on the C 4z rotational axis. By analyzing the Berry curvature field associated with the topological states, we predict that CeB 6 supports a giant anomalous Hall conductivity of -1840Ω -1 cm -1 in the slightly electron-doped regime, which would be larger than the values reported to date in magnetic Weyl semimetals. Our study thus indicates that CeB 6 , which combines ferromagnetic order with topological physics in a heavy-fermion matrix, would provide a promising playground for investigating topological magnetic states with giant anomalous Hall responses.
36 MATERIALS SCIENCE↗
Multiband superconductivity in strongly hybridized 1 T ' - WTe 2 / NbSe 2 heterostructures
The interplay of topology and superconductivity has become a subject of intense research in condensed-matter physics for the pursuit of topologically nontrivial forms of superconducting pairing. An intrinsically normal-conducting material can inherit superconductivity via electrical contact to a parent superconductor via the proximity effect, usually understood as Andreev reflection at the interface between the distinct electronic structures of two separate conductors. However, at high interface transparency, strong coupling inevitably leads to changes in the band structure, locally, owing to hybridization of electronic states. Here, we investigate such strongly proximity-coupled heterostructures of monolayer $1T$' - $\mathrm{WTe_2}$ grown on NbSe 2 by van der Waals epitaxy. The superconducting local density of states, resolved in scanning tunneling spectroscopy down to 500 mK, reflects a hybrid electronic structure well described by a multiband framework based on the McMillan equations which captures the multiband superconductivity inherent to the NbSe 2 substrate and that is induced by proximity to $\mathrm{WTe_2}$ self-consistently. Our material-specific tight-binding model captures the hybridized heterostructure quantitatively and confirms that strong interlayer hopping gives rise to a semimetallic density of states in the two-dimensional $\mathrm{WTe_2}$ bulk, even for nominally band-insulating crystals. The model further accurately predicts the measured order parameter Δ ≅ 0.6 meV induced in the $\mathrm{WTe_2}$ monolayer bulk, stable beyond a 2 T magnetic field. So we believe that our detailed multiband analysis of the hybrid electronic structure provides a useful tool for sensitive spatial mapping of induced order parameters in proximitized atomically thin topological materials.
36 MATERIALS SCIENCE↗
Observation of a van Hove singularity of a surface Fermi arc with prominent coupling to phonons in a Weyl semimetal
A van der Waals coupled Weyl semimetal NbIrTe 4 is investigated by combining scanning tunneling microscopy/spectroscopy and first-principles calculations. Here we observe a sharp peak in the tunneling conductance near the Fermi energy (E F ). Comparison with calculations indicates that the peak originates from a van Hove singularity (vHs) associated with a Lifshitz transition of the surface Fermi-arc state. Interestingly, our tunneling spectroscopy also shows signatures of strong electron-boson coupling. This is potentially due to an anomalously enhanced charge susceptibility coming from the near-E F vHs formation.
36 MATERIALS SCIENCE↗
Layer Hall effect in a 2D topological axion antiferromagnet
While ferromagnets have been known and exploited for millennia, antiferromagnets (AFMs) were only discovered in the 1930s. The elusive nature indicates AFMs' unique properties: At large scale, due to the absence of global magnetization, AFMs may appear to behave like any non-magnetic material; However, such a seemingly mundane macroscopic magnetic property is highly nontrivial at microscopic level, where opposite spin alignment within the AFM unit cell forms a rich internal structure. In topological AFMs, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. Here, we study this exciting possibility in an AFM Axion insulator, even-layered MnBi 2 Te 4 flakes, where spatial degrees of freedom correspond to different layers. Remarkably, we report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under no net electric field, even-layered MnBi 2 Te 4 shows no anomalous Hall effect (AHE); However, applying an electric field isolates the response from one layer and leads to the surprising emergence of a large layer-polarized AHE (~50e 2 /h). Such a layer Hall effect uncovers a highly rare layer-locked Berry curvature, which serves as a unique character of the space-time PT-symmetric AFM topological insulator state. Moreover, we found that the layer-locked Berry curvature can be manipulated by the Axion field, E∙B, which drives the system between the opposite AFM states. Our results achieve previously unavailable pathways to detect and manipulate the rich internal spatial structure of fully-compensated topological AFMs. The layer-locked Berry curvature represents a first step towards spatial engineering of Berry phase, such as through layer-specific moiré potential.
36 MATERIALS SCIENCE↗
Cleaving plane-dependent electronic structures of transition metal diarsenides
Not Available
Dhakal, Gyanendra↗