Condensed Matter

The Hall effect arises when time reversal symmetry is broken by either intrinsic magnetism or an external magnetic field. The latter contribution dominates in non-magnetic materials, in which the angular dependence of the Hall effect is typically a smooth cosine function because only the out-of-plane projection of the field generates the in-plane transverse motion of electrons. Here, we report the observation of an abrupt switching of the Hall effect by field rotation in a non-magnetic material, ZrTe5. The angular dependence of the Hall resistivity approaches a signum function, persisting down to an extremely low field of 0.03 T. By varying the carrier density of ZrTe5 over three orders of magnitude, we show that this singular behavior is due to the anomalous Hall effect generated by the ultra-dilute massive Dirac carriers in the quantum limit of Pauli paramagnetism when the Zeeman energy exceeds the Fermi energy. Our results elucidate the origin of the anomalous Hall effect in ZrTe5, arising owing to the spin-polarized massive Dirac electrons rather than the separation of Weyl nodes.

We demonstrate that a three dimensional time-periodically driven lattice system can exhibit a second-order chiral skin effect and describe its interplay with Weyl physics. This Floquet skin-effect manifests itself, when considering open rather than periodic boundary conditions for the system. Then an extensive number of bulk modes is transformed into chiral modes that are bound to the hinges (being second-order boundaries) of our system, while other bulk modes form Fermi arc surface states connecting a pair of Weyl points. At a fine tuned point, eventually all boundary states become hinge modes and the Weyl points disappear. The accumulation of an extensive number of modes at the hinges of the system resembles the non-Hermitian skin effect, with one noticeable difference being the localization of the Floquet hinge modes at increasing distances from the hinges in our system. We intuitively explain the emergence of hinge modes in terms of repeated backreflections between two hinge-sharing faces and relate their chiral transport properties to chiral Goos-Hänchen-like shifts associated with these reflections. Moreover, we formulate a topological theory of the second-order Floquet skin effect based on the quasi-energy winding around the Floquet-Brillouin zone for the family of hinge states. The implementation of a model featuring both the second-order Floquet skin effect and the Weyl physics is straightforward with ultracold atoms in optical superlattices.

Due to Fermi level pinning (FLP), metal-semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal - an essential requirement for several applications, including light-emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky-Mott limit of de-pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this work, we propose a simple technique to accurately estimate the SBH at the vdW contact interfaces by circumventing several ambiguities associated with SBH extraction. Using this technique on several vdW contacts, including metallic 2H-TaSe 2 , semi-metallic graphene, and degenerately doped semiconducting SnSe 2 , we demonstrate that vdW contacts exhibit a universal de-pinned nature. Superior ambipolar carrier injection properties of vdW contacts are demonstrated (with Au contact as a reference) in two applications, namely, (a) pulsed electroluminescence from monolayer WS 2 using few-layer graphene (FLG) contact, and (b) efficient carrier injection to WS 2 and WSe 2 channels in both nFET and pFET modes using 2H-TaSe 2 contact.

Electron transport in magnetic orders and the magnetic orders dynamics have a mutual dependence, which provides the key mechanisms in spin-dependent phenomena. Recently, antiferromagnetic orders are focused on as the magnetic order, where current-induced spin-transfer torques, a typical effect of electron transport on the magnetic order, have been debatable mainly because of the lack of an analytic derivation based on quantum field theory. Here, we construct the microscopic theory of spin-transfer torques on the slowly-varying staggered magnetization in antiferromagnets with weak canting. In our theory, the electron is captured by bonding/antibonding states, each of which is the eigenstate of the system, doubly degenerates, and spatially spreads to sublattices because of electron hopping. The spin of the eigenstates depends on the momentum in general, and a nontrivial spin-momentum locking arises for the case with no site inversion symmetry, without considering any spin-orbit couplings. The spin current of the eigenstates includes an anomalous component proportional to a kind of gauge field defined by derivatives in momentum space and induces the adiabatic spin-transfer torques on the magnetization. Unexpectedly, we find that one of the nonadiabatic torques has the same form as the adiabatic spin-transfer torque, while the obtained forms for the adiabatic and nonadiabatic spin-transfer torques agree with the phenomenological derivation based on the symmetry consideration. This finding suggests that the conventional explanation for the spin-transfer torques in antiferromagnets should be changed. Our microscopic theory provides a fundamental understanding of spin-related physics in antiferromagnets.

The interfacial band structures of multilayer systems play a crucial role for the ultrafast charge and spin carrier dynamics at interfaces. Here, we study the energy- and momentum-dependent quasiparticle lifetimes of excited states of a lead monolayer film on Ag(111) prior and after the adsorption of a monolayer of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using time-resolved two-photon momentum microscopy, we show that the electron dynamics of the bare Pb/Ag(111) bilayer system is dominated by isotropic intraband scattering processes within the quantum well state as well as interband scattering processes from the QWS into the Pb sideband. After the adsorption of PTCDA on the Pb monolayer, the interband scattering is suppressed and the electron dynamics is solely determined by intraband or inelastic scattering processes. Our findings hence uncover a new possibility to selectively tune and control scattering processes of quantum well systems by the adsorption of organic molecules.

Hinge states are the hallmark of the 3D higher-order topological insulator(HOTI). Here, we show that chiral hinge states can be identified by the magnetic field induced Aharonov-Bohm(AB) oscillation of the electron conductance in the interferometer constructed by HOTI and normal metal. Unlike AB interferometer of 3D topological insulator(TI), we find that there are different AB oscillation frequencies for a given direction of magnetic field in 3D HOTI. And the oscillation frequencies are also strongly depending on the direction of magnetic field. The main conclusion in our work is that there exists a universal linear relation between different oscillation frequencies. Here, by constructing an interference model of hinge states loops, we show both analytically and numerically that the linear relation is fulfilled in the HOTI effective model. The four basic frequencies in the work are labeled as ? x , ? y , ? x+y , ? x?�y and the main linear relations we demonstrate here are ? x±y = ? x ± ? y . These results provide an effective way for the identification of the chiral hinge states, and the oscillation signatures are stable with different sample size and bias.

We present a two-step method specifically tailored for band structure calculation of the small-angle moiré-pattern materials which contain tens of thousands of atoms in a unit cell. In the first step, the self-consistent field calculation for ground state is performed with O(N) Krylov subspace method implemented in OpenMX. Secondly, the crystal momentum dependent Bloch Hamiltonian and overlap matrix are constructed from the results obtained in the first step and only a small number of eigenvalues near the Fermi energy are solved with shift-invert and Lanczos techniques. By systematically tuning two key parameters, the cutoff radius for electron hopping interaction and the dimension of Krylov subspace, we obtained the band structures for both rigid and corrugated twisted bilayer graphene structures at the first magic angle ( θ= 1.08 ??) and other three larger ones with satisfied accuracy on affordable costs. The band structures are in good agreement with those from tight binding models, continuum models, plane-wave pseudo-potential based ab initio calculations, and the experimental observations. This efficient two-step method is to play a crucial role in other twisted two-dimensional materials, where the band structures are much more complex than graphene and the effective model is hard to be constructed.

A simple and unambiguous test has been recently suggested [J. Phys. D: Applied Physics, 52, 01LT01 (2018)] to check experimentally if a resistor with memory is indeed a memristor, namely a resistor whose resistance depends only on the charge that flows through it, or on the history of the voltage across it. However, although such a test would represent the litmus test for claims about memristors (in the ideal sense), it has yet to be applied widely to actual physical devices. In this paper, we experimentally apply it to a current-carrying wire interacting with a magnetic core, which was recently claimed to be a memristor (so-called ` Φ memristor') [J. Appl. Phys. 125, 054504 (2019)]. The results of our experiment demonstrate unambiguously that this ` Φ memristor' is not a memristor: it is simply an inductor with memory. This demonstration casts further doubts that ideal memristors do actually exist in nature or may be easily created in the lab.

We calculate the extremal cross sectional areas and cyclotron masses for the Fermi-surface pockets in Dirac and Weyl topological semimetals. The calculation is carried out for the most general form of the electron energy bands in the vicinity of the Weyl and Dirac points. Using the obtained formulas, one can find parameters characterizing the Dirac and Weyl electrons in the topological semimetals from appropriate experimental data. As an example, we consider the W1 electrons in TaAs.

Van der Waals heterostructures obtained by artificially stacking two-dimensional crystals represent the frontier of material engineering, demonstrating properties superior to those of the starting materials. Fine control of the interlayer twist angle has opened new possibilities for tailoring the optoelectronic properties of these heterostructures. Twisted bilayer graphene with a strong interlayer coupling is a prototype of twisted heterostructure inheriting the intriguing electronic properties of graphene. Understanding the effects of the twist angle on its out-of-equilibrium optical properties is crucial for devising optoelectronic applications. With this aim, we here combine excitation-resolved hot photoluminescence with femtosecond transient absorption microscopy. The hot charge carrier distribution induced by photo-excitation results in peaked absorption bleaching and photo-induced absorption bands, both with pronounced twist angle dependence. Theoretical simulations of the electronic band structure and of the joint density of states enable to assign these bands to the blocking of interband transitions at the van Hove singularities and to photo-activated intersubband transitions. The tens of picoseconds relaxation dynamics of the observed bands is attributed to the angle-dependence of electron and phonon heat capacities of twisted bilayer graphene.