Ordering of the two incommensurate charge density waves (CDW), ${\mathbf{q}}_1=(0.0, 0.243, 0.0)$ and ${\mathbf{q}}_2=(0.5, 0.263, 0.5)$, in the quasi-one-dimensional ${\mathrm{NbSe}}_3$ structure is studied by means of low-temperature scanning tunneling microscopy. Larger (100) van der Waals surfaces are analyzed using one-dimensional Fourier transforms along the three types of trigonal prismatic columns, running parallel to the monoclinic ${\mathbf{b}}_0$ direction. The procedure enables unambiguous differentiation between both CDWs modulating individual columns. In addition, it allows a quantitative comparison of modulation amplitudes along different columns of the same type and, in case of sufficiently large images, also along individual columns. It is confirmed that each CDW modulates both type-III and type-I columns with the narrowest isosceles bases. As a consequence beating is observed between the two slightly different modes. The bridging type-II columns appear not to be modulated. On a large scale the modulation amplitudes along individual columns vary, suggesting formation of CDW nanodomains. The possibility of interchanging both CDWs along columns forming symmetry-related pairs results in a charge difference, which is supposed to be the possible origin of CDW sliding.

Though Weyl fermions have recently been observed in several materials with broken inversion symmetry, there are very few examples of such systems with broken time reversal symmetry. Various ${\mathrm{Co}}_2$-based half-metallic ferromagnetic Heusler compounds are lately predicted to host Weyl-type excitations in their band structure. These magnetic Heusler compounds with broken time reversal symmetry are expected to show a large momentum space Berry curvature, which introduces several exotic magnetotransport properties. In this paper, we present a systematic analysis of experimental results on the anomalous Hall effect (AHE) in ${\mathrm{Co}}_2\mathrm{Ti}X$ ($X=\mathrm{Si}$ and Ge). This study is an attempt to understand the role of Berry curvature on AHE in the ${\mathrm{Co}}_2\mathrm{Ti}X$ family of materials. Anomalous Hall resistivity is observed to scale quadratically with longitudinal resistivity for both compounds. The detailed analysis indicates that in anomalous Hall conductivity, the intrinsic Karplus-Luttinger Berry phase mechanism dominates over the extrinsic skew scattering and side-jump mechanism.

The transition metal chalcogenide ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ undergoes a second-order phase transition at $T_c=328\text{K}$ involving a small lattice distortion. Below $T_c$, a band gap at the center of its Brillouin zone increases up to about 0.35 eV. In this work, we study the electronic structure of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ni $L_3$ edge. In addition to a weak fluorescence response, we observe a collection of intense Raman-like peaks that we attribute to electron-hole excitations. Using density functional theory calculations of its electronic band structure, we identify the main Raman-like peaks as interband transitions between valence and conduction bands. By performing angle-dependent RIXS measurements, we uncover the dispersion of these electron-hole excitations that allows us to extract the low-energy boundary of the electron-hole continuum. From the dispersion of the valence band measured by angle-resolved photoemission spectroscopy, we derive the effective mass of the lowest unoccupied conduction band.

Hyperdoping Si with chalcogens is a topic of great interest due to the strong sub-band-gap absorption exhibited by the resulting material, which can be exploited to develop broadband room-temperature infrared photodetectors using fully Si-compatible technology. Here, we report on the critical behavior of the impurity-driven insulator-to-metal transition in Te-hyperdoped Si layers fabricated via ion implantation followed by nanosecond pulsed-laser melting. Electrical transport measurements reveal an insulator-to-metal transition, which is also confirmed and understood by density functional theory calculations. We demonstrate that the metallic phase is governed by a power-law dependence of the conductivity at temperatures below 25 K, whereas the conductivity in the insulating phase is well described by a variable-range hopping mechanism with a Coulomb gap at temperatures in the range of 2–50 K. These results show that the electron wave function in the vicinity of the transition is strongly affected by the disorder and the electron-electron interaction.

Surface plasmon polaritons in a strained slab of a Weyl semimetal with broken time-reversal symmetry are investigated. It is found that the strain-induced axial gauge field reduces frequencies of these collective modes for intermediate values of the wave vector. Depending on the relative orientation of the separation of Weyl nodes in momentum space, the surface normal, and the direction of propagation, the dispersion relation of surface plasmon polaritons could be nonreciprocal even in a thin slab. In addition, strain-induced axial gauge fields can significantly affect the localization properties of the collective modes. These effects allow for an in situ control of the propagation of surface plasmon polaritons in Weyl semimetals and might be useful for creating nonreciprocal devices.

Classic Amontons’ law states that friction force increases monotonically with the increase of normal force. Here a nonmonotonic variation of interfacial friction with normal force is revealed in two-dimensional van der Waals bilayers by extensive first-principles calculations. The results show that the interlayer sliding energy barriers and shear strengths of $\mathrm{Mo}{\mathrm{S}}_2$, hexagonal boron nitride, and graphene bilayers turn to decrease with the relative movement of top and bottom monolayers from the lowest to the highest energy stacking state under normal pressures beyond 1.8, 14.7, and 7.6 GPa, respectively. Meanwhile, the interlayer binding energy differences between the highest and the lowest energy stacking state are nonmonotonic as the normal pressure increases. This anomalous frictional behavior in two-dimensional crystals is effectively interpreted by the opposite contributions of interlayer van der Waals and Coulomb interactions.

A model description of patterns of atomic displacements in twisted bilayer systems has been proposed. The model is based on the consideration of several dislocation ensembles, employing a language that is widely used for grain boundaries and film/substrate systems. We demonstrate explicitly how the dislocation language relates to vortex lattice motives found in experiment and simulations. We show that three ensembles of parallel screw dislocations are sufficient both to describe the rotation of the layers as a whole, and for the vortexlike displacements resulting from elastic relaxation. The results give a clear explanation of the observed features of the structural state such as vortices, accompanied by alternating stacking.

We propose a different tensor renormalization group algorithm, anisotropic tensor renormalization group (ATRG), for lattice models in arbitrary dimensions. The proposed method shares the same versatility with the higher-order tensor renormalization group (HOTRG) algorithm, i.e., it preserves the lattice topology after the renormalization. In comparison with HOTRG, both the computation cost and the memory footprint of our method are drastically reduced, especially in higher dimensions, by renormalizing tensors in an anisotropic way after the singular value decomposition. We demonstrate the ability of ATRG for the square lattice and the simple cubic lattice Ising models. Although the accuracy of the present method degrades when compared with HOTRG of the same bond dimension, the accuracy with fixed computation time is improved greatly due to the drastic reduction of the computation cost.

An enhanced magnetocaloric effect is being reported in a strain-engineered ferrite-manganite heterostructure driven by a low temperature magnetostructural phase transition. An ultrathin (∼20 nm) epitaxial $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4/{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ (CFO/LSMO) heterostructure was grown on single crystal MgO (100) substrate using pulsed laser deposition. Both temperature dependent x-ray diffraction and magnetization measurements revealed a broad second-order-type magnetostructural phase transition near around 80 K in the CFO/LSMO heterostructure. From detailed theoretical analysis of the experimental data it is confirmed that the phase transition around 80 K is second-order in nature, unlike the first-order transition observed in the bulk CFO materials. Thermodynamic analyses of magnetization reveal extremely broad isothermal entropy changes ΔS(T) about a wide range of temperatures $\left(40<T<160\mathrm{K}\right)$ resulting in enhanced relative cooling powers which are higher than those reported so far on most magnetocaloric materials. We propose that an interfacial strain-induced magnetostructural coupling of the CFO layer with the underlying LSMO layer gives rise to these hitherto unobserved enhanced magnetocaloric effects in the CFO/LSMO heterostructure system. The work provides fundamental insight into the low temperature phase transitions in ferrite-manganite thin films and adds on to the design of artificial heterostructures with novel and enhanced magnetocaloric properties.

Single crystals of ${\mathrm{Nd}}_2{\mathrm{O}}_3$ were grown and characterized using neutron scattering and thermodynamic measurements. ${\mathrm{Nd}}_2{\mathrm{O}}_3$ has long-range antiferromagnetic order below $T_{\mathrm{N}}=0.55$ K and specific-heat measurements have demonstrated that a significant amount of the magnetic entropy is released above $T_{\mathrm{N}}$. Inelastic neutron scattering experiments reveal a magnetic mode(s) with little dispersion peaked at $\approx 0.37\mathrm{meV}$ that is of greatest intensity below $T_{\mathrm{N}}$ but persists above $2T_{\mathrm{N}}$. This persistence of dynamic correlations is likely related to frustrated interactions associated with the nearly ideal stacked triangular lattice geometry of $J_{\text{eff}}=\frac{1}{2}$ spins on ${\mathrm{Nd}}^{3+}$ ions. The magnetization is observed to be strongly anisotropic at all temperatures due to crystal field effects, with easy-plane anisotropy observed. A noncompensated magnetic structure is inferred from the temperature dependence of the magnetization when a magnetic field of sufficient strength is applied within the basal plane near $T_{\mathrm{N}}$, and the evolution of the long-range order is summarized in a temperature-field phase diagram.

The noncollinear $L{1}_2\text{−}{\mathrm{IrMn}}_3$ room-temperature metallic antiferromagnet is one of the most important materials for scientific investigation and for applications in antiferromagnetic spintronics. It has been subject to intense investigation for over two decades, mainly in connection with exchange bias phenomena. Here we present a theoretical calculation of the magnon dispersion relations and the experimental observation of some magnon modes using inelastic Brillouin and Raman light-scattering techniques. The fit of theory to the experimentally measured magnon frequencies yields values for the nearest-neighbor AF exchange, the next-nearest-neighbor FM exchange interaction, and the uniaxial anisotropy parameters that are in good agreement with the ones calculated with atomistic spin models. The knowledge of the magnon dispersion relations revealed here is essential for the study of possible applications of ${\mathrm{IrMn}}_3$ in terahertz and other spintronic devices.

The XYZ antiferromagnetic spin-$1/2$ chain with alternation of the exchange and anisotropy couplings in the presence of uniform and staggered axial magnetic fields is studied. The analysis is done using the effective quadratic fermionic Hamiltonian resulting from the Hartee-Fock approximation. Combining the exact and the mean-field methods, the local and string order parameters on the ground-state phase diagram of the model are identified and calculated. We found a topological phase with oscillating string order with a period of four lattice spacings, not reported before for this model. A detailed analysis of patterns of the string order is given. The special XXZ limit of the model with additional U(1) symmetry brings about, in agreement with the Lieb-Schultz-Mattis theorem and its extensions, plateaux of magnetization and some additional conserving quantities. We have shown that in the XYZ chain, where the plateaux are smeared, the robust oscillating string order parameter is continuously connected to its XXZ limit. Also, the nontrivial winding number and zero-energy localized Majorana edge states, as additional attributes of topological order, are robust in that phase, even off the line of U(1) symmetry.

We present a gauge-invariant density matrix description of nonequilibrium superconductor (SC) states with spatial and temporal correlations driven by intense terahertz (THz) lightwaves. We derive superconductor Bloch-Maxwell equations of motion that extend Anderson pseudospin models to include the Cooper pair center-of-mass motion and electromagnetic propagation effects. We thus describe quantum control of dynamical phases, collective modes, quasiparticle coherence, and high nonlinearities during cycles of carrier wave oscillations, which relates to our recent experiments. Coherent photogeneration of a nonlinear supercurrent with a dc component, achieved via condensate acceleration by an effective lightwave field, dynamically breaks the equilibrium inversion symmetry. Experimental signatures include high harmonic light emission at equilibrium-symmetry-forbidden frequencies, Rabi-Higgs collective modes and quasiparticle coherence, and nonequilibrium moving condensate states tuned by few-cycle THz fields. We use such lightwaves as an oscillating accelerating force that drives strong nonlinearities and anisotropic quasiparticle populations to control and amplify different classes of collective modes, e.g., damped oscillations, persistent oscillations, and overdamped dynamics via Rabi flopping. Recent phase-coherent nonlinear spectroscopy experiments can be modeled by solving the full nonlinear quantum dynamics including self-consistent light-matter coupling.

Elastic neutron scattering on a single crystal combined with powder x-ray diffraction measurements were carried out to investigate how the crystal structure evolves as a function of temperature in the Weyl semimetal $\mathrm{W}{\mathrm{Te}}_2$. A sharp transition from the low-temperature orthorhombic phase ($T_d$) to the high-temperature monoclinic phase ($1T^{\prime }$) was observed at ambient pressure in the single crystal near $\sim 565$ K. Unlike in ${\mathrm{MoTe}}_2$, the solid-solid transition from $T_d$ to $1T^{\prime }$ occurs without the cell doubling of the intermediate $T_d^{*}$ phase with AABB (or ABBA) layer stacking. In powders, however, the thermal transition from the $T_d$ to the $1T^{\prime }$ phase was broadened and a two-phase coexistence was observed until 700 K, well above the structural transition.

Using a numerically exact technique we study spin transport and entanglement growth in the delocalized phase of a disordered spin chain with long-range interactions, decaying as a power law, $r^{-\alpha }$ with distance. For all considered $\alpha$'s and disorder strengths we find that the entanglement entropy grows sublinearly, and the underlying transport is subdiffusive. Since rare-blocking regions, which are central to the Griffiths theory of transport in disordered interacting systems, can be easily circumvented by long-range hops across the lattice, they cannot explain the mechanism of slow transport in long-range systems. Specifically, we show that for long-range systems the Griffiths theory predicts diffusive transport, which is inconsistent with our results.

We theoretically, numerically, and experimentally investigate the feasibility of acoustic doping, i.e., changing one of the effective properties of a medium by adding an impurity, to achieve supersqueezing. This effect, characterized by perfect and zero-phase transmission, can be obtained with zero index media. In acoustics, zero-phase propagation can be achieved with a plate-type acoustic metamaterial (PAM) acting as a density-near-zero metamaterial (DNZ). We point out the possibility of modifying the compressibility of a DNZ medium by mounting a Helmholtz resonator in parallel with the PAM. We are then able to dope the system and to turn it into a density-and-compressibility-near-zero medium, thus allowing supersqueezing.

Using the noninteracting Anderson tight-binding model on the Bethe lattice as a toy model for the many-body quantum dynamics, we propose a novel and transparent theoretical explanation of the anomalously slow dynamics that emerges in the bad metal phase preceding the many-body localization (MBL) transition. By mapping the time decorrelation of many-body wave functions onto directed polymers in random media, we show the existence of a glass transition within the extended regime of loopless Cayley trees separating a metalliclike phase at small disorder, where delocalization occurs on an exponential number of paths, from a bad metal-like phase at intermediate disorder, where resonances are formed on rare, specific, disorder dependent site orbitals on very distant generations. The physical interpretation of subdiffusion and nonexponential relaxation emerging from this picture is complementary to the Griffiths one, although both scenarios rely on the presence of heavy-tailed distribution of the escape times. We relate the dynamical evolution in the glassy phase to the depinning transition of directed polymers, which results in macroscopic and abrupt jumps of the preferred delocalizing paths when a parameter like the energy is varied, and produce a singular behavior of the overlap correlation function between eigenstates at different energies. By comparing the quantum dynamics on loopless Cayley trees and random regular graphs, we discuss the effect of loops, showing that in the latter slow dynamics and apparent power laws extend on a very large time window but are eventually cutoff on a timescale that diverges at the MBL transition.

A one-dimensional metamaterial with parity-time ($\mathcal{PT}$) symmetry that relies on balanced gain and loss is introduced, comprising magnetically coupled split-ring resonators (SRRs). A particular topology that combines a nontrivial (topological) dimer configuration with a trivial (nontopological) dimer configuration separated by a central SRR with neither gain nor loss is investigated. By focusing on the dynamical aspects of such a topological $\mathcal{PT}$ metamaterial (PTMM), the existence of topologically protected interface states which are localized at the central SRR is demonstrated numerically. The solution of the corresponding quadratic eigenvalue problem reveals that the protected state is actually a robust eigenmode of the topological PTMM, whose eigenvalue is isolated in the middle of the gap (midgap state) of the two-band frequency spectrum. Direct numerical simulations have been further used to determine the robustness and dynamic stability of these states in the parameter space of the dimerization strength and the gain-loss coefficient.