We propose an efficient numerical method, which combines the advantages of recently developed tensor-network based methods and standard trial wave functions, to study the ground-state properties of quantum many-body systems. In this approach, we apply a projector in the form of a tensor-product operator to an input wave function, such as a Jastrow-type or Hartree-Fock wave function, and optimize the tensor elements via variational Monte Carlo. The entanglement already contained in the input wave function can considerably reduce the bond dimensions compared to the regular tensor-product state representation. In particular, this allows us to also represent states that do not obey the area law of entanglement entropy. In addition, for fermionic systems, the fermion sign structure can be encoded in the input wave function. We show that the optimized states provide good approximations of the ground-state energy and correlation functions in the cases of two-dimensional bosonic and fermonic systems.

We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{WS}{\mathrm{e}}_2$ and $\mathrm{MoS}{\mathrm{e}}_2/\mathrm{Mo}{\mathrm{S}}_2$ heterobilayers and twisted $\mathrm{Mo}{\mathrm{S}}_2$ bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at $30–40{\mathrm{cm}}^{-1}$) that arise from the layer-breathing mode (LBM) vibration between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and an atomically clean interface. Its frequency also evolves systematically with the relative orientation between the two layers. Our research demonstrates that the LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials.

We have developed a new method that is able to predict the electrical properties of the source and drain contacts in realistic carbon nanotube field effect transistors (CNTFETs). It is based on large-scale ab initio calculations combined with a Green function approach. For the first time, both internal and external parts of a realistic CNT-metal contact are taken into account at the ab initio level. We have developed the procedure allowing direct calculation of the self-energy for an extended contact. Within the method, it is possible to calculate the transmission coefficient through a contact of both finite and infinite length; the local density of states can be determined in both free and embedded CNT segments. We found perfect agreement with the experimental data for Pd and Al contacts. We have explained why CNTFETs with Pd electrodes are $p$-type FETs with ohmic contacts, which can carry current close to the ballistic limit (provided contact length is large enough), whereas in CNT-Al contacts transmission is suppressed to a significant extent, especially for holes.

We elaborate upon the “processed Husimi map” representation for visualizing quantum wave functions using coherent states as a measurement of the local phase space to produce a vector field related to the probability flux. Adapted from the Husimi projection, the processed Husimi map is mathematically related to the flux operator under certain limits but offers a robust and flexible alternative since it can operate away from these limits and in systems that exhibit zero flux. The processed Husimi map is further capable of revealing the full classical dynamics underlying a quantum wave function since it reverse engineers the wave function to yield the underlying classical ray structure. We demonstrate the capabilities of processed Husimi maps on bound systems with and without electromagnetic fields, as well as on open systems on and off resonance, to examine the relationship between closed system eigenstates and mesoscopic transport.

We observe numerically the properties of the infinite-temperature inherent structures of $m$-component vector spin glasses in three dimensions. An increase of $m$ implies a decrease of the amount of minima of the free energy, down to the trivial presence of a unique minimum. For little $m$ correlations are small and the dynamics are quickly arrested, while for larger $m$ low-temperature correlations crop up and the convergence is slower, to a limit that appears to be related with the system size.

The dependence of the spin-pumping effect on the yttrium iron garnet $({\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, YIG) thickness detected by the inverse spin Hall effect (ISHE) has been investigated quantitatively. Due to the spin-pumping effect driven by the magnetization precession in the ferrimagnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ film a spin-polarized electron current is injected into the Pt layer. This spin current is transformed into electrical charge current by means of the ISHE. An increase of the ISHE voltage with increasing film thickness is observed and compared to the theoretically expected behavior. The effective damping parameter of the YIG/Pt samples is found to be enhanced with decreasing ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ film thickness. The investigated samples exhibit a spin mixing conductance of $g_{\mathrm{eff}}^{\uparrow \downarrow }=(3.87\pm 0.21)\times {10}^{18}{\mathrm{m}}^{-2}$ and a spin Hall angle between ${\theta }_{\mathrm{ISHE}}=0.013\pm 0.001$ and $0.045\pm 0.004$ depending on the used spin-diffusion length. Furthermore, the influence of nonlinear effects on the generated voltage and on the Gilbert damping parameter at high excitation powers is revealed. It is shown that for small YIG film thicknesses a broadening of the linewidth due to nonlinear effects at high excitation powers is suppressed because of a lack of nonlinear multimagnon scattering channels. We have found that the variation of the spin-pumping efficiency for thick YIG samples exhibiting pronounced nonlinear effects is much smaller than the nonlinear enhancement of the damping.

We explore possible signatures for observing Majorana fermions in the tunneling spectroscopy of high-$T_c$ superconductors. It is shown that due to the Rashba spin-orbit interaction $\left({\alpha }_R\right)$ generally introduced by contact electrode, in addition to the Heisenberg spin exchange interaction, the Dzyaloshinskii-Moriya and spin dipole-dipole interactions are induced. As a result, $p$-wave superconductivity is induced with the gap-function $d$ vector being not aligned with the internal magnetic field of the spin-orbit interaction. For typical strength of the Rashba interaction, the induced $p$ wave is weak. Hence, the resulting superconductor is still gapless and is not a topological superconductor. However, we find that the ground state undergoes a phase transition to a topological gapless phase with each nodal point originated from pure $d$ wave being split into two stable nodal points characterized by the symmetry class DIII. Due to the splitting nodal structure, zero-energy Majorana modes always exist for any interfaces that are not exactly in $\left(100\right)$ or $\left(010\right)$ directions. Hence, for general interfaces, existence of Majorana modes is a robust feature. In addition, due to the nonaligned $d$ vector, for $\left(110\right)$ interfaces in which $d$ wave is subdominant to $p$ wave, there exist sizable dispersive Majorana edge states. Our results indicate that due to the presence of these Majorana modes, a small plateau in tunneling spectrum near zero-bias peak would be induced. Furthermore, zero-energy Majorana modes result in $4\pi$ periodicity in typical ${\mathrm{SIS}}^{\prime }$ junctions with difference in orientations of S and ${\mathrm{S}}^{\prime }$ being within ${21}^{\circ }–{39}^{\circ }$ for ${\alpha }_R=0.05$–$0.3$ eV. As a result, it is easy for a $\pi$ ring in tricrystal experiments to hold Majorana fermions and exhibit periods of two flux quanta in external magnetic field. These phenomena may have been already observed in experiments and their connections to experimental results are discussed.

Series of high-pressure x-ray diffraction patterns of iron and its high-pressure polymorphs were collected with 0.1–0.2-s exposure time utilizing a membrane diamond anvil cell (DAC) for compression at various loading and unloading rates to a maximum pressure of 70 GPa. Strain rates of ${10}^{-2}{\mathrm{s}}^{-1}$ at a maximum pressurization rate of 4.1 GPa/s were achieved in non-hydrostatic compression of hcp Fe. Linewidth analysis was used to retrieve strain and uniaxial stress of Fe as a function of pressure upon both compression and decompression. Analysis of the lattice parameters ratio $c/a$ of hcp Fe indicates the presence of complex non-hydrostatic stress states, which developed as a function of strain rate, relaxation time, and various levels of hydrostaticity. Our results emphasize the importance of a controlled pressurization in DACs because the experimental loading rate strongly influences the stress state of the sample, particularly on decompression. Our time-resolved x-ray diffraction of the phase transition from bcc Fe to hcp Fe reveals residual grains of bcc Fe capable of surviving to very high pressures ($>35$ GPa) for a few minutes after the transition.

How to understand the order of Floquet stationary states in the presence of external bath coupling and their statistical mechanics is challenging; the answers are important for preparation and control of those Floquet states. Here, we propose a scheme to classify the statistical distribution of Floquet states for time-periodic systems which couple to an external heat bath. If an effective Hamiltonian and a system-bath coupling operator, which are all time independent, can be simultaneously obtained via a time-periodic unitary transformation, the statistical mechanics of the Floquet states is equivalent to the equilibrium statistical mechanics of the effective Hamiltonian. In the large driving frequency cases, we also show that the conditions of this theorem can be weakened to the following: The time-period part in the system Hamiltonian commutes with the system-bath coupling operator. A Floquet-Markov approach is applied to numerically compute the Floquet state occupation distribution of a bosonic chain, and the results agree with the theoretical predictions.

The magnetic phase diagram of the tetragonal ${\mathrm{Ho}}_2{\mathrm{RhIn}}_8$ compound has similar features to many related systems, revealing a zero magnetic field AF1 and a field-induced AF2 phases. Details of the magnetic order in the AF2 phase were not reported yet for any of the related compounds. In addition, only the ${\mathrm{Ho}}_2{\mathrm{RhIn}}_8$ phase diagram contains a small region of the incommensurate zero-field AF3 phase. We have performed a number of neutron diffraction experiments on single crystals of ${\mathrm{Ho}}_2{\mathrm{RhIn}}_8$ using several diffractometers including experiments in both horizontal and vertical magnetic fields up to 4 T. We present details of the magnetic structures in all magnetic phases of the rich phase diagram of ${\mathrm{Ho}}_2{\mathrm{RhIn}}_8$. The Ho magnetic moments point along the tetragonal $c$ axis in every phase. The ground-state AF1 phase is characterized by propagation vector $\mathbf{k}=(1/2,0,0)$. The more complex ferrimagnetic AF2 phase is described by four propagation vectors ${\mathbf{k}}_0=(0,0,0),{\mathbf{k}}_1=(1/2,0,0),{\mathbf{k}}_2=(0,1/2,1/2),{\mathbf{k}}_3=(1/2,1/2,1/2)$. The magnetic structure in the AF3 phase is incommensurate with ${\mathbf{k}}_{AF3}=(0.5,\delta ,0)$. Our results are consistent with theoretical calculations based on crystal field theory.

We report measurements of the cross-plane thermal conductivity of periodic Co/Cu multilayers using time-domain thermoreflectance. The cross-plane thermal conductivity increases from $\sim 18$ $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at remanence to $\sim 32$ $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at saturation fields. This giant magnetothermal resistance (GMTR) effect is consistent with predictions based on the Wiedemann-Franz law. We discuss the role of a spin-dependent temperature, known as spin heat accumulation, in GMTR experiments and develop a three-temperature model capable of predicting the time evolution of the temperatures of majority-spin electrons, minority-spin electrons, and phonons subsequent to pulsed laser heating.

The synthetic mineral urusovite $\mathrm{CuAl}\left(\mathrm{As}{\mathrm{O}}_4\right)\mathrm{O}$ was prepared through the wet chemistry route and characterized over a wide temperature range in terms of studies of magnetization, specific heat, and X-band electron spin resonance. The basic structural units of the compound are distorted square pyramids $\mathrm{Cu}{\mathrm{O}}_5$ assembled into corrugated honeycomb layers separated by $\mathrm{As}{\mathrm{O}}_4$ and $\mathrm{Al}{\mathrm{O}}_4$ tetrahedrons. Both thermodynamic and resonant measurements indicate that $\mathrm{CuAl}\left(\mathrm{As}{\mathrm{O}}_4\right)\mathrm{O}$ is a spin gap system with a gap of $\sim 350\mathrm{K}$. The electronic structure calculations performed within the framework of density functional theory suggest a weakly interacting dimer model with antiferromagnetic signs for both intradimer and interdimer superexchange interactions. This establishes the valence bond solid as the quantum ground state of the title compound. The pronounced discrepancy between experimental data and calculations within the weakly interacting dimer model at elevated temperatures is ascribed in part to the steep increase of the intradimer exchange interaction parameter driven by the thermal expansion effects.

We have performed muon spin rotation and relaxation $\left(\mu \mathrm{SR}\right)$ measurements on single crystals of the chiral helimagnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ at zero to low magnetic field. The transition from the paramagnetic to helical magnetically ordered phase at zero field is marked by the onset of a coherent oscillation of the zero-field muon spin polarization below a critical temperature $T_c$. An enhancement of the muon spin precession frequency is observed below $T\sim 50 \mathrm{K}$, where anomalous behavior has been observed in bulk transport measurements. The enhanced precession frequency indicates a low-temperature modification of the helical magnetic structure. A Landau free-energy analysis suggests that the low-temperature change in the magnetic structure is caused by a structural change. We also suggest a longer periodicity of helicity below $T\sim 50\mathrm{K}$, which can be verified by neutron-scattering experiments.

Density matrix embedding theory (DMET) [Phys. Rev. Lett. 109, 186404 (2012)], introduced an approach to quantum cluster embedding methods whereby the mapping of strongly correlated bulk problems to an impurity with finite set of bath states was rigorously formulated to exactly reproduce the entanglement of the ground state. The formalism provided similar physics to dynamical mean-field theory at a tiny fraction of the cost but was inherently limited by the construction of a bath designed to reproduce ground-state, static properties. Here, we generalize the concept of quantum embedding to dynamic properties and demonstrate accurate bulk spectral functions at similarly small computational cost. The proposed spectral DMET utilizes the Schmidt decomposition of a response vector, mapping the bulk dynamic correlation functions to that of a quantum impurity cluster coupled to a set of frequency-dependent bath states. The resultant spectral functions are obtained on the real-frequency axis, without bath discretization error, and allows for the construction of arbitrary dynamic correlation functions. We demonstrate the method on the one- (1D) and two-dimensional (2D) Hubbard model, where we obtain zero temperature and thermodynamic limit spectral functions, and show the trivial extension to two-particle Green's functions. This advance therefore extends the scope and applicability of DMET in condensed-matter problems as a computationally tractable route to correlated spectral functions of extended systems and provides a competitive alternative to dynamical mean-field theory for dynamic quantities.

We present a general method for retrieving the effective tensorial permittivity of uniaxially anisotropic metamaterials. By relaxing the usually imposed constraint of assuming nonmagnetic metal/dielectric metamaterials, we also retrieve the effective permeability tensor and show that multilayer hyperbolic metamaterials exhibit a strong and broadband diamagnetic response in the visible regime. The method provides the means for designing magnetically anisotropic metamaterials for studying magnetic topological transitions in the visible regime. We obtain orientation-independent effective material parameters, which are distinguishable from mere wave parameters. We analytically validate this method for $\mathrm{Ag}/\mathrm{Si}{\mathrm{O}}_2$ planar metamaterials with a varying number of layers and filling fractions and compare to the results from effective medium theory and Bloch theory.

We demonstrate that there is a strong correlation between the current noise and the inelastic tunneling spectrum of the vibronic current through nanoscale objects. The voltage derivative of the electron-phonon contribution to the noise is found to scale strongly with the contribution to the dynamical conductance when the partition noise vanishes. The scaling relation fails when the partition noise is maximized, i.e., in the case of half transmission, whereas we still find clear vibronic structure in the voltage sweep of the current noise. We have clarified the condition for the scaling relation. The voltage sweep of the current noise can provide us a useful spectroscopy method for the nonequilibrium dynamics of electrons and phonons.