Quantum kinetic equations of motion for the description of the exciton spin dynamics in II-VI diluted magnetic semiconductor quantum wells with laser driving are derived. The model includes the magnetic as well as the nonmagnetic carrier-impurity interaction, the Coulomb interaction, Zeeman terms, and the light-matter coupling, allowing for an explicit treatment of arbitrary excitation pulses. Based on a dynamics-controlled truncation scheme, contributions to the equations of motion up to second order in the generating laser field are taken into account. The correlations between the carrier and the impurity subsystems are treated within the framework of a correlation expansion. For vanishing magnetic field, the Markov limit of the quantum kinetic equations formulated in the exciton basis agrees with existing theories based on Fermi's golden rule. For narrow quantum wells excited at the $1s$ exciton resonance, numerical quantum kinetic simulations reveal pronounced deviations from the Markovian behavior. In particular, the spin decays initially with approximately half the Markovian rate and a nonmonotonic decay in the form of an overshoot of up to $10\%$ of the initial spin polarization is predicted.

The recently discovered topological Dirac semimetal represents a new exotic quantum state of matter. Topological Dirac semimetals can be viewed as three-dimensional analogues of graphene, in which the Dirac nodes are protected by crystalline symmetry. It has been found that the quantum confinement effect can gap out Dirac nodes and convert Dirac semimetal to a band insulator. The band insulator is either a normal insulator or quantum spin Hall insulator, depending on the thin-film thickness. We present the study of disorder effects in a thin film of Dirac semimetals. It is found that moderate Anderson disorder strength can drive a topological phase transition from a normal band insulator to a topological Anderson insulator in a Dirac-semimetal thin film. The numerical calculation based on the model parameters of Dirac semimetal ${\mathrm{Na}}_3\mathrm{Bi}$ shows that in the topological Anderson insulator phase, a quantized conductance plateau occurs in the bulk gap of the band insulator, and the distributions of local currents further confirm that the quantized conductance plateau arises from the helical edge states induced by disorder. Finally, an effective medium theory based on the Born approximation fits the numerical data.

Localized cavity resonances due to nanostructures at material surfaces can greatly enhance radiative heat transfer (RHT) between two closely placed bodies owing to stretching of cavity states in momentum space beyond the light line. Based on such understanding, we numerically demonstrate the possibility of ultrabroadband super-Planckian RHT between two plates patterned with trapezoidal-shaped hyperbolic metamaterial (HMM) arrays. The phenomenon is rooted not only in HMM's high effective index for creating subwavelength resonators but also its extremely anisotropic isofrequency contour. The two properties enable one to create photonic bands with a high spectral density to populate a desired thermal radiation window. At submicron gap sizes between such two plates, the artificial continuum states extend outside the light cone, tremendously increasing overall RHT. Our study reveals that structured HMM offers unprecedented potential in achieving a controllable super-Planckian radiative heat transfer for thermal management at nanoscale.

We report the pressure dependence of the Néel temperature $T_N$ up to $P\approx 27$ GPa for the recently discovered itinerant antiferromagnet (IAFM) TiAu. The $T_N\left(P\right)$ phase boundary exhibits unconventional behavior in which the Néel temperature is enhanced from $T_N\approx 33$ K at ambient pressure to a maximum of $T_N\approx 35$ K occurring at $P\approx 5.5$ GPa. Upon a further increase in pressure, $T_N$ is monotonically suppressed to $\sim 22$ K at $P\approx 27$ GPa. We also find a crossover in the temperature dependence of the electrical resistivity $\rho$ in the antiferromagnetic (AFM) phase that is coincident with the peak in $T_N\left(P\right)$, such that the temperature dependence of $\rho ={\rho }_0+A_n{T}^n$ changes from $n\approx 3$ during the enhancement of $T_N$ to $n\approx 2$ during the suppression of $T_N$. Based on an extrapolation of the $T_N\left(P\right)$ data to a possible pressure-induced quantum critical point, we estimate the critical pressure to be $P_c\approx 45$ GPa.

${\text{La}}_x{\text{Ca}}_{1-x}{\text{MnO}}_3$ (LCMO) has been studied in the framework of density functional theory (DFT) using Hubbard-$U$ correction. We show that the formation of spin polarons of different configurations is possible in the G-type antiferromagnetic phase. We also show that the spin-polaron (SP) solutions are stabilized due to an interplay of magnetic and lattice effects at lower La concentrations and mostly due to the lattice contribution at larger concentrations. Our results indicate that the development of SPs is unfavorable in the C- and A-type antiferromagnetic phases. The theoretically obtained magnetic state diagram is in good agreement with previously reported experimental results.

The phase diagram of the monoaxial chiral helimagnet as a function of temperature ($T$) and magnetic field with components perpendicular ($H_x$) and parallel ($H_z$) to the chiral axis is theoretically studied via the variational mean-field approach in the continuum limit. A phase transition surface in the three-dimensional thermodynamic space separates a chiral spatially modulated phase from a homogeneous forced ferromagnetic phase. The phase boundary is divided into three parts: two surfaces of second-order transitions of instability and nucleation type, in DeGennes terminology, are separated by a surface of first-order transitions. Two lines of tricritical points separate the first-order surface from the second-order surfaces. The divergence of the period of the modulated state on the nucleation transition surface has a logarithmic behavior typical of a chiral soliton lattice. The specific heat diverges on the nucleation surface as a power law with logarithmic corrections, while it shows a finite discontinuity on the other two surfaces. The soliton density curves are described by a universal function of $H_x$ if the values of $T$ and $H_z$ determine a transition point lying on the nucleation surface; otherwise, they are not universal.

We investigate the magnetic properties of the multiferroic quantum-spin system ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ by electron spin resonance (ESR) measurements at $X$- and $Q$-band frequencies in a wide temperature range $(T_{\mathrm{N}1}\le T\le 300$ K). The observed anisotropies of the $g$ tensor and the ESR linewidth in untwinned single crystals result from the crystal-electric field and from local exchange geometries acting on the magnetic ${\mathrm{Cu}}^{2+}$ ions in the zigzag-ladder-like structure of ${\mathrm{LiCu}}_2{\mathrm{O}}_2$. Supported by a microscopic analysis of the exchange paths involved, we show that both the symmetric anisotropic exchange interaction and the antisymmetric Dzyaloshinskii-Moriya interaction provide the dominant spin-spin relaxation channels in this material.

Using angle-resolved photoemission spectroscopy it is revealed that in the vicinity of optimal doping the electronic structure of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ cuprate undergoes an electronic reconstruction associated with a wave vector ${\mathit{q}}_{\mathit{a}}=(\pi ,0)$. The reconstructed Fermi surface and folded band are distinct to the shadow bands observed in BSCCO cuprates and in underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x\le 0.12$, which shift the primary band along the zone diagonal direction. Furthermore, the folded bands appear only with ${\mathit{q}}_{\mathit{a}}=(\pi ,0)$ vector, but not with ${\mathit{q}}_{\mathit{b}}=(0,\pi )$. We demonstrate that the absence of ${\mathit{q}}_{\mathit{b}}$ reconstruction is not due to the matrix-element effects in the photoemission process, which indicates the fourfold symmetry is broken in the system.

A system of fermions forming a Fermi surface exhibits a large degree of quantum entanglement, even in the absence of interactions. In particular, the usual case of a codimension one Fermi surface leads to a logarithmic violation of the area law for entanglement entropy as dictated by the Widom formula. We here generalize this formula to the case of arbitrary codimension, which is of particular interest for nodal lines in three dimensions. We first re-derive the standard Widom formula by calculating an entanglement Hamiltonian for Fermi-surface systems, obtained by repurposing a trick commonly applied to relativistic theories. The entanglement Hamiltonian will take a local form in terms of a low-energy patch theory for the Fermi surface, although it is nonlocal with respect to the microscopic fermions. This entanglement Hamiltonian can then be used to derive the entanglement entropy, yielding a result in agreement with the Widom formula. The method is then generalized to arbitrary codimension. For nodal lines, the area law is obeyed, and the magnitude of the coefficient for a particular partition is nonuniversal. However, the coefficient has a universal dependence on the shape and orientation of the nodal line relative to the partitioning surface. By comparing the relative magnitude of the area law for different partitioning cuts, entanglement entropy can be used as a tool for diagnosing the presence and shape of a nodal line in a ground-state wave function.

Metallic $5f$ materials have very strong coupling of magnetic moments and electrons mediating electrical conduction. It is caused by strong spin-orbit interaction, coming with high atomic number $Z$, together with involvement of the $5f$ states in metallic bonding. We have used the recently discovered class of uranium (ultra)nanocrystalline hydrides, which are ferromagnets with high ordering temperature, to disentangle the origin of negative temperature coefficient of electrical resistivity. In general, the phenomenon of electrical resistivity decreasing with increasing temperature in metals can have several reasons. The magnetoresistivity study of these hydrides reveals that quantum effects related to spin-disorder scattering can explain the resistivity behavior of a broad class of actinide compounds.

In doped manganites, a superlattice (SL) modulation associated with charge/orbital ordering is accepted as a key component in understanding many intriguing properties. It has been reported that the SL modulation always appears on the $a$ axis of the crystals. Here, by using multiple transmission electron microscopic techniques, we observe a type of anomalous nanocluster in which the SL modulation appears on the $c$ axis of $\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{C}{\mathrm{a}}_{2/3}\mathrm{Mn}{\mathrm{O}}_3$. By correlating the thermal evolution of the anomalous nanoclusters to other property measurements, we suggest that strain is responsible for the formation of the anomalous nanoclusters. The phase separation and phase transition scenario in $\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{C}{\mathrm{a}}_{2/3}\mathrm{Mn}{\mathrm{O}}_3$ are also described using electronic-liquid-crystal (ELC) phases. An ELC phase diagram in $\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{C}{\mathrm{a}}_{2/3}\mathrm{Mn}{\mathrm{O}}_3$ is constructed as a function of temperature based on our observations.

${\mathrm{SmB}}_6$, as a topological Kondo insulator, has spin-momentum-locked surface states and fully insulating bulk, which presents promising spintronic applications. Here, we report on the magnetotransport properties of individual ${\mathrm{SmB}}_6$ nanowires. With decreasing temperature below 10 K, the surface states dominate the transport behavior as reflected by the resistance saturation. At 1.5 K, a transition from negative to positive magnetoresistance occurs with gradually increasing the bias current. The nonlocal measurements indicate that the surface state transport is spin polarized, and the spin diffusion length is as long as 0.5 μm. Bias-current-modulated two-channel transport is employed to explain the observed sign reversal of the magnetoresistance.

We develop the self-learning Monte Carlo (SLMC) method, a general-purpose numerical method recently introduced to simulate many-body systems, for studying interacting fermion systems. Our method uses a highly efficient update algorithm, which we design and dub “cumulative update”, to generate new candidate configurations in the Markov chain based on a self-learned bosonic effective model. From a general analysis and a numerical study of the double exchange model as an example, we find that the SLMC with cumulative update drastically reduces the computational cost of the simulation, while remaining statistically exact. Remarkably, its computational complexity is far less than the conventional algorithm with local updates.

The transmission phase across a quantum dot (QD) is expected to show mesoscopic behavior, where the appearance of a phase lapse between Coulomb peaks (CPs) as a function of the gate voltage depends on the orbital parity relation between the corresponding CPs. On the other hand, such mesoscopic behavior has been observed only in a limited QD configuration (a few-electron and single-level transport regime) and universal phase lapses by $\pi$ between consecutive CPs have been reported for all the other configurations. Here, we report on the measurement of a transmission phase across a QD around the crossover between single-level and multilevel transport regimes employing an original two-path quantum interferometer. We find mesoscopic behavior for the studied QD. Our results show that the universal phase lapse, a longstanding puzzle of the phase shift, is absent for a standard QD, where several tens of successive well-separated CPs are observed.

We investigates exciton-exciton annihilation (EEA) in tungsten disulfide $\left(\mathrm{W}{\mathrm{S}}_2\right)$ monolayers encapsulated by hexagonal boron nitride (hBN). It is revealed that decay signals observed by time-resolved photoluminescence (PL) are not strongly dependent on the exciton densities of hBN-encapsulated $\mathrm{W}{\mathrm{S}}_2$ monolayers $(\mathrm{W}{\mathrm{S}}_2/\mathrm{hBN})$. In contrast, the sample without the bottom hBN layer $(\mathrm{W}{\mathrm{S}}_2/\mathrm{Si}{\mathrm{O}}_2)$ exhibits a drastic decrease of decay time with increasing exciton density due to the appearance of a rapid PL decay component, signifying nonradiative EEA-mediated recombination. Furthermore, the EEA rate constant of $\mathrm{W}{\mathrm{S}}_2/\mathrm{hBN}$ was determined as $(6.3\pm 1.7)\times {10}^{-3}\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}$, being about 2 orders of magnitude smaller than that of $\mathrm{W}{\mathrm{S}}_2/\mathrm{Si}{\mathrm{O}}_2$. Thus, the observed EEA rate reduction played a key role in enhancing luminescence intensity at high exciton densities in the $\mathrm{W}{\mathrm{S}}_2$ monolayer.

Motivated by recent experiments, we present here an ab initio study of the impact of the phonon transport on the thermal conductance and thermoelectric figure of merit of ${\mathrm{C}}_{60}$-based single-molecule junctions. To be precise, we combine density functional theory with nonequilibrium Green's-function techniques to compute these two quantities in junctions with either a ${\mathrm{C}}_{60}$ monomer or a ${\mathrm{C}}_{60}$ dimer connected to gold electrodes, taking into account the contributions of both electrons and phonons. Our results show that for ${\mathrm{C}}_{60}$ monomer junctions phonon transport plays a minor role in the thermal conductance and, in turn, in the figure of merit, which can reach values on the order of 0.1, depending on the contact geometry. In ${\mathrm{C}}_{60}$ dimer junctions, phonons are transported less efficiently, but they completely dominate the thermal conductance and reduce the figure of merit as compared to monomer junctions. Thus, claims that by stacking ${\mathrm{C}}_{60}$ molecules one could achieve high thermoelectric performance, which have been made without considering the phonon contribution, are not justified. Moreover, we analyze the relevance of near-field thermal radiation for the figure of merit of these junctions within the framework of fluctuational electrodynamics. We conclude that photon tunneling can be another detrimental factor for the thermoelectric performance, which has been overlooked so far in the field of molecular electronics. Our study illustrates the crucial roles that phonon transport and photon tunneling can play, when critically assessing the performance of molecular junctions as potential nanoscale thermoelectric devices.