The model of thermal rectifier based on the asymmetry of interaction of the molecular chain ends with thermostats is proposed in this work. The rectification mechanism is not related to the chain asymmetry but to the asymmetry of the interaction of chain ends with thermostats, for instance, due to different lengths of the end thermostats. The chain can be homogeneous, it is only important that the thermal conductivity of the chain should depend on temperature strict monotonically. The effect is maximal when convergence of the thermal conductivity with increasing length just begins to manifest itself. In this case, the efficiency of thermal rectification can reach 25%. These conditions are met for carbon nanoribbons and nanotubes. Therefore, they can be ideal objects for the construction of thermal rectifiers based on the asymmetric interaction with thermostats. Numerical simulation of heat transfer shows that the rectification of heat transfer can reach 14% for nanoribbons and 22% for nanotubes.

The notion of fractional charges was up until now reserved for quasiparticle excitations emerging in strongly correlated quantum systems, such as Laughlin states in the fractional quantum Hall effect, Luttinger quasiparticles, or parafermions. Here, the author consider topological transitions in the full counting statistics of standard sequential electron tunneling and find that they lead to charge fractionalization—strikingly without requiring exotic quantum correlations. This conclusion relies on the realization that fundamental integer charge quantization fixes the global properties of the transport statistics, whereas fractional charges can only be well-defined locally. We then show that the reconciliation of these two contradicting notions results in a nontrivially quantized geometric phase defined in the detector space. In doing so, we show that detector degrees of freedom can be used to describe topological transitions in nonequilibrium open quantum systems. Moreover, the quantized geometric phase reveals a profound analogy between the fractional charge effect in sequential tunneling and the fractional Josephson effect in topological superconducting junctions, where likewise the Majorana- or parafermions exhibit a charge which is at odds with the Cooper pair charge as the underlying unit of the supercurrent. To provide means for an experimental verification of our claims, we discuss highly feasible transport models, such as weakly tunnel-coupled quantum dots or charge islands. We show that the geometric phase can be accessed through the detector's waiting-time distribution. Finally, we find that topological transitions in the transport statistics could even lead to new applications, such as the unexpected possibility to directly measure features beyond the resolution limit of a detector.

We introduce an electromagnetic metamaterial—a spin-valley photonic topological insulator (SV-PTI)—that emulates a wide class of theoretically predicted gapped two-dimensional materials with antiferromagnetic order coupled to the valley degree of freedom. First-principles electromagnetic simulations and an analytic model reveal that two geometric parameters control the propagation properties of the chiral kink states at the domain wall between two SV-PTIs: their group velocity, polarization, and the existence/absence of topological protection. We demonstrate that a nonuniform SV-PTI structure can compress the electromagnetic energy carried by the kink states.

The spin crossover in organometallic molecules constitutes one of the most promising routes towards the realization of molecular spintronic devices. In this paper, we explore the hybridization-induced spin crossover in metal-organic complexes. We propose a minimal many-body model that captures the essence of the spin-state switching, thus providing insight into the underlying physics. Combining the model with density functional theory (DFT), we then study the spin crossover in isomeric structures of Ni-porphyrin (Ni-TPP). We show that metal-ligand charge transfer plays a crucial role in the determination of the spin state in Ni-TPP. Finally, we propose a spin-crossover mechanism based on mechanical strain, which does not require a switch between isomeric structures.

Phase transitions to a long-range-ordered state driven by a softened phonon mode are ubiquitous across condensed matter physics, but the evolution of such a mode as the system is tuned to or from the transition has never been explicitly measured until now. We report the effect of pressure on the soft mode associated with ferroelectricity in the archetypal quantum critical paraelectric ${\mathrm{SrTiO}}_3$. This is an ideal, clean, model system for exploring these effects, with pressure directly addressing the phonon modes only. We measure and report the effect of quantum critical fluctuations on the pressure and temperature dependence of the ferroelectric soft phonon mode as the system is tuned away from criticality. We show that the mean-field approximation is confirmed experimentally. Furthermore, using a self-consistent model of the quantum critical excitations including coupling to the volume strain and without adjustable parameters, we determine logarithmic corrections that would be observable only very close to the quantum critical point. Thus, the mean-field character of the pressure dependence is much more robust to the fluctuations than is the temperature dependence. We predict stronger corrections for lower dimensionalities. The same calculation confirms that the Lydanne-Sachs-Teller relation is valid over the whole pressure and temperature range considered. Therefore, the measured dielectric constant can be used to extract the frequency of the soft mode down to 1.5 K and up to 20 kbar of applied pressure. The soft mode is observed to stiffen further, raising the low-temperature energy gap and returning towards the expected shallow temperature dependence of an optical mode. This behavior is consistent with the existence of a ferroelectric quantum critical point on the pressure-temperature phase diagram of ${\mathrm{SrTiO}}_3$, which applied pressure tunes the system away from. This work represents an experimental measurement of the stiffening of a zone center soft phonon mode as a system is tuned away from criticality, a potentially universal phenomenon across a variety of phase transitions and systems in condensed matter physics.

Recently, investigation and control of the properties of propagating spin waves (SWs) in ultrathin ferromagnetic films have attracted considerable attention because of their possible technological impact on future nanoscale magnonic devices. Control of SW properties using voltage-controlled magnetic anisotropy (VCMA) may reduce the power consumption of future magnonic devices significantly. Here, we report an experimental study of manipulation of uniform ferromagnetic resonance (UFMR) and dipole-exchange SWs by VCMA in ultrathin $\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ (CoFeB) films with thicknesses of 1.6, 1.8, and 2.0 nm. The UFMR and the SWs are excited using microwave antennae and detected by spin pumping and inverse spin Hall effect techniques. A significant change in the SW frequency caused by VCMA is observed, particularly in the 1.6-nm-thick CoFeB film, where the effective value of the net anisotropy is quite small because of the presence of strong interfacial perpendicular magnetic anisotropy. Additionally, micromagnetic simulations are performed to demonstrate that the SWs in the 1.6-nm-thick CoFeB film with its in-plane easy axis of magnetization can be guided through virtual nanochannels formed by VCMA.

In this work we explain the hour-glass magnetic dispersion in underdoped cuprates. The dispersion arises due to the Lifshitz-type magnetic criticality. Superconductivity also plays a role, but the role is secondary. We list six major experimental observations related to the hour glass and explain all of them. The theory provides a unified picture of the evolution of magnetic excitations in various cuprate families, including “hour-glass” and “wine-glass” dispersions and an emergent static incommensurate order. We propose the Lifshitz spin-liquid “fingerprint” sum rule, and show that the latest data confirm the validity of the sum rule.

Herein, we discuss the identification of vanadium trimers in layered ${\mathrm{LiVO}}_2$ and its sulfide analog of ${\mathrm{LiVS}}_2$ with two-dimensional triangular lattices. Our comprehensive structural studies using synchrotron x-ray-diffraction experiments clarified that vanadium trimers are randomly aligned along the $c$-axis direction in ${\mathrm{LiVO}}_2$, while the long-range ordering of vanadium trimers along the $c$-axis direction appears in ${\mathrm{LiVS}}_2$. Our results solve the long-standing issue of cluster patterns in ${\mathrm{LiVO}}_2$ and provide an experimental basis for identifying the mechanism of trimer formation.

We study Coulomb drag between an active layer with a clean electron liquid and a passive layer with a pinned electron lattice in the regime of fast intralayer equilibration. Such a two-fluid system offers an experimentally realizable way to disentangle the fast rate of intralayer electron-electron interactions from the much slower rate of momentum transfer between both layers. We identify an intermediate temperature range above the Fermi energy of the electron fluid but below the Debye energy of the electronic crystal where the hydrodynamic drag resistivity is directly proportional to a fast electron-electron scattering rate. The results are compatible with the conjectured scenario for strong electron-electron interactions which poses that a linear temperature dependence of resistivity originates from a “Planckian” electron relaxation time ${\tau }_{eq}\sim \hslash /k_{B}T$. We compare this to the better known semiclassical case, where the diffusion constant is found to be not proportional to the microscopic timescale.

We present a detailed investigation of different excitonic states weakly confined in single GaAs/AlGaAs quantum dots obtained by the Al droplet-etching method. For our analysis we make use of temperature-, polarization-, and magnetic-field-dependent $\mu$-photoluminescence measurements, which allow us to identify different excited states of the quantum dot system. Besides that, we present a comprehensive analysis of $g$-factors and diamagnetic coefficients of charged and neutral excitonic states in Voigt and Faraday configuration. Supported by theoretical calculations by the configuration-interaction method, we show that the widely used single-particle Zeeman Hamiltonian cannot be used to extract reliable values of the $g$-factors of the constituent particles from excitonic transition measurements.

The crystal-field (CF) splitting of the ${}^6{H}_{5/2}$ Hund's rule ground state of ${\mathrm{Sm}}^{3+}$ in the strongly correlated topological insulator ${\mathrm{SmB}}_6$ has been determined with high-resolution resonant inelastic x-ray scattering (RIXS) at the Sm $M_5$ edge. The valence selectivity of RIXS allows isolating the crystal-field-split excited multiplets of the ${\mathrm{Sm}}^{3+}$ ($4f^5$) configuration from those of ${\mathrm{Sm}}^{2+}$ ($4f^6$) in intermediate valent ${\mathrm{SmB}}_6$. We find that the quartet ${\mathrm{\Gamma }}_8$ ground state and the doublet ${\mathrm{\Gamma }}_7$ excited state are split by ${\mathrm{\Delta }}_{{}^6{H}_{5/2}}^{\mathrm{CF}}=20\pm 10$ meV which sets an upper limit for the $4f$ bandwidth. This indicates an extremely large mass renormalization from the band-structure value, pointing out the need to consider the coefficients of fractional parentage for the hopping of the $4f$ electrons. The tiny bandwidth explains the small value of the indirect gap and puts constraints on the energies of the topological surface states.

We numerically investigate the momentum-space entanglement entropy and entanglement spectrum of the random-dimer model and its generalizations, which circumvent Anderson localization, after a quench in the Hamiltonian parameters. The type of dynamics that occurs depends on whether or not the Fermi level of the initial state is near the energy of the delocalized states present in these models. If the Fermi level of the initial state is near the energy of the delocalized states, we observe an interesting slow logarithmiclike growth of the momentum-space entanglement entropy followed by an eventual saturation. Otherwise, the momentum-space entanglement entropy is found to rapidly saturate. We also find that the momentum-space entanglement spectrum reveals the presence of delocalized states in these models for long times after the quench and the many-body entanglement gap decays logarithmically in time when the Fermi level is near the energy of the delocalized states.

We propose a computationally efficient approach to account for the nonadiabatic effects in time-dependent density functional theory (TDDFT) based on a representation of the frequency-dependent exchange-correlation kernel as a response of a set of damped oscillators. The requirements to computational resources needed to implement our approach do not differ from those of the standard real-time TDDFT in the adiabatic local density approximation. Thus, our result offers an exciting opportunity to take into account temporal nonlocality and memory effects in calculations with TDDFT in quantum chemistry and solid state physics for unprecedentedly low costs.

This study investigates the topological behavior of a continuous elastic phononic structure characterized by a three-term Kekulé distortion. The elastic waveguide consists of a hexagonal unit cell whose geometric dimensions are intentionally perturbed according to a generalized Kekulé scheme. The resulting structure exhibits an effective Hamiltonian that resembles a quantum spin Hall system, hence suggesting that the waveguide can support helical topological edge states. Using first-principles calculations, we show the existence of nondegenerate pseudospin states and a very peculiar six-lobe pseudospin texture and Berry curvature pattern. Important insights are also provided concerning the topological states in the Kekulé lattice, so far considered indistinguishable, and their critical role in enabling unique gapless edge states, typically not achievable in phononic systems.

An ab initio effective Hamiltonian is developed and used to reveal the effect of misfit strain on magnetic properties of epitaxial (001) films made of a prototypical rare earth iron garnet: Gadolinium iron garnet. It is found that both the magnetization compensation temperature ($T_{\mathrm{M}}$) and the Curie temperature ($T_{\mathrm{C}}$) can be significantly and linearly enhanced when varying the strain from tensile to compressive values. Consequently, $T_{\mathrm{M}}$ can be designed to coincide with room temperature, and the magnitude of the magnetization can be optimized at around 300 K when choosing the appropriate substrate. The microscopic origins of these technologically promising effects are further revealed.

In our proof-of-principle study we examine the influence of skyrmions on magnetoresistive transport. In particular, we show that magnetic tunnel junctions are a technologically appealing and promising way for electrical detection of noncollinear magnetic structures. The calculated effect is shown to originate from scattering between different $k$ states and cannot be identified through densities of states alone. Our results suggest that the detection efficiency strongly depends on the utilized materials.

We study frustrated spin-1/2 dimer systems in two dimensions with a bilayer structure, where spins are ferromagnetically coupled in dimers. Our model includes frustrated two-spin exchange interactions as well as four-spin interactions. We pay particular attention to the spin nematic phase, which does not exhibit any magnetic (spin-dipole) order but has a spin-quadrupolar long-range order. Employing a perturbation calculation, a mean-field approximation, and a numerical many-variable variational Monte Carlo method, we determine ground-state phase diagrams on various two-dimensional lattices. It is found that the model exhibits the spin nematic phase with ferroquadrupolar order in a wide parameter region, in addition to conventional magnetically ordered phases. In particular, it is shown that even when the four-spin interactions are absent, frustrated two-spin exchange interactions can realize the spin nematic phase as a result of strong interdimer correlations. It is also found that the phase transitions between the spin nematic phase and antiferromagnetic phases can be continuous. Furthermore, we present some exact arguments that various phases including the spin nematic phase and the vector chiral (p-type nematic) phase emerge from an SU(4) symmetric point in the model by the addition of appropriate perturbative interactions. The spin nematic phase generated from the SU(4) point is connected with the spin nematic phase found numerically in the system with only two-spin interactions.

In this paper we investigate the magneto-optical (MO) and magnetic properties of bismuth iron garnet $\mathrm{B}{\mathrm{i}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ thin films over a wide range of photon energies (1.6–3.5 eV) and temperatures (5–740 K). Depending on the photon energy range, the Faraday rotation (${\mathrm{\Theta }}_F$) and ellipticity (${\varepsilon }_F$) vary nonmonotonously with temperature. This behavior cannot be explained by a magnetization variation that can only decrease with increasing temperature. ${\mathrm{\Theta }}_F$ and ${\varepsilon }_F$ spectra have therefore been analyzed using a model based on two optical transitions of a diamagnetic nature, representing the tetrahedral and octahedral iron sites. Thus, the contribution of each magnetic sublattice has been extracted from the global macroscopic MO response and investigated as a function of temperature. The magnetic properties of octahedral and tetrahedral sublattices depend differently on temperature, suggesting a different anisotropy due to oxygen coordination. We have demonstrated that this relatively simple macroscopic measurement with a subsequent analysis can grant access to the information on the properties at a microscopic level. These results can advance the fundamental understanding of MO properties in multisublattice magnetic materials.