Journal of Physics: Condensed Matter最新文献

Lighter isotopes typically diffuse faster than heavier isotopes; however, the case is not necessarily true for H. Predicting the kinetics of H isotope transport and reactions in substances remains a fundamental challenge in material and condensed matter physics. The peculiar experimentally observed isotope effect on H diffusivities in face-centred cubic (fcc) metals has long been an unresolved problem. Using anab initiopath-integral approach to explore the quantum mechanical nature of both electrons and nuclei, this study successfully predicts H isotope diffusivities in fcc Pd over a wide temperature range. The temperature dependence of the diffusivities follows an unusual 'reversed-S' shape on Arrhenius plots. This irregular behaviour, arising from the competition between different nuclear quantum effects (NQEs) with different temperature dependencies, reveals the mechanism of anomalous crossovers between normal and reversed isotope effects. The results illustrate that this phenomenon is common in other fcc metals (e.g. Cu and Ag), where H atoms prefer to occupy octahedral (O) sites. Conversely, in Al, where H atoms prefer to occupy tetrahedral (T) sites, the dependence of H diffusivities on temperature exhibits a familiar 'C' shape. A lattice expansion of approximately 1%-2% causes the stable position of H atoms dissolved in Pd to shift from the O to T sites, and H diffusion in expanded Pd is no longer suppressed by NQEs, as observed in Al. This finding has important implications for interpreting kinetic processes involving the crossover from classical to quantum behaviour of H atoms moving between different interstitial sites. Path-integral simulation results describing the approximate quantum dynamics of the Pd-H system, using a machine-learning-based interatomic potential with accuracy similar to the density functional theory calculations, are presented. This computational approach paves the way for elucidating the quantum behaviour of H isotopes in various materials.

When compressed, certain lattices undergo phase transitions that may allow nuclei to gain significant kinetic energy. To explore the dynamics of this phenomenon, we develop a methodology to study Coulomb coupledN-body systems constrained to a sphere, as in the Thomson problem. We initializeNtotal Boron nuclei as point particles on the surface of the sphere, allowing them to equilibrate via Coulomb scattering with a viscous damping term. To simulate a phase transition, we removeNrmparticles, forcing the system to rearrange into a new equilibrium. With this model, we consider the Thomson problem as a dynamical system, providing a framework to explore how non-zero temperature affects structural imperfections in Thomson minima. We develop a scaling relation for the average peak kinetic energy attained by a single particle as a function ofNandNrm. For certain values ofN, we find an order of magnitude energy gain when increasingNrmfrom 1 to 6. The model may help to design a lattice that maximizes the energy output.

We use low-temperature scanning tunneling microscopy (LT-STM) to characterize the early stages of silver fluorination. On Ag(100), we observe only one adsorbate species, which shows a bias-dependent STM topography. Notably, at negative bias voltages,VB<0, the apparent shape can be described as a round protrusion surrounded by a moat-like depression (sombrero). As the voltage increases, the apparent shape changes, eventually evolving into a round depression. From the STM images, we determine the adsorption site to be the hollow position. On Ag(110) we find adsorbates with three distinct STM topographies. One type exhibits the same shape change withVBas observed on Ag(100), that is, from asombreroshape to a round depression as the voltage changes from negative to positive values; the other two types are observed as round depressions regardless ofVB. From the STM images, we find the three adsorbates to be sitting on the short-bridge, hollow and top position on the Ag(110) surface, with a relative abundance of 60%, 35% and 5%.

The Mpemba effect has initially been noticed in macroscopic systems-namely that hot water can freeze faster than cold water-but recently its extension to open quantum systems has attracted significant attention. This phenomenon can be explained in the context of nonequilibrium thermodynamics of Markovian systems, relying on the amplitudes of different decay modes of the system dynamics. Here, we study the Mpemba effect in a single-level quantum dot coupled to a thermal bath, highlighting the role of the sign and magnitude of the electron-electron interaction in the occurrence of the Mpemba effect. We gain physical insights into the decay modes from a dissipative symmetry of this system called fermionic duality. Based on this analysis of the relaxation to equilibrium of the dot, we derive criteria for the occurrence of the Mpemba effect using two thermodynamically relevant measures of the distance to equilibrium, the nonequilibrium free energy and the dot energy. We furthermore compare this effect to a possible exponential speedup of the relaxation. Finally, we propose experimentally relevant schemes for the state preparation and explore different ways of observing the Mpemba effect in quantum dots in experiments.

It is difficult to completely eliminate disorder during the fabrication of graphene-based nanodevices. From a simulation perspective, it is straightforward to determine the electronic transport properties of disordered devices if complete information about the disorder and the Hamiltonian describing it is available. However, to do the reverse and determine information about the nature of the disorder purely from transport measurements is a far more difficult task. In this work, we apply a recently developed inversion technique to identify important structural information about edge-disordered zigzag graphene nanoribbons. The inversion tool decodes the electronic transmission spectrum to obtain the overall level of edge vacancies in this type of device. We also consider the role of spin-polarised states at the ribbon edges and demonstrate that, in addition to edge roughness, the inversion procedure can also be used to detect the presence of magnetism in such nanoribbons. We finally show that if the transmission for both spin orientations is available, for example by using ferromagnetic contacts in a transport measurement, then additional structural information about the relative concentration of defects on each edge can be derived.

Due to the time reversal symmetry, the linear anomalous Hall effect (AHE) usually vanishes inMoS2monolayer. In contrast, the nonlinear AHE plays an essential role in such system when the uniaxial strain breaks theC3vsymmetry and eventually results in the nonzero Berry curvature dipole (BCD). We find that not only the magnitude of the AHE but also the nonlinear Hall angle can be tuned by the strain. Especially the nonlinear Hall angle exhibits a deep relationship which is analogy to the birefraction phenomenon in optics. It actually results from the pseudotensor nature of the BCD moment. Besides the ordinary positive and negative crystals in optics, there are two more birefraction-like cases corresponding to an imaginary refraction index ratio in monolayerMoS2. Our findings shed lights on the strain controlled electronic devices based on the two-dimensional materials with BCD.

Ferro-rotation (FR) phenomena-arising from intrinsic crystallographic rotational distortions-have only recently gained significant attention, despite the long-known presence of FR distortions in various compounds. Such phenomena include multiferroicity linked to helical spin order and electric field-induced optical activity. However, the broader relationship between FR and magnetic ordering remains largely unexplored and poorly understood. This study delves into the interplay between magnetic order and FR identifies potential materials capable of hosting magnetic FR and explores the interaction between magnetic FR and external stimuli. Our analysis reveals that out of the 122 magnetic point groups, 43 of them exhibit magnetic FR. Materials belonging to these magnetic point groups hold promise for demonstrating magnetic order-induced FR. Notably, the combination of parity/time reversal-odd antiferromagnetic order and FR can lead to the emergence of directionally nonreciprocal spin waves, as exemplified in MnTiO₃. Moreover, our investigation uncovers novel magnetic order-induced switchable chirality, when magnetic FR objects are exposed to external electric fields or temperature gradients. Overall, this research elucidates the intricate relationship between FR, magnetic order, and external perturbations, revealing the untapped potential and functionalities of magnetic FR materials.

We investigate a flexible polymer chain made up of chiral active Brownian particles in two dimensions using computer simulations. In the presence of chiral active Brownian forces, the radius of gyration of the chain reduces significantly. We further identify the formation of spirals using the tangent-tangent correlation to characterize the internal structure of the chain. The polymer chain forms a pair of spirals with opposite spiral turns on both ends of the polymer. We compute the number of turns of both spirals, and find that the total number of turns increases with angular frequency as well as Péclet number. However, the spirals become weak and the number of turns decreases at a very high Péclet number. We draw a phase diagram using the turn number. The end-to-end correlation displays oscillatory behavior, which signifies the rotational dynamics of the chain. We quantify the rotation frequency from the end-to-end vector, which follows a power law behavior with exponent3/2. We also provide a scaling relation between the radius of gyration and the chain length, and the exponent decreases significantly in the presence of chiral active forces.

An excitonic insulator (EI) phase is a consequence of collective many-body effects where an optical band gap is formed by the condensation of electron-hole pairs or excitons. We report pressure-dependent optical pump-optical probe spectroscopy of EI Ta₂NiSe₅up to 5 GPa. The differential reflectivity as a function of delay time between the pump and probe pulses shows two relaxation processes with their time constants and amplitudes revealing changes at PC1∼1 GPa (transition from EI phase to semiconductor) and PC2∼3 GPa (from semiconductor to semimetallic phase). The pressure dependence of the fast relaxation time and corresponding amplitude in the EI phase are captured by the Rothwarf-Taylor model, bringing out the decrease of the bandgap under pressure, with a pressure coefficient of 65 meV GPa^-1, closely agreeing with our first principle calculations. The decrease of the slow relaxation time in the EI phase with pressure is due to enhanced electron-phonon coupling as confirmed by our calculations. The fluence dependence of the relaxation parameters at different pressures corroborates the semi-metallic nature above PC2. Our experiments combined with first principle calculations thus provide additional insights into different high-pressure phases of Ta₂NiSe₅.

SOP (superconducting order parameter) in topological superconductors is characterized by an IR (irreducible representation) of a point group and a phase winding. In the Ginzburg-Landau theory and in phenomenological symmetry approaches wavefunction of a Cooper pair is identified as a SOP, which is expressed in terms of Legendre polynomials ink-space. In the space-group approach a Cooper pair wavefunction is constructed on the basis of exact one-electron functions ink-space. In the present work a connection between an IR of individual pair and possible phase winding in a pair condensate is investigated group-theoretically. It has been shown that the nodal structure of SOP forD4hsymmetry is defined group-theoretically for one-dimensional IRs and for two-dimensional IRs in the basal plane. For two-dimensional IRs nodes in vertical planes are defined by the additional quantum numbers, i.e. the intermediate group and the labels of its IRs. Experimental nodal structure of SOP in Sr₂RuO₄with nodal basal plane and nodes and dips in vertical planes corresponds to IRE_g. It has been shown using group theory, that when the Ginzburg-Landau theory is extended on two-dimensional IRs of point groups each one of the basis functions has a half of phase winding±πmand total phase winding may be equal to zero or to2πm. Phase winding2πmcorresponds to the magnetic group4/mm'm'. Our results explain a possibility of half quantized flux in Sr₂RuO₄in the presence of external magnetic field and may be used for description of recently obtained phase winding0.3×2πin Ba1-xK_xFe₂As₂.