We study a dynamical spin response of surface Majorana modes in a topological superconductor–magnet hybrid under microwave irradiation. We find a method to toggle between dissipative and nondissipative Majorana Ising spin dynamics by adjusting the external magnetic field angle and the microwave frequency. This reflects the topological nature of the Majorana modes, enhancing the Gilbert damping of the magnet, thereby providing a detection method for the Majorana Ising spins. Our findings illuminate a magnetic probe for Majorana modes, paving the path to innovative spin devices.

Bismuth, the heaviest of all stable group V elements with strong spin-orbit coupling, is famously known to exhibit many interesting transport properties, and effects such as Shubnikov-de Haas and de Haas-van Alphen were first revealed in its bulk form. However, the transport properties have not yet been fully explored experimentally in thin bismuth nor in its two-dimensional limit. In this work, bismuth flakes with average thicknesses ranging from 29 to 69 nm were mechanically exfoliated by a microtrench technique and were used to fabricate four-point devices. Due to the mixing of components, Onsager's relations were used to extract the longitudinal ($R_{xx}$) and Hall ($R_{xy}$) resistances where the latter shows a Hall anomaly that is consistent with the anomalous Hall effect. Our work strongly suggests that that there could be a hidden mechanism for time-reversal symmetry breaking in pure bismuth thin films.

The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a combined high-pressure powder synchrotron x-ray diffraction experiment and $abinitio$ study on the bilayered Ruddlesden-Popper phases ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ and ${\mathrm{Ca}}_3{\mathrm{Ti}}_2{\mathrm{O}}_7$. In both compounds we observe a first-order phase transition, that in combination with our density functional theory calculations, we can confidently assign as being between polar $A{2}_{1}am$ and nonpolar $Acaa$ structures. Interestingly, we show that while the application of pressure ultimately favors a nonpolar phase, as is commonly observed for proper ferroelectrics, regions of response exist where pressure actually acts to increase the polar mode amplitudes. The reason for this can be untangled by considering the varied response of octahedral tilts and rotations to hydrostatic pressure and their trilinear coupling with the polar instability.

Magnetic and atomic ordering in equiatomic FeNi alloy is studied by different ab initio techniques and methods based on density functional theory in order to clarify the main driving forces and their interplay behind these transitions and possibility of their accurate description within standard density functional theory calculations. The Curie temperature is obtained in Monte Carlo simulations using magnetic exchange interactions obtained by applying the magnetic force theorem within multiple scattering theory for different magnetic and atomic configurational states, including account for the thermal atomic displacements and exchange-correlation potential. The calculations show a very strong sensitivity of the results upon exchange-correlation potential, atomic order, and thermal atomic displacements. The calculated Curie temperature of a completely random alloy with the account of thermal lattice displacement is at least about 200 K below the known experimental data (780–800 K) depending on the above mentioned factors. The atomic order-disorder transition temperature is determined from effective chemical interactions, which apart from the chemical contribution (on the ideal fcc lattice) include contributions from lattice thermal vibrations and local lattice relaxations. The effective chemical interactions are strongly affected by the magnetic state, so the order-disorder transition temperature changes between 1000 and 140 K in the fully ordered ferromagnetic and paramagnetic states, respectively. For the reduced magnetization 0.7 (close to the experimental order-disorder transition temperature at 600 K), the order-disorder transition temperature varies between 550 and 700 K depending mostly on the exchange-correlation potential. The latter effect is the uncertainty in the choice of the exchange-correlation approximation in density functional theory calculations.

This paper undertakes a comprehensive exploration of multiphysics phenomena involving electromagnetic (EM) radiation emitted by a nanospherical piezoelectric scatterer embedded in a polymer matrix, subjected to P waves within the THz frequency spectrum. A nanosized piezoelectric scatterer as a potential antenna possesses a significant surface-to-volume ratio. Consequently, it becomes imperative to meticulously account for the nanoscopic polarization vector, residual polarization vector, stress field, and residual stress field at its interface with the surrounding matrix. The conventional electrodynamics theory falls short in addressing the problem of interest, primarily due to its oversight of the magneto-electro-elastic interface. One of the aims of the current work is to extend the previously devised mathematical framework of surface/interface elasticity theory for purely elastic medium at the nanoscale, to magneto-electro-elastic (MEE) medium. By harnessing the versatility of spherical harmonics, we have simultaneously solved the fully coupled elastodynamics and Maxwell's equations within the mathematical framework of spectral MEE surface/interface theory. In this study, we scrutinize specific nuances related to the introduced surface/interface characteristic lengths. This exploration enables us to examine the influence of size on EM radiated power, the fundamental resonance frequency, and the distribution of the magnetic field. The insights gained from our findings hold promise for the design of acoustically actuated nanospherical antennas, nanosensors, and nanoresonators utilizing piezoelectric nanospheres.

We investigate the melting properties of the bcc refractory metals V and W, and the disordered equiatomic VW alloy from first principles. We show that thermal vibrations have a large impact on the electronic density of states (DOS) and thus considerably affect the electronic contribution to the free energy. For W, the impact of vibrations on the electronic free energy of solid and liquid is different. This difference substantially impacts the computed melting point and also triggers a large electronic heat capacity difference between solid and liquid. For V, although vibrations likewise affect the electronic free energy, the effect on the melting properties cancels out to a large degree. For the binary VW alloy we observe a similar impact as for W, but slightly weaker. The underlying physics is explained in terms of the electronic DOS of the solid and liquid phases. Based on our accurate first-principles results, we reveal critical limitations of the Sommerfeld approximation in predicting the electronic heat capacity difference between solid and liquid. Our results thus prompt us to scrutinize this approximation which is often used in phase diagram parametrizations in the CALPHAD approach, as well as for materials, such as W, that have a large electronic DOS difference between solid and liquid at the melting temperature.

We theoretically investigate the steady-state transmission of continuous terahertz (THz) wave across a freestanding ferroelectric slab. Based on the Landau-Ginzburg-Devonshire theory of ferroelectrics and the coupled equations of motion for polarization and electromagnetic (EM) waves, we derive the analytical expressions of the frequency- and thickness-dependent dielectric susceptibility and transmission coefficient at the thin slab limit in the harmonic excitation regime. When the slab thickness is much smaller than the THz wavelength in the ferroelectric, the analytical predictions agree well with the numerical simulations from a dynamical phase-field model that incorporates the coupled dynamics of strain, polarization, and EM wave in multiphase systems. At larger thicknesses, the transmission is mainly determined by the frequency-dependent attenuation of THz waves in the ferroelectric and the formation of a standing polarization/THz wave. Our results advance the understanding of the interaction between THz wave and ferroelectrics and suggest the potential of exploiting ferroelectrics to achieve low-heat-dissipation, nonvolatile voltage modulation of THz transmission for high-data-rate wireless communication.

We study entanglement transitions in a periodically driven Ising chain in the presence of an imaginary transverse field $\gamma$ as a function of drive frequency ${\omega }_D$. In the high drive amplitude and frequency regime, we find a critical value $\gamma ={\gamma }_c$ below which the steady-state half-chain entanglement entropy, $S_{L/2}$, scales with chain length $L$ as $S_{L/2}\sim lnL$; in contrast, for $\gamma >{\gamma }_c$, it becomes independent of $L$. In the small $\gamma$ limit, we compute the coefficient, $\alpha$, of the $lnL$ term analytically using a Floquet perturbation theory and trace its origin to the presence of Fisher-Hartwig jump singularities in the correlation function of the driven chain. We also study the frequency dependence of ${\gamma }_c$ and show that ${\gamma }_c\to 0$ at special drive frequencies; at these frequencies, which we analytically compute, $S_{L/2}$ remain independent of $L$ for all $\gamma$. This behavior can be traced to an approximate emergent symmetry of the Floquet Hamiltonian at these drive frequencies which we identify. Finally, we discus the behavior of the driven system at low and intermediate drive frequencies. Our analysis shows the presence of volume law behavior of the entanglement in this regime $S_{\ell }\sim \ell$ for small subsystem length $\ell \le {\ell }^{*}\left({\omega }_D\right)$. We identify ${\ell }^{*}\left({\omega }_D\right)$ and tie its existence to the effective long-range nature of the Floquet Hamiltonian of the driven chain for small subsystem size. We discuss the applicability of our results to other integrable non-Hermitian models.

Lattice vibrations can greatly reduce the charge-carrier mobilities of organic semiconductors by introducing dynamic structural disorder. The low-frequency regime is expected to contribute the most to this disorder since these vibrational modes result in the largest structural distortions and electronic coupling changes in time. In this work, a finite-displacement Boltzmann transport theory ($\mathrm{\Delta }$-BTE) method is developed to evaluate vibration-induced mobility effects in periodic boundary conditions. $\mathrm{\Delta }$-BTE is tested on a model organic semiconductor crystal, i.e., tetracene, and it predicts that particular high-frequency ring-breathing modes significantly reduce its mobility through a transient carrier localization process, even when restricted to the zero-point vibrational energy level.

Hybrid ferroelectrics have attracted much attention recently due to their low processing cost and superior piezoelectric responses. However, their photovoltaic properties are less explored. For better performance, ferroelectric semiconductors with small band gaps are desired. Here, we report on an organic-inorganic hybrid material (MV) $\left[{\mathrm{SbI}}_5\right]$ ($\mathrm{M}{\mathrm{V}}^{2+}=\mathrm{N},{\mathrm{N}}^{\prime }$-dimethyl-4,4′-bipyridinium or methylviologen), with a band gap of 1.47 eV, which exhibits ferroelectricity at room temperature. Careful analysis shows that a flat band formed by the MV not only enhances light absorption but also allows for the simultaneous manifestation of small band gap and ferroelectricity in the same material. Under the irradiation of a 445 nm laser, we observed an open circuit voltage of ∼5 V, far greater than its band gap. The light polarization-dependent photocurrent confirms that the above-band-gap photovoltage is caused by the bulk photovoltaic effect (BPVE). Further investigations revealed that the contribution of the MV group to the conduction band leads to two distinct electron excitation pathways for (MV) $\left[{\mathrm{SbI}}_5\right]$ under visible and infrared light illumination, resulting in photocurrents in opposite directions. In this paper, we offer a strategy for designing hybrid ferroelectrics with narrow band gaps and improve our understanding of the BPVE.

Strong long-range hoppings up to third-nearest neighbors may induce a topological phase transition in one-dimensional chains. Unlike the Su-Schrieffer-Heeger model, this transition from trivial to topological phase occurs with the emergence of a pseudospin valley structure and a twofold nontrivial topological phase. Within a tight-binding approach, these topological phases are analyzed in detail and it is shown that the low-energy excitations follow a modified Dirac equation, in which the dynamics of particles with positive and negative mass occur differently. An experimental realization in a one-dimensional elastic chain, where it is feasible to tune directly the third-nearest-neighbor hoppings, is proposed.

The locally self-consistent multiple scattering (LSMS) method can perform efficient first-principles calculations of systems with a large number of atoms. In this paper, we combine the Kubo-Greenwood equation with LSMS, enabling us to calculate the first-principles residual resistivity of large systems. This has been implemented in the open-source code lsms. We apply this method to selected pure elements and binary random alloys. The results compare well with experiment, and with values obtained from a first-principles effective medium technique (the Korringa-Kohn-Rostoker coherent potential approximation). We discuss future applications of this method to complex systems where other methods are not applicable.

To add to the rapidly progressing field of ultrafast photocurrent, we propose a universal method to generate photocurrent in normal and topological materials using a pair of multicycle linearly polarized laser pulses. The interplay of the fundamental and its second harmonic pulses is studied for the generation of photocurrent in Weyl semimetals by varying the angle between the polarization direction, relative intensity, and relative phase delay. It has been found that the presence of a comparatively weaker second harmonic pulse is sufficient to generate substantial photocurrent. Moreover, significant photocurrent is generated even when polarization directions are orthogonal for certain ratios of the lasers' intensities. In addition, the photocurrent is found to be susceptible to the delay between the two pulses. We have illustrated that all our findings are extendable to nontopological and two-dimensional materials, such as graphene and molybdenum disulfide.

Synthetic antiferromagnets host spin waves that are highly tunable. It is of practical interest to analyze the symmetry of their coupling to surface acoustic waves with the design of hybrid devices in view. For this we calculate the layer-resolved susceptibility tensor of a synthetic antiferromagnet, the effective magnetoelastic and magnetorotation fields associated with a traveling elastic wave, and the power irreversibly transferred by the elastic wave to the magnetic layers. We consider Rayleigh-type surface acoustic waves (a) that travel in an elastically isotropic, nonpiezoelectric substrate, or (b) that propagate along the X direction at the surface of a Z-cut ${\mathrm{LiNbO}}_3$ substrate, or (c) that are guided in a thin Z-cut ${\mathrm{LiNbO}}_3$ film grown on a sapphire substrate. In particular, we show that the complementary angular dependencies of the acoustic and optical spin wave modes in synthetic antiferromagnets make it possible to excite spin waves for any relative orientation of magnetization and acoustic wave vector. In addition, we discuss the symmetries of the driving fields and of the energy transferred to the magnetic degree of freedom. We give evidence of new interaction channels coupling the magnetization eigenmodes when elastic anisotropy and piezoelectricity of the substrate are considered.

Among the family of odd-parity topological superconductors derived from ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Cu}}_x{\left(\mathrm{PbSe}\right)}_5{\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)}_6$ (CPSBS) has been elucidated to have gap nodes. Although the nodal gap structure has been established by specific-heat and thermal-conductivity measurements, there has been no direct observation of the superconducting gap of CPSBS using scanning tunneling spectroscopy (STS). Here we report the first STS experiments on CPSBS down to 0.35 K, which found that the vortices generated by out-of-plane magnetic fields have an elliptical shape, reflecting the anisotropic gap structure. The orientation of the gap minima is found to be aligned with the bulk direction when the surface lattice image shows twofold symmetry, but, surprisingly, it is rotated by ${30}^{\circ }$ when twofold symmetry is absent. In addition, the superconducting gap spectra in zero magnetic field suggest that the gap nodes are most likely lifted. We argue that only an emergent symmetry at the surface, allowing for a linear superposition of gap functions with different symmetries in the bulk, can lead to the rotation of the gap nodes. The absence of inversion symmetry at the surface additionally lifts the nodes. This result establishes the subtle but crucial role of crystalline symmetry in topological superconductivity.

With recent discoveries of superconductivity in infinite–layer nickelates and in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure, new opportunities appeared that yet another family of high-temperature superconductors based on Ni element may exist in nature, as was previously the case of cuprates and iron-based materials. With their famous strong Coulomb correlations among $3d$ electrons and the proximity to antiferromagnetic instability, these systems represent a challenge for their theoretical description, and most previous studies of superconductivity relied on the solutions of simplified few-orbital model Hamiltonians. Here, on the other hand, we use a recently developed combination of density functional theory with momentum and frequency-resolved self-energies deduced from the so-called fluctuational-exchange–type random phase approximation to study spin fluctuation mediated pairing tendencies in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under pressure. This methodology uses first-principles electronic structures of an actual material and is free of tight-binding parametrizations employed in the model Hamiltonian approach. Based on our numerical diagonalization of the BCS gap equation, we show that competing $d_{xy}$ and ${\mathrm{s}}_{\pm }$-pairing symmetries emerge in superconducting ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ with the corresponding coupling constants becoming large in the proximity of spin-density-wave instability. The results presented here are discussed in light of numerous other calculations and provide ongoing experimental efforts with predictions that will allow further tests of our understanding of unconventional superconductors.

A major challenge in the field of correlated electrons is the computation of dynamical correlation functions. For comparisons with experiment, one is interested in their real-frequency dependence. This is difficult to compute because imaginary-frequency data from the Matsubara formalism require analytic continuation, a numerically ill-posed problem. Here, we apply quantum field theory to the single-impurity Anderson model using the Keldysh instead of the Matsubara formalism with direct access to the self-energy and dynamical susceptibilities on the real-frequency axis. We present results from the functional renormalization group (fRG) at the one-loop level and from solving the self-consistent parquet equations in the parquet approximation. In contrast with previous Keldysh fRG works, we employ a parametrization of the four-point vertex which captures its full dependence on three real-frequency arguments. We compare our results to benchmark data obtained with the numerical renormalization group and to second-order perturbation theory. We find that capturing the full frequency dependence of the four-point vertex significantly improves the fRG results compared with previous implementations, and that solving the parquet equations yields the best agreement with the numerical renormalization group benchmark data but is only feasible up to moderate interaction strengths. Our methodical advances pave the way for treating more complicated models in the future.

Copper (Cu) is ubiquitously utilized in industry owing to its exceptional electrical conductivity and serves as a standard material in shock compression experiments. However, a comprehensive understanding of the electrical and thermal transport properties of Cu under extreme pressure-temperature ($P-T$) conditions remains a significant challenge due to limited experiments and theoretical constraints. In this work, we have developed a robust methodology for achieving high-quality electrical resistivity measurements of transition metals at ultrahigh $P-T$ conditions under shock compression. We conducted electrical resistivity measurements on Cu utilizing a four-probe method in a diamond anvil cell up to 50 GPa at ambient temperature, and in a two-stage light-gas gun up to 118 GPa and 1800 K. Simultaneously, we computed the electrical and thermal conductivity of face-centered cubic (fcc) Cu over a wide $P-T$ range using first-principles molecular dynamics simulations. Notably, our experimental and theoretical results are overall consistent with each other. Our results reveal that the electrical resistivity of fcc Cu diminishes with increasing pressure and displays a linear augmentation with rising temperature. The relationship between the electrical resistivity of fcc Cu and temperature can be described by the Bloch-Grüneisen formula, indicating that electron-phonon scattering governs its electrical conductivity.

Photocarrier dynamics of the ${\mathrm{ZrTe}}_3$ under pressure are investigated using optical pump-probe (OPOP) spectroscopy in combination with a diamond anvil cell. A prominent laser heating effect is manifested, characterized by significant changes in the profiles and an elongation of the echo period as the pump fluence is increased. Furthermore, this heating effect is found to be enhanced at pressures below 2 GPa, gradually diminishing until it completely disappears at 6 GPa. Additionally, the estimated sound velocity at high pressures indicates a rapid increase with pressure. This study not only assesses the potential application of ${\mathrm{ZrTe}}_3$ in the field of ultrafast optoelectronic devices but also provides fundamental understanding on the electron structural transition under pressure.