Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) measurements have previously been applied to the study of the heavy-fermion system $\mathrm{CeCo}{\mathrm{In}}_5$ to examine the superconducting gap structure and band dispersions via quasiparticle intereference. Here we directly measure the dispersing electron bands with angle-resolved photoelectron spectroscopy (ARPES) and compare with first-principles electronic structure calculations. By autocorrelating the ARPES-resolved bands with themselves we can measure the potential $q$ vectors and discern exactly which bands the STM is measuring. We find that the STM results are dominated by scattering associated with a cloverleaf shaped band centered at the zone corners. This same band is also a viable candidate to host the superconducting gap. The electronic structure calculations indicate that this region of the Fermi surface involves significant contributions from the Co $d$ electrons, an indication that the superconductivity in these materials is more three dimensional than that found in the related unconventional superconductors, the cuprates and the pnictides.

An efficient machine-learning-based method combined with a conventional local optimization technique has been proposed for exploring local energy minima of interstitial species in a crystal. In the proposed method, an effective initial point for local optimization is sampled at each iteration from a given feasible set in the search space. The effective initial point is here defined as the grid point that most likely converges to a new local energy minimum by local optimization and/or is located in the vicinity of the boundaries between energy basins. Specifically, every grid point in the feasible set is classified by the predicted label indicating the local energy minimum that the grid point converges to. The classifier is created and updated at every iteration using the already-known information on the local optimizations at the earlier iterations, which is based on the support vector machine (SVM). The SVM classifier uses our original kernel function designed as reflecting the symmetries of both host crystal and interstitial species. The most distant unobserved point on the classification boundaries from the observed points is sampled as the next initial point for local optimization. The proposed method is applied to three model cases, i.e., the six-hump camelback function, a proton in strontium zirconate with the orthorhombic perovskite structure, and a water molecule in lanthanum sulfate with the monoclinic structure, to demonstrate the high performance of the proposed method.

The electrical and thermal transport properties of metals become complicated in the intermediate and low temperature range ($0.1{\mathrm{\Theta }}_D-{\mathrm{\Theta }}_D$, with ${\mathrm{\Theta }}_D$ being the Debye temperature) due to electron-phonon inelastic scattering. For the Wiedemann-Franz law, a notable feature is that the Lorenz ratio significantly deviates from the Sommerfeld value. Although qualitatively theoretical understanding has been developed for decades, a mode-level first-principles analysis is still lacking in this temperature range and a better understanding of inelastic scattering and thermal transport mechanisms is desirable. In this work, we take aluminum and copper as examples. We find that two factors are essential to correctly predict the thermal conductivity and Lorenz ratio in the intermediate temperature range. First, the momentum relaxation time should be used for electrical conductivity calculations, while the energy relaxation time should be used for electronic thermal conductivity calculations. Second, proper choice of broadening parameter and fine sampling in the Brillouin zone is vital. Using the mode-level description of inelastic electron-phonon scattering at intermediate temperatures, the correct Lorenz ratio can be obtained within the present scheme, while using only the energy or momentum relaxation time cannot capture the correct trend of Lorenz ratio. The calculation scheme can be expanded to other metallic systems and is valuable for a better understanding of the transport properties of metals.

Tricritical phenomenon appearing in multiple phases is a fundamental and attractive issue in condensed-matter physics. Monopnictide DySb, a candidate for potential magnetic topological semimetal with an antiferromagnetic ground state, has been found to possess complex and intriguing field-induced magnetic phase transitions. In this work, critical behaviors of single-crystal DySb are investigated systematically, which generate a series of critical exponents including $\beta =0.244\left(2\right)$, $\gamma =0.827\left(2\right)$, and $\delta =4.425\left(1\right)$ for $H\parallel \left[001\right]$. The deduced critical exponents verified by Widom law and scaling equations are close to a tricritical mean-field model, suggesting a field-induced tricritical phenomenon in DySb. Based on the universality principle, a detailed $H\text{−}T$ phase diagram around the phase transition is constructed for $H\parallel \left[001\right]$, in which a tricritical point is revealed at temperature and field of (7.5 K, 51.4 kOe) on intersected boundaries of antiferromagnetic, forced ferromagnetic, and paramagnetic phases. Moreover, a triple point is found at the intersection (9.2 K, 19.7 kOe) of paramagnetic and two kinds of antiferromagnetic states. Such a fascinating phase diagram is indicative of delicate competition and balance between multiple magnetic interactions in this system, and lays a solid foundation for future research in topological transition and criticality.

${\mathrm{CeIr}}_3$ is a Ce-based superconductor with a superconducting transition temperature $T_{\mathrm{c}}=3.4$ K in an intermediate-valence state. We grew high-quality single crystals of ${\mathrm{CeIr}}_3$ using the Czochralski method, and measured the electrical resistivity, magnetic torque, and specific heat. The anisotropy of the superconducting upper critical field $H_{\mathrm{c}2}$ was determined. The temperature dependence of $H_{\mathrm{c}2}$, obtained from the resistivity measurements, suggests the multiband character of the superconductivity in ${\mathrm{CeIr}}_3$. Different field-angle dependencies of $H_{\mathrm{c}2}$ in electrical transport and thermodynamic measurements indicate the robust surface effect in the bulk superconductivity of ${\mathrm{CeIr}}_3$. We analyzed the surface superconductivity based on simple models to reveal the bulk superconducting properties. Our results support the isotropic bulk superconducting state and robust surface superconductivity in ${\mathrm{CeIr}}_3$ single crystals.

In the nematic state of iron-based superconductors, twin formation often obscures the intrinsic, anisotropic, in-plane physical properties. Relatively high in-plane external magnetic fields $H_{\mathrm{ext}}$ greater than the typical laboratory-scale magnetic fields 10–15 T are usually required to completely detwin a sample. However, recently a very small in-plane $H_{\mathrm{ext}}\sim 0.1$ T was found to be sufficient for detwinning the nematic domains in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$. To explain this behavior, a microscopic theory based on biquadratic magnetic interactions between the Eu and Fe spins has been proposed. Here, using ${}^{153}\mathrm{Eu}$ nuclear magnetic resonance (NMR) measurements below the ${\mathrm{Eu}}^{2+}$ ordering temperature, we show experimental evidence of the detwinning under small in-plane $H_{\mathrm{ext}}$. Our NMR study also reveals the evolution of the angles between the Eu and Fe spins during the detwinning process, which provides experimental evidence for the existence of biquadratic coupling in the system.

X-ray diffraction measurements performed in a diamond anvil cell under quasihydrostatic conditions up to 142 GPa at 300 K evidence an $\alpha$-Zr $\to$ (17 GPa) $\omega$-Zr $\to$ (35 GPa) $\beta$-Zr phase transitions sequence. Ab initio molecular dynamics calculations performed on the body-centered cubic $\beta$-Zr at 300 and 1000 K and between 0 and 100 GPa produced an equation of state in excellent agreement with the experiments. The stability of $\beta$-Zr under pressure has been verified by numerical heating-quenching experiments, and the anharmonicity of the thermal vibrations has been evaluated. No dynamical instability due to a soft mode is evidenced between 25 and 100 GPa, in line with the experimental finding of a wide stability range for $\beta$-Zr.

We report on the finding of a hard carbon structure in body centered cubic $\left(I{2}_{1}3\right)$ symmetry that possesses an extremely high bulk modulus (386 GPa) and Vickers hardness (60 GPa) comparable to that of $c$-BN and diamond. This carbon phase has a 14-atom primitive cell in an all-${sp}^3$ bonding network comprising five-membered rings, making it a truly full-fledged pentadiamond. Total energy calculations show that it is more stable than the previously reported diamondlike six-membered-ring bonded BC8 and BC12 carbon phases. Electronic band structure calculations show that it is an insulator with an indirect band gap of 5.64 eV. Simulated x-ray diffraction patterns and lattice parameters provide an excellent match to the previously unexplained distinct diffraction peaks found in carbon soot. The present results establish a distinct type of carbon phase and offer insights into its outstanding structural, mechanical, and electronic properties.

To investigate the magnetic properties of $\mathrm{Sm}{\mathrm{Fe}}_{12}$, we construct an effective spin model, where magnetic moments, crystal-field (CF) parameters, and exchange fields at 0 K are determined by first-principles calculations. Finite-temperature magnetic properties are investigated by using this model. We further develop an analytical method with strong mixing of states with a different quantum number of angular momentum $J$ ($J$-mixing), which is caused by a strong exchange field acting on the spin component of $4f$ electrons. Comparing our analytical results with those calculated by Boltzmann statistics, we clarify that the previous analytical studies for Sm transition-metal compounds overestimate the $J$-mixing effects. The present method enables us to perform a quantitative analysis of the temperature dependence of magnetic anisotropy (MA) with high reliability. The analytical method with model approximations reveals that the $J$-mixing caused by the exchange field increases the spin angular momentum, which enhances the absolute value of the orbital angular momentum and MA constants via spin-orbit interaction. It is also clarified that these $J$-mixing effects remain even above room temperature. Magnetization of $\mathrm{Sm}{\mathrm{Fe}}_{12}$ shows a peculiar field dependence known as the first-order magnetization process (FOMP), where the magnetization shows an abrupt change at a certain magnetic field. The result of the analysis shows that the origin of FOMP is attributed to competitive MA constants between positive $K_1$ and negative $K_2$. The sign of $K_{1\left(2\right)}$ appears due to an increase in the CF potential denoted by the parameter $A_2^0\langle r^2\rangle$ ($A_4^0\langle r^4\rangle$) caused by hybridization between $3d$-electrons of Fe on the $8i$ ($8j$) site and $5d$ and $6p$ valence electrons on the Sm site. It is verified that the requirement for the appearance of FOMP is given as $-K_2<K_1<-6K_2$.

We have developed a phenomenological model that connects the direct and inverse Edelstein effects. Our model implies a trade-off relation between the conversion coefficients for the direct and inverse effects. Thus, a large conversion coefficient for the inverse effect does not necessarily bring a large conversion coefficient for the direct effect. Instead of these coefficients, we propose a figure of merit of Edelstein effects, which consists of two factors; one of them represents the magnitude of the spin-orbit coupling. The other represents the strength of the hybridization between bulk and interface states. Both of them are important for efficient conversion through Edelstein effects. To test our model, we measured the inverse and direct Edelstein effects at the ${\mathrm{Bi}}_2{\mathrm{O}}_3/\mathrm{Cu}$ interface using the spin absorption method with a nonlocal spin valve structure and calculated the conversion coefficients. The effective spin Hall angle reaches $\sim 0.09$ in this system. This relatively large value of the spin Hall angle is attributable to the large spin-orbit coupling and the strong hybridization between the interface and bulk states at the ${\mathrm{Bi}}_2{\mathrm{O}}_3/\mathrm{Cu}$ interface.

We demonstrate that $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ can be a platform for observing the bulk odd-frequency superconducting state owing to its multiorbital/multiband nature. We consider a three-orbital tight-binding model for $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ in the vicinity of a ferroelectric critical point. Assuming an intraorbital spin-singlet $s$-wave superconducting order parameter, it is shown that the odd-frequency pair correlations are generated due to the intrinsic $LS$ coupling which leads to local orbital mixing. Furthermore, we show the existence of additional odd-frequency pair correlations in the ferroelectric phase, which is induced by an odd-parity orbital hybridization term proportional to the ferroelectric order parameter. We also perform a group theoretical classification of the odd-frequency pair amplitudes based on the fermionic and space group symmetries of the system. The classification table enables us to predict the dominant components of the odd-frequency pair correlations based on the symmetry of the normal state Hamiltonian that we take into account. Furthermore, we show that experimental signatures of odd-parity orbital hybridization, which is an essential ingredient for ferroelectricity-induced odd-frequency pair correlations, can be observed in the spectral functions and density of states.

Quantum many-body scarring is believed to be the mechanism behind long-lived coherent oscillations in interacting Rydberg atom chains. These persistent oscillations are due to the large overlap of the many-body scars with certain initial states. We show that the “effective dimension” is a useful measure for identifying nonthermalizing initial states in many-body scarred systems. By minimising the effective dimension we find physically reasonable initial states of the Rydberg chain that lead to more pronounced and longer-lived oscillations, accentuating the effect of the many-body scars on the dynamics.

We investigate the roles of spin-orbit coupling and the Van Hove singularity in the dynamical properties of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, which become prominent at zero and very low temperature, by means of density functional theory plus dynamical mean-field theory with an exact diagonalization solver. We examine the crossover between a Fermi liquid and a Hund's metal for a wide range of temperatures and Hund's coupling. In the absence of doping, we confirm that the Fermi liquid persists at zero temperature even with nonzero Hund's coupling. The freezing-moment mechanism suggests that thermal fluctuations lead to a suppression of the Fermi liquid phase and promote Hund's metallicity with incoherence. We show that the Van Hove singularity is an additional key ingredient to drive the suppression at very low temperature by observing a doping dependence of the freezing or long-lived paramagnetic moments. The role of spin-orbit coupling is marked by an amplified Van Vleck contribution of spin susceptibility, significantly promoting Hund's metallicity. Together with the known doping dependence of Hund's metallicity, the additional Van Hove singularity doping dependence found here may allow for the control of the Hund's metallicity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4\left(t_{2g}^4\right)$ by a fine tuning of the doping or possibly even strain.

The compelling original idea of a time crystal has referred to a structure that repeats in time as well as in space, an idea that has attracted significant interest recently. While obstructions to realize such structures became apparent early on, focus has shifted to seeing a symmetry breaking in time in periodically driven systems, a property of systems referred to as discrete time crystals. In this work, we introduce Stark time crystals based on a type of localization that is created in the absence of any spatial disorder. We argue that Stark time crystals constitute a phase of matter coming very close to the original idea and exhibit a symmetry breaking in space and time. Complementing a comprehensive discussion of the physics of the problem, we move on to elaborating on possible practical applications, and we argue that the physical demands of witnessing genuine signatures of many-body localization in large systems may be lessened in such physical systems.

The spin degree of freedom is crucial for both understanding and exploiting the particular properties of the edges of two-dimensional topological insulators. In the absence of superconductivity and magnetism, Rashba coupling is the most relevant single-particle perturbation in this system. Since Rashba coupling does not break time reversal symmetry, its influence on transport properties is visible only if processes that do not conserve the single-particle energy are included. Paradigmatic examples of such processes are electron-electron interactions and time-dependent external drivings. We analyze the effects of a periodically driven Rashba impurity at the helical edge, in the presence of electron-electron interactions. Interactions are treated by means of bosonization, and the backscattering current is computed perturbatively up to second order in the impurity strength. We show that the backscattering current is nonmonotonic in the driving frequency. This property is a fingerprint of the Rashba impurity, being absent in the case of a magnetic impurity in the helical liquid. Moreover, the nonmonotonic behavior allows us to directly link the backscattering current to the Luttinger parameter $K$, encoding the strength of electron-electron interactions.

Self-organized silicide nanowires are considered as building blocks of future nanoelectronics and have been intensively investigated. In nanostructures, the lattice vibrational waves (phonons) deviate drastically from those in bulk crystals, which gives rise to anomalies in thermodynamic, elastic, electronic, and magnetic properties. Hence a thorough understanding of the physical properties of these materials requires a comprehensive investigation of the lattice dynamics as a function of the nanowire size. We performed a systematic lattice dynamics study of endotaxial $\mathrm{Fe}{\mathrm{Si}}_2$ nanowires, forming the metastable, surface-stabilized $\alpha$ phase, which are in-plane embedded into the Si(110) surface. The average widths of the nanowires ranged from 24 to 3 nm, and their lengths ranged from several micrometers to about 100 nm. The Fe-partial phonon density of states, obtained by nuclear inelastic scattering, exhibits a broadening of the spectral features with decreasing nanowire width. The experimental data obtained along and across the nanowires unveiled a pronounced vibrational anisotropy that originates from the specific orientation of the tetragonal $\alpha \text{−}\mathrm{Fe}{\mathrm{Si}}_2$ unit cell on the Si(110) surface. The results from first-principles calculations are fully consistent with the experimental observations and allow for a comprehensive understanding of the lattice dynamics of endotaxial silicide nanowires.

We examine the far-field optical response, under plane-wave excitation in the presence of a static magnetic field, of core-shell nanoparticles involving a gyroelectric component, either as the inner or the outer layer, through analytic calculations based on appropriately extended Mie theory. We focus on absorption and scattering of light by bismuth-substituted yttrium iron garnet (Bi:YIG) nanospheres and nanoshells, combined with excitonic materials such as organic-molecule aggregates or two-dimensional transition-metal dichalcogenides, and discuss the hybrid character of the modes emerging from the coupling of the two constituents. We observe the excitation of strong magneto-optic phenomena and explore, in particular, the response and tunability of a magneto-transverse light current, indicative of the photonic Hall effect. We show how interaction between the Bi:YIG and excitonic layers leads to a pair of narrow bands of highly directional scattering, emerging from the aforementioned hybridization, which can be tuned at will by adjusting the geometrical or optical parameters of the system. Our theoretical study introduces optically anisotropic media as promising templates for strong coupling in nanophotonics, offering a means to combine tunable magnetic and optical properties, with potential implications both in the design of all-dielectric photonic devices but also in novel clinical applications.