Systems that have been prepared to undergo a second-order phase transition at zero Kelvin, the so-called quantum critical systems, appear to fall into two categories: (chemically) heavily doped systems where the unusual properties can be related to a disorder-induced distribution of Kondo shielding temperatures, and (almost) stoichiometric systems where the departures from Fermi-liquid theory have been attributed to intrinsic instabilities. Here we show that this distinction is not as clear-cut and that magnetic clusters associated with a distribution of Kondo shielding temperatures are also present in ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$, a system close to a quantum critical point. By revisiting the published data on this system and comparing them to the results for heavily doped quantum critical $\mathrm{Ce}{\left({\mathrm{Ru}}_{0.755}{\mathrm{Fe}}_{0.245}\right)}_2{\mathrm{Ge}}_2$, we show that clusters exist in both systems at low temperatures and that the moments of the Ce ions within these clusters have all lined up with their neighbors. This implies that the dominant physics that drives heavily doped systems, namely, the spontaneous formation of magnetic clusters, should also play a leading role in the response of homogenous systems. This represents a notable departure from how the physics that governs quantum critical points is treated in the literature.

We study theoretically transverse photoconductivity induced by circularly polarized radiation, i.e., the photovoltaic Hall effect, and linearly polarized radiation causing intraband optical transitions in the two-dimensional electron gas (2DEG). We develop a microscopic theory of these effects based on the analytical solution of the Boltzmann equation for arbitrary electron spectrum and scattering mechanisms. We calculate the transverse photoconductivity of 2DEG with parabolic and linear dispersion for short-range and Coulomb scatterers at different temperatures. We show that the transverse electric current is significantly enhanced at frequencies comparable to the inverse energy relaxation time, whereas at higher frequencies the excitation spectrum and the direction of current depend on the scattering mechanism. We also analyze the effect of thermalization processes caused by electron-electron collisions on the photoconductivity.

Due to the strong coupling between magnetism and ferroelectricity, ${\left({\mathrm{ND}}_4\right)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$ exhibits several intriguing magnetic and electric phases. In this Letter, we include higher-order onsite anisotropic spin interactions to explain the ferroelectric phase transitions of ${\left({\mathrm{ND}}_4\right)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$ in a magnetic field and to produce the large weights of high-order harmonic components in the cycloid structure that are observed from neutron diffraction experiments. Moreover, we predict a new ferroelectric phase sandwiched between the FE II and FE III phases in a magnetic field. By emphasizing the importance of the higher-order spin anisotropic interactions, our work provides a framework to understand multiferroic materials with rich phase diagrams.

Investigating the pressure dependence of the superconducting (SC) transition temperature $T_{\mathrm{c}}$ is crucial for understanding the SC mechanism. Herein, we report on the pressure dependence of $T_{\mathrm{c}}$ in the nonmagnetic topological line-nodal material ${\mathrm{CaSb}}_2$ based on measurements of electric resistance and alternating current magnetic susceptibility. $T_{\mathrm{c}}$ initially increases with increasing pressure and peaks at $\sim$ 3.1 GPa. With a further increase in pressure, $T_{\mathrm{c}}$ decreases and finally becomes undetectable at 5.9 GPa. Because no signs of phase transition or Lifshitz transition are observed in the normal state, the peculiar peak structure of $T_{\mathrm{c}}$ suggests that ${\mathrm{CaSb}}_2$ has an unconventional SC character.

We report that spin supercurrents can be induced in diffusive superconductor-ferromagnet-superconductor Josephson junctions without any magnetic misalignment or intrinsic spin-orbit coupling. Instead, the pathway to spin-triplet generation is provided via geometric curvature, and results in a long-range Josephson effect. In addition, the curvature is shown to induce a dynamically tunable $0\text{−}\pi$ transition in the junction. We provide the analytic framework and discuss potential experimental and innovation implications.

We study disorder operator, defined as a symmetry transformation applied to a finite region, across a continuous quantum phase transition in $(2+1)d$. We show analytically that, at a conformally invariant critical point with U(1) symmetry, the disorder operator with a small U(1) rotation angle defined on a rectangle region exhibits power-law scaling with the perimeter of the rectangle. The exponent is proportional to the current central charge of the critical theory. Such a universal scaling behavior is due to the sharp corners of the region and we further obtain a general formula for the exponent when the corner is nearly smooth. To probe the full parameter regime, we carry out systematic computation of the U(1) disorder parameter in the square lattice Bose-Hubbard model across the superfluid-insulator transition with large-scale quantum Monte Carlo simulations, and confirm the presence of the universal corner correction. The exponent of the corner term determined from numerical simulations agrees well with the analytical predictions.

The out-of-plane thermal conductivity and elastic constant of epitaxial [100] ${}^{12}\mathrm{C}/{}^{13}\mathrm{C}$ superlattice diamonds with layer thicknesses of 1, 30, and 100 nm are evaluated by picosecond ultrasound spectroscopy. The measured elastic constants of the superlattices are equivalent to those of single-layer diamond thin films. This result confirms our success in synthesizing superlattice specimens with few defects at the interfaces. Therefore, the phonon transport behavior is governed by the mass difference, not the interfacial defects. The measured thermal conductivity of the superlattices is lower than that of a pure ${}^{12}\mathrm{C}$ isotope diamond thin film. We estimated the lattice thermal conductivity using the lattice dynamics calculation, attributing the lowered thermal conductivity to the decrease in the phonon group velocity in superlattices. We further consider the effect of mini-umklapp scattering in the superlattice, which explains the dependence of the thermal conductivity on the layer thickness. We reveal that the mini-umklapp scattering effect becomes significant only for an isotope diamond superlattice because of the high Debye temperature and large relative mass difference.

Three-dimensional topological insulator (TI) nanowires with quantized surface subband spectra are studied as a main component of Majorana bound states (MBS) devices. However, such wires are known to have large concentration $N\sim {10}^{19}{\mathrm{cm}}^{-3}$ of Coulomb impurities. It is believed that a MBS device can function only if the amplitude of long-range fluctuations of the random Coulomb potential $\mathrm{\Gamma }$ is smaller than the subband gap $\mathrm{\Delta }$. Here we calculate $\mathrm{\Gamma }$ for recently experimentally studied large-dielectric-constant ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ wires in a small-dielectric-constant environment (no superconductor). We show that provided by such a dielectric-constant contrast, the confinement of electric field of impurities within the wire allows more distant impurities to contribute into $\mathrm{\Gamma }$, leading to $\mathrm{\Gamma }\sim 3\mathrm{\Delta }$. We also calculate a TI wire resistance as a function of the Fermi level and carrier concentration due to scattering on Coulomb and neutral impurities and do not find observable discrete subband-spectrum-related oscillations at $N\gtrsim {10}^{18}{\mathrm{cm}}^{-3}$.

Although quantum phase transitions involved with Anderson localization have been investigated for more than a half century, the role of spin polarization in these metal-insulator transitions has not been clearly addressed as a function of both the range of interactions and the range of energy scales. Based on the Anderson-Hartree-Fock study, we reveal that the spin polarization has nothing to do with Anderson metal-insulator transitions in three dimensions as long as effective interactions between electrons are long-ranged Coulomb type. On the other hand, we find that metal-insulator transitions appear with magnetism in the case of Hubbard-type local interactions. In particular, we show that the multifractal spectrum of spin-$\uparrow$ electrons differs from that of spin-$\downarrow$ electrons at the high-energy mobility edge, which indicates the existence of spin-dependent universality classes for metal-insulator transitions. One of the most fascinating and rather unexpected results is the appearance of half metals at intermediate energy scales above the high-energy mobility edge in Anderson-type insulators of the Fermi energy; that is, only spin-$\uparrow$ electrons are delocalized, while spin-$\downarrow$ electrons are Anderson type localized.

Temperature-dependent thermal conductivity of epitaxial silicon (Si)/ultrathin silica multilayers film with epitaxial germanium (Ge) nanodot inclusions is measured over the range of temperature from 50 K to room temperature using time-domain thermoreflectance. The measured thermal conductivity with 5-nm Ge nanodots is much smaller than the reported values for Si/Ge superlattices, bulk SiGe, and nanostructured SiGe in the entire temperature range. The thermal conductivity of the film is analyzed with a kinetic model incorporating multiple phonon scattering processes, where intrinsic three-phonon scattering inside the Si layers is calculated by first principles, boundary scattering at the ultrathin silica layer is calculated by the atomistic Green's function, and scattering by the Ge nanodots is approximated with nanovoids. The analysis reveals that summing the multiple scattering rates by Matthiessen's rule cannot explain the extremely low thermal conductivity. The Monte Carlo ray tracing calculation that incorporates the multiple scattering effect reveals that the synergistic effect of ultrathin silica interfaces and Ge nanodots enhances phonon scattering. This suggests the merit in synergistically designing multiple nanostructures to reduce thermal conductivity, which is beneficial for developing thermoelectric materials.

We present a simple physical picture and precise model for the low-frequency acoustic modes of phononic crystals with multiple local resonances within each unit cell and their coupling to spatially separated resonators. The physical picture is a generalization of the widely quoted mass-in-a-box representation of resonant acoustic metamaterials. Our model consists of an array of frequency-dependent effective masses and moments of inertia coupled to near and distant neighbors by a wave-vector-dependent effective spring constant matrix. We demonstrate, using several two-dimensional models, that our simple representation accurately recaptures exact phononic band structure involving coupled translational and rotational modes. Our simplified but precise description is ideally suited for resonators consisting of multiple rigid cores and shells embedded in a softer elastic background. This enables a rich spectrum of acoustic modes and band gaps at audible frequencies, using millimeter to centimeter scale resonators. Our model is readily generalized to three-dimensional phononic crystals. It is suggested that suitable modifications of the spring tensor may enable description of disordered resonant acoustic media.

The fine control of magnetism and electronic structure in a magnetic topological insulator is crucial in order to realize various novel magnetic topological states including axion insulators, magnetic Weyl semimetals, Chern insulators, etc. Through crystal growth, transport, thermodynamic, neutron diffraction measurements, we show that under Sb doping the newly discovered intrinsic antiferromagnetic (AFM) topological insulator ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ evolves from AFM to ferromagnetic (FM) and then ferrimagnetic. We attribute this to the formation of ${\mathrm{Mn}}_{(\mathrm{Bi},\mathrm{Sb})}$ antisites upon doping, which results in additional Mn sublattices that modify the delicate interlayer magnetic interactions and cause the dominant Mn sublattice to go from AFM to FM. We further investigate the effect of antisites on the band topology using the first-principles calculations. Without considering antisites, the series evolves from AFM topological insulator ($x=0$) to FM axion insulators. In the exaggerated case of 16.7% of periodic antisites, the band topology is modified and a type-I magnetic Weyl semimetal phase can be realized at intermediate dopings. Therefore, this doping series provides a fruitful platform with continuously tunable magnetism and topology for investigating emergent phenomena, including quantum anomalous Hall effect, Fermi arc states, etc.

We report an infinite number of orthonormal eigenfunction bases for the quantum problem of a free electron in presence of an applied external magnetic field, suitable to describe doubly periodic electronic densities. The completeness of these bases is shown here and so, a single basis, labeled by the number of magnetic fluxons trapped in the unit cell (integer $p$), expands any function in the unit cell. The present framework unveils for the electronic density an egg-box pattern that displays fractional charge and magnetic flux. In case of electrons confined to the lowest Landau level we obtain an analytic expression for the local magnetic field created by their own motion and find that it yields an attractive magnetic interaction. The well-known de Haas–van Alphen oscillations are retrieved, thus showing the correctness of the present theoretical framework.

We propose a mechanism for the photoinduced superconductinglike response recently reported in cuprates and other strongly correlated materials. This mechanism relies on quantum-fluctuating bosons consisting of electron pairs. With periodic drive, the electron pairs and vacancies of pairs form a coherent nonequilibrium condensate, different from conventional superconductors, yet showing a superconductinglike response in some regime even with dissipation. Unlike the case of driven fermionic bands, which results in the familiar Floquet bands with hybridization gaps, for driven bosons the “gap” opens up in the momentum direction, resulting in a resonant region in momentum space where the eigenvalues are complex. We give a simple physical argument for why this picture leads to a “perfect conductor” which exhibits superconductinglike frequency-dependent conductivity but no Meissner response. While our model is quite general, in the case of cuprates, a quantum-fluctuating pair density wave in the pseudogap region may serve as the origin of the quantum-fluctuating electron pairs.

The dynamic structure factor $S(Q,E)$ of liquid ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ has been obtained by inelastic x-ray scattering, where $Q$ and $E$ are momentum and energy transfer, respectively. The dispersion curve of the longitudinal acoustic excitation energy exhibits flat-topped $Q$ dependence similarly to that in liquid GeTe and liquid Bi, where the local structure is modulated by Peierls distortion. The flat-topped energy in liquid ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ is split into low and high energy parts arising from Sb-Te and Ge-Te correlations, respectively. Furthermore, the high energy part depends on $Q$ like an optical mode with decreasing $Q$ to zero, and it approaches the vibrational energy of fourfold coordinated Ge atoms with octahedral configurations. The result indicates that a majority of Ge in the liquid stays in octahedral order as predicted by first-principles molecular dynamics simulations.

Quantum geometry has been identified as an important ingredient for the physics of quantum materials and especially of flat-band systems, such as moiré materials. On the other hand, the coupling between light and matter is of key importance across disciplines and especially for Floquet and cavity engineering of solids. Here we present fundamental relations between light-matter coupling and quantum geometry of Bloch wave functions, with a particular focus on flat-band and moiré materials, in which the quenching of the electronic kinetic energy could allow one to reach the limit of strong light-matter coupling more easily than in highly dispersive systems. We show that, despite the fact that flat bands have vanishing band velocities and curvatures, light couples to them via geometric contributions. Specifically, the intraband quantum metric allows diamagnetic coupling inside a flat band; the interband Berry connection governs dipole matrix elements between flat and dispersive bands. We illustrate these effects in two representative model systems: (i) a sawtooth quantum chain with a single flat band and (ii) a tight-binding model for twisted bilayer graphene. For (i) we highlight the importance of quantum geometry by demonstrating a nonvanishing diamagnetic light-matter coupling inside the flat band. For (ii) we explore the twist-angle dependence of various light-matter coupling matrix elements. Furthermore, at the magic angle corresponding to almost flat bands, we show a Floquet-topological gap opening under irradiation with circularly polarized light despite the nearly vanishing Fermi velocity. We discuss how these findings provide fundamental design principles and tools for light-matter-coupling-based control of emergent electronic properties in flat-band and moiré materials.

In Weyl semimetals subjected to torsion, there are two different kinds of chirality: (i) The (coordinate-space) shape of the twisted crystal is chiral and (ii) the momentum space contains chiral quasiparticles. Here we construct a general kinetic theory of anomalous transport using the phase space (coordinate and momentum spaces combined) Berry curvature induced by torsion in Weyl systems. We describe how torsion generates the chiral chemical potential and thus leads to the chiral magnetic effect (CME) in the presence of a background magnetic field. We propose to measure the CME current induced by the torsion as a way to detect the anomalous coupling between the coordinate-space and momentum-space chiralities.

The spin resonance of electrons can be coupled to a microwave cavity mode to obtain a photon-magnon hybrid system. These quantum systems are widely studied for both fundamental physics and technological quantum applications. In this paper, the behavior of a large number of ferrimagnetic spheres coupled to a single cavity is tested. We use second-quantization modeling of harmonic oscillators to theoretically describe our experimental setup and understand the influence of several parameters. The magnon-polariton dispersion relation is used to characterize the system, with a particular focus on the vacuum Rabi mode splitting due to multiple spheres. We combine the results obtained with simple hybrid systems to analyze the behavior of a more complex one and show that it can be devised in such a way to minimize the degrees of freedom needed to completely describe it. By studying single-sphere coupling two possible size effects related to the sample diameter have been identified, while multiple-sphere configurations reveal how to upscale the system. This characterization is useful for the implementation of an axion-to-electromagnetic field transducer in a ferromagnetic haloscope for dark matter searches. Our dedicated setup, consisting of ten 2-mm-diameter yttrium iron garnet spheres coupled to a copper microwave cavity, is used for this aim and studied at millikelvin temperatures. Moreover, we show that applications of optimally controlled hybrid systems can be foreseen for setups embedding a large number of samples.

Two-dimensional magnetic insulators can be promising hosts for topological magnons. In this study, we show that $ABC$-stacked honeycomb lattice multilayers with alternating Dzyaloshinskii-Moriya interaction (DMI) reveal a rich topological magnon phase diagram. Based on our band-structure and Berry curvature calculations, we demonstrate jumps in the thermal Hall behavior that corroborate with topological phase transitions triggered by adjusting the DMI and interlayer coupling. We connect the phase diagram of generic multilayers to a bilayer and a trilayer system. We find an even-odd effect among the multilayers where the even layers show no jump in thermal Hall conductivity, but the odd layers do. We also observe the presence of topological proximity effect in our trilayer. Our results offer new schemes to manipulate Chern numbers and their measurable effects in topological magnonic systems.