CuTe is a two-dimensional (2D) layered material; yet it forms a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) along the $a$ axis in the $ab$ plane at high transition temperature $T_{\mathrm{CDW}}=335\mathrm{K}$. However, the anisotropic properties of CuTe remain to be explored. Here, we performed combined transport, polarized infrared reflectivity, and ultrafast pump-probe spectroscopy studies to investigate the underlying CDW physics of CuTe. Polarized optical measurements clearly revealed that an energy gap gradually forms along the $a$ axis upon cooling, while optical evidence of a gap signature is absent along the $b$ axis, suggesting pronounced electronic anisotropy in this quasi-2D material. Time-resolved optical reflectivity measurements revealed that the amplitude and relaxation time of photoexcited quasiparticles change dramatically across the CDW phase transition. Performing fast Fourier transformation of the oscillation signals arising from collective excitations, we identify the 1.65-THz mode as the CDW amplitude mode, whose energy softens gradually at elevated temperatures. Consequently, we provide further evidence for the formation of completely anisotropic CDW order in CuTe, which is quite rare in quasi-2D materials.

The formation process of core-shell bimetallic nanoparticles synthesized by sputtering onto a substrate is observed in real time using an originally developed acoustic technique. The technique enables us to evaluate the structural change of nanoparticles at room temperature without contacting the nanoparticles or substrate. In the experiments, the sputtering of metal A followed by metal B tended to form B-shell/A-core nanoparticles. However, in Pd-Au alloy system, notable restructuring occurred during synthesis, resulting in the formation of A-shell/B-core nanoparticles. The formation process is analyzed using the molecular dynamics simulation, revealing that this restructuring occurs on a short timescale, and high diffusivity of Au plays an important role.

We report on the formation of triple-atomic-layer metal films on Si(111) by deposition of Mg onto In double-layer films. The deposited Mg atoms are intercalated between the In layers and Si substrate and form a buffer layer with $(\sqrt{3}\times \sqrt{3})$ periodicity on an unreconstructed Si(111) surface. Two metal layers with nearly hexagonal close-packed arrangement are stacked in an ABC sequence on the buffer layer. The (In, Mg) triple-layer phase shows free-electron-like electronic structure with circular Fermi surface. Unlike the single Fermi circle of the In double layers, the Fermi surface of the (In, Mg) triple layers is composed of two circles with different radii. The larger and smaller Fermi circles are found to come from bonding and antibonding states between the top and middle layers of the three layers. The bottom layer acts as a buffer layer to saturate the Si dangling bonds and realize a nearly freestanding double-layer metal.

We generalize the optimized effective potential (OEP) formalism in the quantum electrodynamical density functional theory (QEDFT) to the case of continuous distribution of photon modes, and study its applicability to dissipative dynamics of electron systems interacting with photons of lossy cavities. Specifically, we test whether this technique is capable of capturing the quantum features of electron-photon interaction related to spontaneous emission and the corresponding energy transfer from the electrons to cavity photons. For this purpose, we analyze a discrete three-site system with one electron coupled to photons of the cavity, which, in fact, is a minimal model allowing us to eliminate classical radiation and the corresponding energy loss, but still have nontrivial density dynamics. By considering two typical spectral densities of photon modes, modeling (i) lossy cavity with Lorentzian broadening of photon peaks, and (ii) the Ohmic bath, and several representative dynamical regimes, we find that OEP-QEDFT demonstrates a good qualitative and quantitative performance, especially in the case when the dissipation is dominated by one-photon processes.

We establish a general formulation for spin current pumped by magnons at the interface between a normal metal and a magnetic insulator, valid for any noncollinear magnetic configuration in the linear spin wave regime. This current is generated by driving the system in a nonequilibrium state, covering setups such as thermal spin injection (spin Seebeck effect) or spin voltage by irradiation of the insulator (spin pumping). We illustrate the formula in the special case of a honeycomb topological ferromagnet, for simplicity, and cover both the spin Seebeck and spin pumping setups. We show how the topological parameters influence the spin current and propose a way to obtain a contribution mainly from the topological edge magnons in the spin pumping case.

Atomic-scale control of spins by electric fields is highly desirable for future technological applications. Magnetically doped Aurivillius-phase oxides present one route to achieve this, with magnetic ions substituted into the ferroelectric structure at dilute concentrations, resulting in spin-charge coupling. However, there has been minimal exploration of the ferroelectric switching pathways in this materials class, limiting predictions of the influence of an electric field on magnetic spins in the structure. Here, we determine the ferroelectric switching pathways of the end member of the Aurivillius phase family, ${\mathrm{Bi}}_2{\mathrm{WO}}_6$, using a combination of group theoretic analysis and density functional theory calculations. We find that in the ground state $P{2}_{1}ab$ phase, a two-step switching pathway via $C2$ and $Cm$ intermediate phases provides the lowest energy barrier. Considering iron substitutions on the W site in ${\mathrm{Bi}}_2{\mathrm{WO}}_6$, we determine the spin easy axis. By tracking the change in spin directionality during ferroelectric switching, we find that a ${90}^{\circ }$ switch in the polarization direction leads to a ${112}^{\circ }$ reorientation of the spin easy axis. The low-symmetry crystal-field environment of ${\mathrm{Bi}}_2\mathrm{W}{\mathrm{O}}_6$ and magnetoelastic coupling on the magnetic dopant provide a route to spin control via an applied electric field.

A simple mean-field description of frustrated antiferromagnets on hexagonal lattices, aimed to describe the high-temperature part of the temperature-magnetic field phase diagram, is proposed. It is shown that an interplay between modulation vector symmetry, Zeeman energy, and magnetodipolar interaction leads to stabilization of the triple-$Q$ skyrmion lattice in a certain region of the phase diagram. The corresponding analytical expressions for phase boundaries are derived. The possible relevance to the high-temperature part of the ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$ phase diagram is discussed.

Exploring superconductors which can possess a pairing mechanism other than the BCS predicted $s$ wave have continually attracted considerable interest. Superconductors with low-lying phonons may exhibit unconventional superconductivity as the coupling of electrons with these low-lying phonons can potentially affect the nature of the superconducting ground state, resulting in strongly coupled superconductivity. In this work, by using magnetization, AC transport, specific heat, and muon spin rotation/relaxation ($\mu \mathrm{SR}$) measurements, we report a detailed investigation on the superconducting ground state of the strongly coupled superconductor, IrGe, that has a transition temperature, $T_C$, at 4.7 K. Specific heat (SH), and transverse field $\mu \mathrm{SR}$ is best described with an isotropic $s$-wave model with strong electron-phonon coupling, indicated by the values of both $\mathrm{\Delta }\left(0\right)/k_B{T}_C=2.3,2.1$ (SH,$\mu \mathrm{SR}$), and $\mathrm{\Delta }{C}_{el}/{\gamma }_n{T}_C=2.7$. Zero-field $\mu \mathrm{SR}$ measurements confirm the presence of time-reversal symmetry in the superconducting state of IrGe.

We formulate a theory of thermal expansion based on the self-consistent phonon (SCP) theory, which nonperturbatively considers the anharmonic effect. We show that the Grüneisen formula holds within the SCP theory by replacing the phonon frequency by the SCP frequency. By comparing it with the quasiharmonic approximation (QHA), we derive explicit formulas of the correction to the QHA result. We show that the phonon anharmonicity gives a small correction of $O(\langle {\widehat{U}}_4\rangle /\langle {\widehat{U}}_2\rangle )$ to the thermal expansion coefficient $\alpha$, where ${\widehat{U}}_2$ and ${\widehat{U}}_4$ are the harmonic and the quartic terms of the potential energy surface. On the other hand, we show that the phonon anharmonicity gives two correction terms to the temperature ($T$)-dependent phonon frequency shift which are comparable to the original QHA term. In strongly anharmonic materials such as NaCl and MgO, these two correction terms tend to cancel out each other, which explains why QHA sometimes gives reasonable values for the $T$-dependent phonon frequency shift while it fails for thermal expansion.

We study entanglement dynamics in hybrid ${\mathbb{Z}}_2$-symmetric quantum automaton circuits subject to local composite measurements. We show that there exists an entanglement phase transition from a volume-law phase to a critical phase by varying the measurement rate $p$. By analyzing the underlying classical bit-string dynamics, we demonstrate that the critical point belongs to parity-conserving universality class. We further show that the critical phase with $p>p_c$ is related to the diffusion-annihilation process and is protected by the ${\mathbb{Z}}_2$-symmetric measurement. We give an interpretation of the entanglement entropy in terms of a two-species particle model and identify the coefficient in front of the critical logarithmic entanglement scaling as the local persistent coefficient. The critical behavior observed at $p\ge p_c$ and the associated dynamical exponents are also confirmed in the purification dynamics.

Ferromagnets with uniaxial magnetocrystalline anisotropy have been a significant platform to explore applications in magnetic devices. Although it is commonly recognized that the magnetic anisotropy is crucial for the formation of noncollinear domain patterns, the effects of the anisotropy direction are usually neglected in recent studies. Here, we imaged stray fields generated by a polycrystalline MnNiGa sample with a nitrogen-vacancy color center in diamond and found that various stripe domains and magnetic bubbles exist in different regions. These domain patterns could be explained by the difference in anisotropy directions among crystallites via micromagnetic simulation. It can be inferred that the magnetic anisotropy direction plays a key role in energetically favorable domain structures based on our results. The magnetic anisotropy direction may be exploited as an ingredient to modulate domains and develop devices.

We investigate the electron spin resonance of the organic spin-Peierls chain ($o$-DMTTF)${}_{2}X$ with $X$ = Cl, Br, and I. We describe the temperature dependence of the spin gap during the phase transition and quantify the dimerization parameter $\delta$. At the lowest temperatures, the susceptibility is governed by defects in the spin dimerized chain. Such strongly correlated defects are the consequence of breaks in the translational symmetry of the chain. In the vicinity of the defects the spins are polarized antiferomagnetically forming a magnetic soliton: a spin $\frac{1}{2}$ quasiparticle of size ruled by $\delta$ pinned to the defects. For ($o$-DMTTF)${}_2\mathrm{Br}$ and ($o$-DMTTF)${}_2\mathrm{Cl}$, we show that the one-half of the total number of solitons are in isolation (as singles) whereas the other half form pairs (soliton dimers) with a strong magnetic coupling. The Rabi oscillations of both the single soliton and the soliton dimer are observed, which is a prerequisite in the context of quantum information.

Single-crystal x-ray diffraction recently identified the space group of the weakly correlated yet exotic time reversal symmetry (TRS) breaking superconductor $\mathrm{La}\mathrm{Ni}{\mathrm{Ga}}_2$ to be nonsymmorphic $Cmcm$. This symmetry causes band sticking on a zone face pierced by four Fermi surfaces, leading to two nodal Fermi lines and a nodal Fermi loop on that zone face $k_z=\frac{\pi }{c}$. These line singularities are examples of perfectly flat (in energy and in geometry) nodal structures that lie precisely at a single energy, with that energy being the Fermi level $E_F$, even under variation of the carrier density. Projections onto surfaces perpendicular to that zone face produce collapsed drumhead state regions of zero area on the edges of the surface Brillouin zone. Although small by most measures, spin-orbit coupling splits the line and loop degeneracies on the Fermi surfaces (FSs) in the normal state except at two symmetry-related Dirac points, which topologically are locally dispersionless in one direction (zero velocity and infinite mass) while linear in the other two directions. The band sticking and distinctive Fermi surface placed Dirac points are most impactful in establishing $\mathrm{La}\mathrm{Ni}{\mathrm{Ga}}_2$ as a topological superconductor in the bulk, with the degenerate FSs providing a natural platform for the superconducting order-parameter symmetry necessary for describing this TRS breaking superconductor. Unlike most other crystal symmetries, spin-orbit coupling leaves intact a nodal line piercing the FS, resulting in Dirac points situated at $E_F$ lying along nodal lines. Additionally, the degeneracy exactly at the Fermi energy is central in placing $\mathrm{La}\mathrm{Ni}{\mathrm{Ga}}_2$ precisely at a Lifshitz FS instability, independent of band filling and protected by $Cmcm$ symmetry. The eight-band low-energy Bogoliubov–de Gennes quasiparticle spectrum is presented along one dispersive direction for varying strengths of TRS breaking. We include a discussion of energetics of gauge symmetry and magnetic order resulting from TRS breaking, incorporating information from experimental data. These results suggest a scenario where TRS breaking rather than gauge-symmetry breaking (superconductivity) might be the driving order parameter.

We examine the effect of repulsion on superconductivity in a three-dimensional system with a Bardeen-Pines-like interaction in the low-density limit, where the chemical potential $\mu$ is much smaller than the phonon frequency ${\omega }_L$. We parametrize the strength of the repulsion by a dimensionless parameter $f$, and find that the superconducting transition temperature $T_c$ approaches a nonzero value in the $\mu =0$ limit as long as $f$ is below a certain threshold $f^{*}$. In this limit, we find that $T_c$ goes to zero as a power of $f^{*}-f$, in contrast to the high density limit, where $T_c$ goes to zero exponentially quickly as $f$ approaches $f^{*}$. For all nonzero $f$, the gap function $\mathrm{\Delta }\left({\omega }_m\right)$ changes sign along the Matsubara axis, which allows the system to partially overcome the repulsion at high frequencies. We trace the position of the gap node with $f$ and show that it approaches zero frequency as $f$ approaches $f^{*}$. To investigate the robustness of our conclusions, we then go beyond the Bardeen-Pines model and include full dynamical screening of the interaction, finding that $T_c$ still saturates to a nonzero value at $\mu =0$ when $f<f^{*}$.

We investigate the scaling behavior between Hall resistivity (${\rho }_{xy}$) and longitudinal resistivity (${\rho }_{xx}$) in the mixed state and the vortex phase diagram for ${\mathrm{FeSe}}_{0.7}{\mathrm{Te}}_{0.3}$ (FST) thin films. The ${\rho }_{xy}$ and the ${\rho }_{xx}$ are simultaneously measured as functions of temperature and magnetic field, and ${\rho }_{xy}(H,T)$ is expressed using the power law relation ${\rho }_{xy}\left(H,T\right)=A{\rho }_{xx}^{\beta }\left(H,T\right)$. Interestingly, FST thin films show a two-slope behavior in the power law relation with different exponent values of ${\beta }_1$ and ${\beta }_2$. For the temperature sweep ($T$ sweep) at a fixed magnetic field, ${\beta }_1 (=2.0\pm 0.16)$ is insensitive to the magnetic field, whereas for the magnetic field sweep ($H$ sweep) at a fixed temperature, the value significantly increases from 1.93 to 3.82 with an increase in temperature. On the other hand, changes in the ${\beta }_2$ value are relatively small for both cases. Two ${\beta }_1$ and ${\beta }_2$ values result in two vortex liquid regimes, which could be ascribed to different pinning strengths within the ${\beta }_1$ and ${\beta }_2$ regions. In addition, the tangent of the Hall angle $\left(\tan {\theta }_{\mathrm{H}}\right)$ with respect to the magnetic field exhibits a crossover behavior at the critical field $H^{*}$, which coincides with the magnetic field corresponding to the boundary of the ${\beta }_1$ and ${\beta }_2$ regimes. These results suggest that various β values are possible in FST thin films and are closely related to the flux pinning characteristics.

We establish the nonequilibrium thermal phases of a voltage driven antiferromagnetic Mott insulator in three dimensions, realized at steady state under a voltage bias. Starting from the Keldysh action for the half-filled Hubbard model, we derive an effective Langevin equation for the “slow” magnetic variables. The coupling of electrons to these degrees of freedom determine the transport properties. At low temperature we find a voltage-driven discontinuous insulator-metal transition, along with hysteresis. We map the suppression of the Néel temperature $T_N$ and pseudogap temperature $T_{\text{pg}}$ with increasing voltage, and discover that the biased Mott insulator has a finite temperature insulator-metal transition. The low temperature results resolve an experimental puzzle about hysteresis, and the thermal results make testable predictions on spectra and nonlinear transport.

Pressure in halide perovskites attracts extensive attention recently as an effective tool for band-gap engineering. Here, combining first-principles calculations and symmetry-mode analyses, we give a general insight into the role of pressure in inorganic halide perovskites and provide a complete and consistent description of the evolution of band gap that observed in high-pressure experiments. We reveal that strain-phonon and phonon-phonon couplings are the essential factors determining the band-gap evolution. The subtle interplay between strain-phonon and phonon-phonon couplings triggers the increase of out-of-phase tilt at a larger pressure, which results in the simultaneous increase of the band gap. Additionally, we point out that the bond lengths vary continuously, and their nonlinear behaviors originate from strain-phonon coupling instead of the stiffening of the volume. With this knowledge, we propose that epitaxial compressive strain continuously decreases the tilt distortion, and reduction of band gap of 0.5 eV is achieved in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_3$ by 5% compressive strain, which may dramatically enhance the energy conversion efficiency.

In order to explore the properties of a two-sublattice ferroelectric, we measured the infrared and Raman scattering response of ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ across the ferroelectric and glassy transitions and compared our findings to a symmetry analysis, calculations of phase stability, and lattice dynamics. In addition to uncovering displacive character and a large hysteresis region surrounding the ferroelectric transition temperature $T_{\mathrm{C}}$, we identify the vibrational modes that stabilize the polar phase and confirm the presence of two ferroelectric variants with opposite polarizations. Below $T_{\mathrm{C}}$, a poorly understood relaxational or glassy transition at $T_{\mathrm{g}}$ is characterized by local structure changes in the form of subtle peak shifting and activation of low frequency out-of-plane Cu- and In-containing modes. The latter are due to changes in the Cu/In coordination environments and associated order-disorder processes. Moreover, $T_{\mathrm{g}}$ takes place in two steps with another large hysteresis region and significant underlying scattering. Combined with imaging of the room temperature phase separation, this effort lays the groundwork for studying ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ under external stimuli and in the ultrathin limit.

A two-dimensional electron gas in a static external magnetic field exhibits two distinct collective excitation modes. The lower frequency mode propagates along the periphery of the domain almost freely with an extended lifetime, which is referred to as edge magnetoplasmons. Peculiar phenomena caused by a capacitive interaction between nearest-neighbor domains are known, such as the emergence of Tomonaga-Luttinger liquid and charge-density fractionalization. Meanwhile, the number of coupled domains investigated in the past has been limited to a small number. Here, we performed calculations using a continuum model of edge magnetoplasmons, the band structures of planar crystals composed of an arbitrary number of domains, including a chain, ladder, and honeycomb network, with the general interaction strength. We explain the band structures in terms of the fundamental collective modes of a molecule composed of two equivalent domains. These are the extended chiral propagation modes that yield a linear dispersion band and the standing wave modes localized in the coupled regions that cause a flat band. The chain's band structures resemble the miniband structures calculated from the Kronig-Penney model for the electron in a semiconductor superlattice. We point out that a geometrical deformation of the chain does not change the band structures as it can be expressed as a gauge degree of freedom that only causes a shift in the wave number.