Spin triplet superconductivity in the Kondo lattice ${\mathrm{UTe}}_2$ appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of superconductivity under pressure, which discontinuously transitions to magnetic order, likely of ferromagnetic nature, at higher pressures. The application of a magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in ${\mathrm{UTe}}_2$. As the superconductivity and magnetism exist on two opposite sides of the first-order phase boundary, our results indicate other microscopic mechanisms may be playing a role in stabilizing spin triplet superconductivity in addition to spin fluctuations associated with magnetism.

Structural characterization is essential to material engineering, but few tools can detect structural properties in the time domain. High harmonic generation (HHG) emerges as a new frontier that touches the heart of condensed-matter physics from the symmetry to quantum geometrical nature of electrons, but its capability in structural characterization has not been materialized. Here, we establish a crucial connection between the symmetry of a material and the helicity of light. We employ monolayer ${\mathrm{MoS}}_2$ as an example. We show that a linearly polarized laser pulse used in experiments is not ideal for structural characterization because it only generates in-plane anisotropy. It is the circularly polarized laser field that is capable of producing four distinctive HHG signals from the four phases of ${\mathrm{MoS}}_2$. This finally links the laser helicity to the crystal structure. The results are generic and are not affected by the Berry curvature, the interband or intraband contribution. Our study unleashes the power of HHG as a structural characterization tool for technologically important materials.

Cerium electronic multipoles in trigonal symmetry viewed in magnetic neutron diffraction are investigated. Analytic expressions for all the magnetic multipoles, and radial integrals calculated with an established atomic code, are used to scrutinize a recent claim to have observed an octupole-ice configuration in a powder sample of pyrochlore $\mathrm{C}{\mathrm{e}}_2\mathrm{S}{\mathrm{n}}_2{\mathrm{O}}_7$. Three equally plausible suites of multipoles belonging to uncorrelated cerium ions give equally satisfactory accounts of the available diffraction pattern. Our multipoles are suitable for future experiments using Bragg diffraction from a compound that supports long-range magnetic order.

Using micromagnetic simulations, we have modeled the formation of imprinted merons and antimerons in cobalt overlayers of different thickness (1–8 nm), stabilized by interfacial exchange with antiferromagnetic vortices in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. Structures similar to those observed experimentally could be obtained with reasonable exchange parameters, also in the presence of surface roughness. We produce simulated meron and antimeron images by magnetic force microscopy and nitrogen-vacancy center microscopy, and established signatures of these topological structures in different experimental configurations.

Partial bosonization of the two-dimensional Hubbard model focuses the functional renormalization flow on channels in which interactions become strong and local order sets in. We compare the momentum structure of the four-fermion vertex, obtained on the basis of a patching approximation, to an effective bosonic description. For parameters in the antiferromagnetic phase near the onset of local antiferromagnetic order, the interaction of the electrons is indeed well described by the exchange of collective bosonic degrees of freedom. The residual four-fermion vertex after the subtraction of the bosonic-exchange contribution is small. We propose that similar partial bosonization techniques can improve the accuracy of renormalization flow studies also for the case of competing order.

Motivated by recent experimental studies that have found signatures of a correlated insulator phase and tuning superconductivity in twisted bilayer graphene, we study the temperature-dependent conductivity, the spin correlation, and the superconducting pairing correlation within a two-orbital Hubbard model on an emergent honeycomb lattice. The evaluation of the temperature dependence of the conductivity demonstrates that there is a metal-insulator transition and the Mott phase at strong coupling is accompanied by antiferromagnetic order. The electronic correlation drives a $d+id$ superconducting pairing to be dominant over a wide filling region. All of the dc conductivity, the spin correlation, and the superconductivity are suppressed as the interlayer coupling strength increases, and the critical $U_c$ for the metal-insulator transition is also reduced. Our intensive numerical results reveal that twisted bilayer graphene should be a uniquely tunable platform for exploring strongly correlated phenomena.

We investigate the metal to nonmagnetic insulator (MI) transition of MnP-type Ru pnictide RuP using hard x-ray and ultraviolet photoemission spectroscopies. The spectral weight at $E_{\mathrm{F}}$ is suppressed below the MI transition temperature $T_{\mathrm{MI}}$, while there is no appreciable change across the pseudogap temperature $T_{\mathrm{PG}}$. The estimated energy scale of the gap is $\sim 110$ meV, which is in good correspondence to the spin gap opening observed in the previous NMR study. According to the band structure calculation, the density of states at $E_{\mathrm{F}}$ mainly originates from narrow bands of the Ru $4d_{xy}$ orbitals. Our results suggest that the fourfold-degenerate Ru $4d_{xy}$ orbitals are deeply related to the MI transition. Based on the photoemission results and the band structure calculation, we argue the possible origin of the MI transition for polycrystalline RuP.

Absence of quasiparticle-level anticrossing in random non-Hermitian systems is demonstrated. As a result, the general Wigner-Dyson distributions of energy-level spacing of diffusive metals in the usual Hermitian systems is replaced by the Poisson distribution for quasiparticle-level (the real parts of complex eigenenergies) spacing of non-Hermitian disordered metals in the thermodynamic limit of infinite system size. This is a very surprising result because Poisson statistics is universally true for the Anderson insulators where energy eigenstates do not overlap with each other, so energy levels are independent from each other. For disordered metals where different eigenstates overlap with each other, one should expect different levels trying to stay away from each other, so the Poisson distribution should not apply there. Our results show that the larger non-Hermitian energy (dissipation) can invalidate the quasiparticle-level anticrossing that holds dearly in Hermitian metals. Thus, our theory provides a unified picture for recently discovered “level attraction” in various systems, as well as a theoretical basis for manipulating quasiparticle energy levels.

We demonstrate the strong coupling of a quantum dot and a graphene spherical shell coating it. Our simulations are based on solutions of three-dimensional Maxwell equations, using a boundary element method approach. Interaction between the nanostructures produces sharp hybrid modes, even when the two are off-resonant. The coupling of the light to these “very sharp” plexcitonic resonances is an order of magnitude larger than its coupling to a quantum dot, and they are voltage tunable (continuously) in an 80-meV interval. Hence, our results are very attractive for sensing applications and graphene display technologies with sharper colors. Moreover, on a simple theoretical model, we explain why such sharp highly tunable hybrid resonances emerge.

Microwave-induced resistance oscillations (MIROs) and zero-resistance states (ZRSs) occur in high-mobility two-dimensional electron gas exposed to microwave (MW). We observe that the velocity shift ($\mathrm{\Delta }v/v$) oscillates in anticorrelation with MIRO, and $\mathrm{\Delta }v/v$ shows peaks at the minimal resistance of MIRO or at ZRS. The SAW velocity features of ZRS remain robust even in the absence of external driving current, which suggests the involvement of intrinsic mechanism in the nonequilibrium phase. In addition, under high-power MW, the phase (${\phi }_{\mathrm{ac}}$) of ZRS stays constant at about 1/4, whereas the phase of the transitions in MIRO is reduced to below 0.10. We argue that the peaks of SAW velocity at ZRS may result from the inhomogeneity of superposed current domain structures. Moreover, a multiphoton process around $\varepsilon =1/2$ is observed in the SAW measurements.

We employ a first-principles cluster-expansion method in combination with canonical Monte Carlo simulations to study the effect of spin-orbit coupling on the alloying behavior of multilayer ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$. Our simulations reveal that spin-orbit coupling plays an essential role in determining the configurational thermodynamics of Bi and Sb atoms. Without the presence of spin-orbit coupling, ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ is predicted to exhibit at low-temperature chemical ordering of Bi and Sb atoms, leading to formation of an ordered structure at $x\approx 0.5$. Interestingly, the spin-orbit-coupling effect intrinsically induced by the existence of Bi and Sb results in the disappearance of chemical ordering of the constituent elements within an immiscible region existing at $T$ < 370 K, and consequently ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ displays merely a tendency toward local segregation of Bi and Sb atoms, resulting in coexistence of Bi-rich and Sb-rich ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ solid solutions without the formation of any ordered structure of ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ as predicted in the absence of spin-orbit coupling. These findings distinctly highlight an influence of spin-orbit coupling on the alloying behavior of ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ and probably other alloys composed of heavy elements, where the spin-orbit-coupling effect is supposed to be robust.

Surface roughness plays an important role on the magnetotransport properties of thin films, especially in ultrathin films. In this work, we prepared Fe thin films with various surface roughness by using different seed layers and studied the electrical transport and anomalous Hall effect. By tuning surface roughness scattering, the longitudinal resistivity (${\rho }_{xx}$) measured at 5 K increases by one order of magnitude and the corresponding anomalous Hall resistivity (${\rho }_{\mathrm{AHE}}$) increases by three times with increasing roughness. The intrinsic, skew-scattering, and side-jump contributions to ${\rho }_{\mathrm{AHE}}$ were separated from our data. The anomalous Hall angle depends on the surface roughness, which may be of importance to the material engineering for achieving large spin Hall angle.

Complex magnetic orders in frustrated magnets may exhibit rich melting processes when the magnet is heated toward the paramagnetic phase. We show that one may tune such melting processes by quantum fluctuations. We consider a kagome lattice dipolar Ising model subject to transverse field and focus on the thermal transitions out of its magnetic ground state, which features a $\sqrt{3}\times \sqrt{3}$ magnetic unit cell. Our quantum Monte Carlo (QMC) simulations suggest that, at weak transverse field, the $\sqrt{3}\times \sqrt{3}$ phase melts by way of an intermediate magnetic charge ordered phase where the lattice translation symmetry is restored while the time reversal symmetry remains broken. By contrast, at stronger transverse field, QMC simulations suggest the $\sqrt{3}\times \sqrt{3}$ order melts through a floating Kosterlitz-Thouless phase. The two distinct melting processes are separated by either a multicritical point or a short line of first order phase transition.

Both the vibrational and electrical transport properties of $2H\text{−}{\mathrm{TaS}}_2$ have been investigated at high pressures and low temperatures. The collapse of the charge-density-wave order at pressures above 7.3 GPa has been verified by Raman scattering, resistivity, and Hall coefficient measurements. For pressures above the critical pressure of 7.3 GPa, the superconducting transition temperature continues to increase and reaches its maximum value at 11.5 GPa, suggesting that it is not a simple competition between the charge-density-wave order and superconductivity. Through the standard resistivity fit in the normal state, the decline of the superconducting transition temperature with increasing pressure up to 47.0 GPa is due to the decrease of interaction strength and the increase of the impurity scattering. These results are very important in understanding the superconducting mechanism of transition-metal dichalcogenides.

The ground state of the $S=\frac{1}{2}$ kagome Heisenberg antiferromagnet is now recognized as a spin liquid, but its precise nature remains unsettled, even if more and more clues point towards a gapless spin liquid. We use high-temperature series expansions (HTSEs) to extrapolate the specific heat $c_V\left(T\right)$ and the magnetic susceptibility $\chi \left(T\right)$ over the full temperature range, using an improved entropy method with a self-determination of the ground-state energy per site $e_0$. Optimized algorithms give HTSE coefficients up to unprecedented orders (20 in $1/T$) and as exact functions of the magnetic field. Three extrapolations are presented for different low-$T$ behaviors of $c_V$: exponential (for a gapped system), and linear or quadratic (for two different types of gapless spin liquids). We study the effects of various perturbations to the Heisenberg Hamiltonian: Ising anisotropy, Dzyaloshinskii-Moriya interactions, second- and third-neighbor interactions, and randomly distributed magnetic vacancies. We propose an experimental determination of $\chi (T=0)$, which could be nonzero, from $c_V$ measurements under different magnetic fields.

We consider a Hamiltonian description of the vibrations of a clamped, elastic circular plate. The Hamiltonian of this system features a potential energy with two distinct contributions: one that depends on the local mean curvature of the plate and a second that depends on its Gaussian curvature. We quantize this model using a complete, orthonormal set of eigenfunctions for the clamped, vibrating plate. The resulting quanta are the flexural phonons of the thin circular plate. As an application, we use this quantized description to calculate the fluctuations in the displacement of the plate for arbitrary temperature. We compare the fluctuation profile with that from an elastic membrane under tension. At low temperature, we find that while both profiles have a circular ring of local maxima, the ring in the membrane profile is much more pronounced and sharper. We also note that with increasing temperature the plate profile develops two additional rings of extrema.

Measurements of induced thermal convection have been used to study fluid viscosity at simultaneous high pressure and temperature conditions. Direct observations of flow were made by tracking entrained particles in samples melted by laser heating during high-pressure confinement. Finite element models confirmed thermal convection as the origin of the detected motions, and were refined to assess the fluid viscosity. Observations of flow in ethanol partially melted in the laser-heated diamond anvil cell at 2–3 GPa point to a sharply rising viscosity at room temperature above the equilibrium solidification pressure, similar to that seen previously in methanol. The analysis shows that measurement of viscosity from convective flow in laser-heated fluids under static pressure is a promising strategy to determine viscosity at ultrahigh pressures, where high melting temperatures and small samples preclude application of traditional viscometric techniques. The data confirm theoretical predictions of detectable natural convection at ultralow Rayleigh numbers ($\mathrm{Ra}\ll 1$) in a microscopic system having sufficiently large temperature gradients.

The magnetic hedgehog lattice (HL) is a noncoplanar magnetic texture with a periodic array of magnetic monopoles and antimonopoles. Despite phenomenological and numerical studies thus far, there remain open issues on the microscopic origin, especially with respect to the recent experimental findings of two different types of HLs even at zero magnetic field. Here, we study the stability of the HLs for an effective spin model with long-range interactions arising from the itinerant nature of electrons. By variational calculations and simulated annealing, we find that the HLs are stabilized in the ground state at zero magnetic field by the synergetic effect of the antisymmetric exchange interactions generated by the spin-orbit coupling and the multiple-spin interactions generated by the spin-charge coupling. We also clarify the phase diagram in the magnetic fields, which includes topological phase transitions with pair annihilation of the monopoles and antimonopoles depending on the field directions.