Reentrant superconductivity has been observed in the candidate spin-triplet superconductor ${\mathrm{UTe}}_2$ as a function of the magnetic field applied along the hard axis. As resistivity measurements have shown, a broadened superconducting transition appears near the minimal $T_c$, highlighting the importance of superconducting fluctuations in this regime. We present a phenomenological study assuming a field-driven first-order transition between two superconducting states. We show that with quenched randomness, inhomogeneity-enhanced superconducting fluctuations near the transition could naturally account for both the reentrant superconductivity as well as the broadened superconducting transition.

Conformal field theories (CFTs) have been used extensively to understand the physics of critical lattice models at equilibrium. However, the applicability of CFT calculations to the behavior of the lattice systems in the out-of-equilibrium setting is not entirely understood. In this work, we compare the CFT results of the evolution of the entanglement spectrum after a quantum quench with numerical calculations of the entanglement spectrum of the three-state Potts model using matrix product state simulations. Our results lead us to conjecture that CFT does not describe the entanglement spectrum of the three-state Potts model at long times, contrary to what happens in the Ising model. We thus numerically simulate the out-of-equilibrium behavior of the Potts model according to the CFT protocol, i.e., by taking a particular product state and “cooling” it, then quenching to the critical point and finding that, in this case, the entanglement spectrum is indeed described by the CFT at long times.

Motivated by the recently synthesized layered material ${\mathrm{MoSi}}_2{\mathrm{N}}_4$, we investigate excitonic response of quasiparticle of monolayer ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ by using $G_0{W}_0$ and Bethe-Salpeter equation calculations. With a dually sandwiched structure consisting of a central ${\mathrm{MoN}}_2$ layer analog of $2H\text{−}{\mathrm{MoS}}_2$ capped with silicon-nitrogen (SiN) honeycomb outer layers, ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ possesses frontier orbitals confined at the central ${\mathrm{MoN}}_2$ layer with similar subvalley at the $K$ point as $2H\text{−}{\mathrm{MoS}}_2$. The valley splitting ($\sim 130\mathrm{meV}$) due to the spin-orbital coupling gives rise to a doublet in the spectrum. Excitons in ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ show a strong binding energy up to 0.95 eV with the optical band gap of 2.44 eV. Both electronic and optical gaps are highly sensitive to tensile strains and become redshifted, albeit a marginal change of exciton binding energy. With the protection of capped SiN layers, quantum confined excitons in ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ without the need of additional passivation layer like BN would provide a bright platform for robust emission with partially screened disturbance from environment.

The multipartite non-Hermitian Su-Schrieffer-Heeger model is explored as a prototypical example of one-dimensional systems with several sublattice sites for unveiling intriguing insulating and metallic phases with no Hermitian counterparts. These phases are characterized by composite cyclic loops of multiple complex-energy bands encircling single or multiple exceptional points (EPs) on the parametric space of real and imaginary energy. We show the topology of these composite loops is similar to well-known topological objects like Möbius strips and Penrose triangles, and can be quantified by a nonadiabatic cyclic geometric phase which includes contributions only from the participating bands. We analytically derive a complete phase diagram with the phase boundaries of the model. We further examine the connection between encircling of multiple EPs by complex-energy bands on parametric space and associated topology.

Tensor network states (TNSs) offer an efficient representation for the ground state of quantum many-body systems and play an important role in the simulations of them. Numerous TNSs have been proposed in the past few decades. However, due to the high cost of TNS for two-dimensional systems, a balance between the encoded entanglement and computational complexity of TNS is yet to be reached. In this work we introduce a new tree tensor network-based (TTN-based) TNS dubbed a “fully augmented tree tensor network (FATTN) by releasing the constraint in augmented tree tensor network (ATTN). When disentanglers are augmented in the physical layer of TTN, FATTN can provide more entanglement than TTN and ATTN. At the same time, FATTN maintains the scaling of computational cost with bond dimension in TTN and ATTN. Benchmark results on the ground-state energy for the transverse Ising model are provided to demonstrate the improvement of accuracy of FATTN over TTN and ATTN. Moreover, FATTN is quite flexible, which can be constructed as an interpolation between TTN and the multiscale entanglement renormalization ansatz to reach a balance between the encoded entanglement and the computational cost.

Intervalence charge transfer (IVCT) refers to the transfer of electrons between two metal ($M$) sites with different oxidation states, through a bridging ligand: $M^{n+1}+M^{\prime m}\to M^n+M^{\prime m+1}$. It is considered that the IVCT is related to the hopping probability of electrons (or electron mobility) in solids. Controlling the conductivity of ferromagnetic semiconductors (FMSs) is critical for device applications, and thus, the manipulation of conductivity through IVCT may be an approach of band engineering in FMSs. In Ru-doped $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4$ (CFO), which shows ferrimagnetism and semiconducting transport properties, the reduction in electric resistivity is attributed to both carrier doping caused by the Ru substitution of Co and increase in carrier mobility due to hybridization between the wide Ru $4d$ and Fe $3d$ orbitals. The latter indicates the so-called IVCT mechanism, i.e., charge transfer between the mixed valence ${\mathrm{Fe}}^{2+}/{\mathrm{Fe}}^{3+}$ states, facilitated by the bridging Ru $4d$ orbital, expressed as ${\mathrm{Fe}}^{2+}+{\mathrm{Ru}}^{4+}\leftrightarrow {\mathrm{Fe}}^{3+}+{\mathrm{Ru}}^{3+}$. To elucidate the emergence of the IVCT state, we conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) measurements on undoped CFO and Ru-doped ${\mathrm{Co}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ (CRFO) thin films. The XAS and RPES spectra indicated the presence of mixed valence ${\mathrm{Fe}}^{2+}/{\mathrm{Fe}}^{3+}$ states and hybridization between the Fe $3d$ and Ru $4d$ states. These results provide experimental evidence for the IVCT state in CRFO, demonstrating a mechanism that controls electron mobility through hybridization between the $3d$ transition metal cations with intervening $4d$ states.

We investigate ferromagnetic resonance (FMR) modulation in $d$-wave superconductor (SC)/ferromagnetic insulator (FI) bilayer systems theoretically. The modulation of the Gilbert damping in these systems reflects the existence of nodes in the $d$-wave SC and shows power-law decay characteristics within the low-temperature and low-frequency limit. Our results indicate the effectiveness of the use of spin pumping as a probe technique to determine the symmetry of unconventional SCs with high sensitivity for nanoscale thin films.

We study magneto-optical (MO) properties of a semi-Dirac system in presence of a perpendicular magnetic field using kernel polynomial method (KPM) based on the Keldysh formalism in few hundreds of the terahertz (THz) frequency regime. For the semi-Dirac case, the band structure demonstrates a linear (Dirac-like) dispersion along the $y$ direction, while it has a quadratic (nonrelativistic) behavior along the $x$ direction in the Brillouin zone. For comparison, we have also included results for the Dirac systems as well, so that the interplay of the band structure deformation and MO conductivities can be studied. We have found that the MO conductivity shows features in the semi-Dirac system that are quite distinct from the Dirac case in the above mentioned frequency regime. The real parts of the longitudinal MO conductivities, namely $\mathrm{Re}\left({\sigma }_{xx}\right)$ and $\mathrm{Re}\left({\sigma }_{yy}\right)$ (which are different in the semi-Dirac case, as opposed to a Dirac one), present a series of resonance peaks as a function of the incident photon energy. We have also found that the absorption peaks corresponding to the $y$ direction are larger (roughly one order of magnitude) than those corresponding to the $x$ direction for the semi-Dirac case. In the case of the MO Hall conductivity, that is, $\mathrm{Re}\left({\sigma }_{xy}\right)$, there are extra peaks in the spectra compared to the Dirac case, which originate from the distinct optical transitions of the carriers from one Landau level to another. These peaks are otherwise absent in a Dirac system, where some of these peaks result from transitions between pairs of Landau levels that differ by the same energy and hence those peaks can not be resolved. We have also explored how the carrier concentration influences the MO conductivities. In the semi-Dirac case, there is the emergence of additional peaks yet again in the absorption spectrum underscoring the presence of an asymmetric dispersion compared to the Dirac case. Further, we have explored the interplay between the polarization of the incident beam and the features of the absorption spectra, which can be probed in experiments. Finally, we evaluate the MO activity of the medium by computing the Faraday rotation angle ${\theta }_F$. The semi-Dirac case shows two maxima and two minima of ${\theta }_F$ at different photon energies, which can be captured in experiments.

CePdAl is a prototypical frustrated Kondo lattice with partial long-range order (LRO) at $T_{\mathrm{N}}=2.7$ K. Previous bulk experiments under hydrostatic pressure found signatures for a quantum critical regime that extends from $p_{\mathrm{c}}\approx 0.9$ GPa, where LRO disappears, up to $\sim 1.7$ GPa. We employed extensive muon spin relaxation and rotation experiments under pressure. The continuous and complete suppression of LRO at $p_{\mathrm{c}}$ is confirmed. Above $T_{\mathrm{N}}\left(p\right)$ and beyond $p_{\mathrm{c}}$, an additional crossover scale $T^{*}\left(p\right)$ characterizes the change from pure to stretched exponential relaxation in zero field. Remarkably, $T^{*}\left(p\right)$ agrees with previously determined signatures of entropy accumulation above LRO. This coincidence microscopically evidences fluctuating frustrated spins at $T\le T^{*}$ with spin-liquid behavior. Power-law divergences of the temperature and longitudinal field dependences of the relaxation rate, with time-field scaling, at pressures between $p_c$ and 1.7 GPa characterize this regime as quantum critical.

Quantum states are usually fragile, making quantum computation less stable than classical computation. Quantum correction codes can protect quantum states but need many physical qubits to encode a single logical qubit. Alternatively, protecting quantum states at the hardware level has been recently developed to maintain the coherence of the quantum information by using symmetry. However, it generally has to pay the expense of increasing the complexity of the quantum devices. In this work, we propose to approach the protection of quantum states at the hardware level without increasing the complexity of the devices. The interplay between the spin-orbit coupling and the Zeeman splitting in the semiconductor allows us to tune the Josephson coupling in terms of the spin degree of freedom of Cooper pairs, the hallmark of the superconducting spintronics. This leads to the implementation of the parity-protected 0-$\pi$ superconducting qubit with only one highly transparent superconductor-semiconductor Josephson junction, which makes our proposal immune from the various fabrication imperfections.

We theoretically explore twisted cuprate multilayers, a moiré material family where the individual layers are themselves strongly correlated. We study the twisted t-J model, using a slave-boson mean field treatment that is compatible with Mott physics at small doping. Furthermore, we incorporate the interlayer tunneling form-factor dictated by the symmetry of the Cu $d_{x^2-y^2}$ orbital. Including both these features leads to a phase diagram distinct from earlier weak-coupling treatments that predicted large gap spontaneous topological superconductors. Instead, we find that spontaneous time reversal (T) breaking occurs around twist angle of $\theta ={45}^{\circ }$, but only in a narrow window. Moreover, a nearly gapless superconductor is obtained, whose spectroscopic features parallels that in monolayer cuprates, despite the broken time reversal and reflection symmetries. At smaller $\theta$ however, driving an interlayer current can lead to a gapped topological phase. The energy-phase relation of the interlayer Josephson junction displays notable double-Cooper-pair tunneling which dominates around ${45}^{\circ }$. The $\theta$ dependence of the Josephson critical current and the Shapiro steps are consistent with recent experiments. Utilizing the moiré structure as a probe of correlation physics, e.g., the pair density wave state, is discussed.

We theoretically investigated the fundamental distinction between the intrinsic and extrinsic nonlinear thermal Hall effects in the presence of disorder at the second-order response to the temperature gradient in terms of the semiclassical Boltzmann equation. We found that, at low temperatures, the intrinsic contribution of the nonlinear thermal Hall conductivity is proportional to the square of temperature, whereas the extrinsic contributions (side jump and skew scattering) are independent of temperature. This distinct dependency on temperature provides an approach to readily distinguish between the intrinsic and extrinsic contributions. Specifically, we analyzed the nonlinear thermal Hall effect for a tilted two-dimensional massive Dirac material. In particular, we showed that when the Fermi energy is located at the Dirac point, the signal is solely from the intrinsic mechanism; when the Fermi energy is higher, the extrinsic contributions are dominant, which are two to three orders of magnitude larger than the intrinsic contribution.

We present an exact full symmetry analysis of the $0\text{−}\pi$ superconducting circuit. We identify points in control parameter space of enhanced anomalous symmetry, which imposes robust twofold degeneracy of its ground state, that is for all values of the energy parameters of the model. We show, both analytically and numerically, how this anomalous symmetry is maintained in the low-energy sector, thus providing us with a strong candidate for robust qubit engineering.

We consider a system of fermions with local interactions on a lattice (Hubbard model) and apply a novel extension of Laplace's method (saddle-point approximation) for evaluating the corresponding partition function. There, we introduce dual free bosonic fields, with a propagator corresponding to an effective (renormalized) interaction with Maki-Thompson and Aslamazov-Larkin type corrections and beyond, and demonstrate that the superconducting pairing originates as an instability of the effective interaction. We derive the corresponding Bethe-Salpeter equation (instability criterion) and show that the interaction enters the equation only in its effective form to all orders, including the exchange part of the self-energy. An important implication of this result is that the effective interaction always remains finite, even at phase-transition points, directly contradicting the often used assumption of linear relationship between the interaction and susceptibility, established within the random-phase approximation. By analyzing the Bethe-Salpeter equation in the context of unconventional superconductivity, we find that the presence of a flat band close the Fermi level, found in materials such as twisted bilayer graphene, has a twofold favorable impact persisting beyond the weak-coupling approximation: a reduced kinetic energy cost of the gap formation and an increased anisotropy of the effective interaction, favoring a momentum dependent order parameter.

Phonon engineering is a novel and effective approach to tailor the thermal conductivity for the thermoelectric performance and heat dissipation. In general, the acoustic phonons rather than the optical phonons are dominant in heating carriers. Here we report an unprecedented large contribution, 47% overall, from the low-frequency optical phonons to the in-plane thermal conductivity in 2H molybdenum disulfide, revealed by low-wave-number high-pressure Raman technology assisted with first-principles calculations. The analysis of phonon dispersion curves and Grüneisen parameters of individual phonon modes reveals that the large contribution originates in a joint effect of the large group velocity of low-frequency optical phonons and their strong anharmonic effects. The joint effect is continuously maintained when pressure increases, up to 20 GPa. Our work provides new insights into the optical phonon transport, paving the way for the phonon engineering and thermal management.

Periodically driven (Floquet) systems are said to prethermalize when their energy absorption is very slow for a long time. This effect was first discovered in quantum spin models, where the heating rate is exponentially small in the ratio between the driving frequency and the spin bandwidth. Recently, it was shown that prethermalization occurs also in classical systems with an infinite bandwidth. Here, we address the open question of which small parameter controls the lifetime of the prethermal state in these systems. We show that the lifetime is controlled by the temperature of the prethermal state, which is quasiconserved when the heating is slow. We substantiate this finding in systems of periodically driven coupled rotors, by studying the dependence of the prethermal lifetime on both the initial conditions and the connectivity. This result allows us to develop a simple analytical model that describes the crossover from prethermalization to chaos in many-body classical systems.

Chiral magnetism defines the spin structure sense of rotation in magnetic films and stabilized by the interfacial Dzyaloshiniskii-Moriya interaction (DMI), which can be used to generate the chiral nature of magnetic textures like spin spirals and skyrmions. Here, the direct evidence of the interlayer DMI was observed at room temperature, by designing the synthetic system with a ferromagnet/insulating spacer/ferromagnet structure whose magnetic chirality can be effectively manipulated between ferromagnetic coupling and antiferromagnetic coupling by changing spacer thickness. The interlayer DMI breaks the symmetry of the magnetic reversal process, leading to chiral exchange-biased Hall loops, where the noncollinear magnetic states were systematically characterized and quantified by using polarized neutron reflectometry (PNR). PNR results indicate that the maximum angle of the canted magnetic moments for ferromagnetic coupling can reach as high as $11.5{}^{\circ }$, which is stronger than that for antiferromagnetic coupling, suggesting the higher energy excitation of magnetic chirality. This canted spin structure is verified by first-principles calculation. Our findings should be greatly useful for the interfacial design of spintronic devices to control and tailor the magnetic chirality for the formation of the spin texture in high-density memory.

We determine the zero- and finite temperature phase diagram of the fully frustrated quantum Ising model on the bathroom tile (4-8) lattice. The phase diagram exhibits a wealth of $2+1\mathrm{d}$ physics, including (1) classical Coulomb dimer liquids of both square and triangular lattice types; (2) quantum order-by-disorder induced phases breaking ${\mathbb{Z}}_4, {\mathbb{Z}}_6$, and ${\mathbb{Z}}_8$ symmetries; (3) finite temperature Kosterlitz-Thouless phases floating over the ${\mathbb{Z}}_6$ and ${\mathbb{Z}}_8$ orders; and (4) staircases of (in)-commensurate symmetry-breaking phases at intermediate coupling. We establish this elaborate phase diagram using a combination of dimer model mapping, perturbation theory, Landau analysis and Stochastic Series Expansion Quantum Monte Carlo. Our results provide a baseline for studying frustrated magnetism with D-Wave architecture annealers, where the 4-8 lattice can be embedded naturally without “cloning,” reducing the number of competing energy scales. Simulations with the D-Wave 2000Q demonstrate qualitative agreement with the high-temperature portion of the phase diagram, but are unable to access the low-temperature phases.