We consider a finite quantum system under slow driving and weakly coupled to thermal reservoirs at different temperatures. We present a systematic derivation of the quantum master equation for the density matrix and the out-of-time-order correlators. We start from the microscopic Hamiltonian and we formulate the equations ruling the dynamics of these quantities by recourse to the Schwinger-Keldysh nonequilibrium Green's function formalism, performing a perturbative expansion in the coupling between the system and the reservoirs. We focus on the adiabatic dynamics, which corresponds to considering the linear response in the ratio between the relaxation time due to the system-reservoir coupling and the time scale associated to the driving. We calculate the particle and energy fluxes. We illustrate the formalism in the case of a qutrit coupled to bosonic reservoirs and of a pair of interacting quantum dots attached to fermionic reservoirs, also discussing the relevance of coherent effects.

We propose a unified scheme to identify phase transitions out of the ${\mathbb{Z}}_2$ Abelian topological order, including the transition to a non-Abelian chiral spin liquid. Using loop gas and and string gas states [H.-Y. Lee, R. Kaneko, T. Okubo, and N. Kawashima, Phys. Rev. Lett. 123, 087203 (2019)] on the star lattice Kitaev model as an example, we compute the overlap of minimally entangled states through transfer matrices. We demonstrate that, similar to the anyon condensation, continuous deformation of a ${\mathbb{Z}}_2$-injective projected entangled-pair state (PEPS) also allows us to study the transition between Abelian and non-Abelian topological orders. We show that the charge and flux anyons defined in the Abelian phase transmute into the $\sigma$ anyon in the non-Abelian topological order. Furthermore, we show that contrary to the claim in [Phys. Rev. B 101, 035140 (2020)], both the LG and SG states have infinite correlation length in the non-Abelian regime, consistent with the no-go theorem that a chiral PEPS has a gapless parent Hamiltonian.

Weyl particles exhibit chiral transport property under external curved space-time geometry. This effect is called the chiral gravitational effect, which plays an important role in quantum field theory. However, the absence of real Weyl particles in nature hinders the observation of such interesting phenomena. In this paper, we show that chiral gravitational effect can be manifested in Weyl metamaterials with spatially controlled nonlocality. This inhomogeneous modulation results in a spatially dependent group velocity in the Weyl cone dispersion, which is equivalent to introducing a curved background space-time (or gravitational field) for Weyl pseudospinors according to the Clifford algebra. The Weyl equation describing the Weyl cone dispersions is reformulated into the covariant form that contains a nonAbelian gauge field by considering the parallel transport of pseudospinors. The synthetic gravitational field (i.e., space-time curvature) leads to the quantization of energy levels, including chiral zeroth order energy modes (or simply chiral zero modes) that determine the chiral transport property of pseudospinors. Inhomogeneous Weyl metamaterials can potentially become a platform for investigating the interaction between Weyl particles and gravitational fields in future table-top experiments. The theoretical model and the associated results may serve as a guidance for experimentalists to observe such chiral transport effects in photonic Weyl systems.

The ultrafast transport processes of electrons and holes excited by femtosecond laser pulses are essential for a better understanding of carrier dynamics in semiconductors. Transient optical techniques are commonly used to investigate the carrier excitation and transport in these materials. Recently, ultrafast scanning electron microscopy has been employed to elucidate the ballistic dynamics of such hot carriers near a $p\text{−}n$ junction. We provide here a model not only to explain the previously reported superfast expansion of electron-hole clouds but also to predict plasma wave behavior at high laser fluences. In addition to a bipolar junction, we also investigate the well structure of a $p\text{−}n\text{−}p$ junction sandwiched by insulating layers. By confining hot carriers inside junctions, these carriers begin to oscillate as a result of Coulomb interactions among carriers and with the intrinsic field at the depletion zone, leading to the occurrence of terahertz-scale plasma waves for potential applications as pulsed terahertz light sources.

Asymmetric heterointerfaces that bridge two nonisostructural oxides provide valuable opportunities for novel emergent phenomena that may be unavailable for symmetric interfaces. Here we present a theoretical investigation on three different asymmetric interfaces consisting of the infinite-layer nickelate $\mathrm{LaNi}{\mathrm{O}}_2$ and the perovskite manganite $\mathrm{LaMn}{\mathrm{O}}_3$ (type A, B and C). An alternative crystal geometry, pyramid, is introduced when the planar-type $\mathrm{LaNi}{\mathrm{O}}_2$ and the $\mathrm{LaMn}{\mathrm{O}}_3$ are jointed at the interface, resulting in strong charge and orbital reconstruction. For type A interface, the magnetic moment per Mn ion has increased by 10% due to the replacement of $\mathrm{Mn}{\mathrm{O}}_6$ by $\mathrm{Mn}{\mathrm{O}}_5$. For type B interface, in contrast, the magnetic moment grew by 26% for the interfacial Ni ions due to the strong charge transfer between center nickel and apical oxygen. For type C interface, only slightly enhanced $\mathrm{Mn}{\mathrm{O}}_6$ distortions are observed and thus the change of charge and orbital occupancy are negligible. Our results demonstrated that an interface-selective orbital occupancy, where the Mn $e_{\mathrm{g}}$ orbital preferential occupation alternated from the out-of-plane ${\mathrm{d}}_{3z^2-r^2}$ state at type A interface to nearly degenerate at type C interface and then to in-plane ${\mathrm{d}}_{x^2-y^2}$ state at type B interface. The values of relative change of Mn $e_{\mathrm{g}}$ orbital occupancy are 15%, 2%, and −21%, respectively. The values of relative change at type A and B interface are larger than that achieved by strain $(\sim 5\%)$ or symmetric interface design (10%). Therefore, interface reconstructions lead to unusual electronic properties, opening space for the advancement of oxide electronics.

We calculate the density-density response function of biased twisted bilayer graphene (BTBG) and study its plasmon dispersion within the random phase approximation (RPA). At long wavelengths ($q\to 0$), plasmon dispersion shows local classical behavior $\omega ={\omega }_0\sqrt{q}$. Unlike the situation in conventional two-dimensional electron gas (2DEG), where the density dependence of the plasmon energy is of the form ${\omega }_0\propto \sqrt{n}$ ($n$ is the carrier concentration), the plasmon energy ${\omega }_0$ is independent of the carrier concentration ($n$) in biased twisted bilayer graphene. Furthermore, the plasmon energy (${\omega }_0$) is also independent of the Fermi energy ($\mu$) which is decided by the carrier concentration ($n$).

Scaling up spin qubit systems requires high-fidelity single-qubit and two-qubit gates. Gate fidelities exceeding $98\%$ were already demonstrated in silicon-based single and double quantum dots, whereas for the realization of larger qubit arrays, crosstalk effects on neighboring qubits must be taken into account. We analyze qubit fidelities impacted by crosstalk when performing single-qubit and two-qubit operations on neighbor qubits with a simple Heisenberg model. Furthermore, we propose conditions for driving fields to robustly synchronize Rabi oscillations and avoid crosstalk effects. In our analysis, we also consider crosstalk with two neighbors and show that double synchronization leads to a restricted choice for the driving field strength, exchange interaction, and thus gate time. Considering realistic experimental conditions, we propose a set of parameter values to perform a nearly crosstalk-free cnot gate and so open up the pathway to scalable quantum computing devices.

The topological characterization of nonequilibrium topological matter is highly nontrivial because familiar approaches designed for equilibrium topological phases may not apply. In the presence of crystal symmetry, Floquet topological insulator states cannot be easily distinguished from normal insulators by a set of symmetry eigenvalues at high-symmetry points in the Brillouin zone. This work advocates a physically motivated, easy-to-implement approach to enhance the symmetry analysis to distinguish between a variety of Floquet topological phases. Using a two-dimensional inversion-symmetric periodically driven system as an example, we show that the symmetry eigenvalues for anomalous Floquet topological states, of both first order and second order, are the same as for normal atomic insulators. However, the topological states can be distinguished from one another and from normal insulators by inspecting the occurrence of stable dynamical symmetry inversion points in their microscopic dynamics. The analysis points to a position-space picture for understanding how topological boundary states can coexist with localized bulk states in anomalous Floquet topological phases.

${\mathrm{VI}}_3$ is a ferromagnet with planar honeycomb sheets of bonded ${\mathrm{V}}^{3+}$ ions held together by van der Waals forces. We apply neutron spectroscopy to measure the two-dimensional ($J/J_c\approx 17$) magnetic excitations in the ferromagnetic phase, finding two energetically gapped ($\mathrm{\Delta }\approx k_B{T}_c\approx 55\mathrm{K}$) and dispersive excitations. We apply a multilevel spin-wave formalism to describe the spectra in terms of two coexisting domains hosting differing ${\mathrm{V}}^{3+}$ orbital ground states built from contrasting distorted octahedral environments. This analysis fits a common nearest-neighbor in-plane exchange coupling ($J=-8.6\pm 0.3\mathrm{meV}$) between ${\mathrm{V}}^{3+}$ sites. The distorted local crystalline electric field combined with spin-orbit coupling provides the needed magnetic anisotropy for spatially long-ranged two-dimensional ferromagnetism in ${\mathrm{VI}}_3$.

In nonperturbative regimes, the superfluid instability in the two-dimensional (2D) Hubbard model can be described by an emergent BCS theory with small effective pairing constants. We compute the effective couplings using a controlled bold-line diagrammatic Monte Carlo approach, which stochastically sums all skeleton Feynman diagrams dressed in the one- and two-particle channels to high expansion orders, and map out the resulting superfluid ground-state phase diagram in a range of next-nearest-neighbor hopping $0\le t^{\prime }\le 0.3t$, interaction strength $0\le U\le 3t$, and lattice filling $0\le n\le 2$. The phase diagram is dramatically transformed in the hole-doped region and becomes particularly rich at larger doping and $t^{\prime }$. At $t^{\prime }=0.3$, the weak-coupling picture with the dominant triplet pairing sharply peaked at $n\approx 0.73$ due to the Van Hove singularity is replaced by a plateau of the singlet $d_{x^2-y^2}$ paring, while for $U\gtrsim 3t$ the effective couplings are consistent with the $d_{x^2-y^2}$ high-temperature superconductivity in the hole-doped region near cuprates' optimal doping.

We investigate how symmetries and conserved quantities relate to the occurrence of the boundary time crystal (BTC) phase in a generalized spin model with Lindblad dissipation. BTCs are a nonequilibrium phase of matter in which the system, coupled to an external environment, breaks the continuous time translational invariance. We perform a detailed mean-field study aided by a finite-size analysis of the quantum model of a $p,q$-spin-interaction system, a generalized $p$-spin-interaction system, which can be implemented in fully connected spin-1/2 ensembles. We find the following conditions for the observation of the BTC phase. First, the BTC appears when the discrete symmetry held by the Hamiltonian, ${\mathbb{Z}}_2$ in the considered models, is explicitly broken by the Lindblad jump operators. Second, the system must be coupled uniformly to the same bath in order to preserve the total angular momentum during the time evolution. If these conditions are not satisfied, any oscillatory behavior appears only as a transient in the dynamics and a time-independent stationary state is eventually reached. Our results suggest that these two elements may be general requirements for the observation of a stable BTC phase relating symmetries and conserved quantities in arbitrary spin models.

Thermal ammonolysis of quasi-two-dimensional (quasi-2D) $\mathrm{Co}{\mathrm{Ta}}_2{\mathrm{O}}_6$ yields the ${\mathrm{O}}^{2-}/{\mathrm{N}}^{3-}$ and anionic vacancy-ordered ${\mathrm{Co}}^{2+}{\mathrm{Ta}}_2^{5+}{\mathrm{O}}_{6-x}{\mathrm{N}}_{2x/3}{\square }_{x/3}$ ($x\le 0.15$), which exhibits a transition from antiferromagnetism to defect-engineered above-room-temperature ferromagnetism. First-principles calculations reveal that the origin of ferromagnetism is a particular $\mathrm{Co}{\mathrm{O}}_5\mathrm{N}$ configuration with N located at Wyckoff position $8j$, which breaks mirror symmetry about the $ab$ plane. A pressure-induced electronic phase transition is also predicted at $\sim 24.5\mathrm{GPa}$, accompanied by insulator-to-metal transition and magnetic moment vanishing.

We study the intrinsic superconductivity in a dissipative Floquet electronic system in the presence of attractive interactions. Based on the functional Keldysh theory beyond the mean-field treatment, we find that the system shows a time-periodic bosonic condensation and reaches an intrinsic dissipative Floquet superconducting (SC) phase. Due to the interplay between dissipations and periodic modulations, the Floquet SC gap becomes “soft” and contains the diffusive fermionic modes with finite lifetimes. However, bosonic modes of the bosonic condensation are still propagating even in the presence of dissipations.

Based on first-principles calculations and effective model analysis, all possible straight nodal-line (NL) phonons in symmorphic and nonsymmorphic space groups are uncovered. A classification of two-band $\mathbf{k}·\mathbf{p}$ models at an arbitrary point on the NLs is carried out, and these NL phonons are found to locate along the high-symmetry lines of the Brillouin zone. According to our classification, there are two types of NL phonons. The first type is topologically nontrivial with a linear dispersion, while the second type is topologically trivial, exhibiting a quadratic dispersion. The second type hosts the highest order of NLs in phonon systems, and it is a promising platform for investigating topological phase transitions. All the minimum symmetries that guarantee the existence of straight NL phonons are uncovered, which can be used to distinguish the types of straight NLs. This paper not only classifies NL phonons but also provides guidance to search for topological NLs in bosonic systems.

Antiferromagnetic (AFM) thin films are promising materials for engineering the magnetic anisotropy of adjacent ferromagnetic (FM) films. In this work, we explored the effects of triggering perpendicular magnetic anisotropy (PMA) of FM films by applying a series of NiMn-based AFM films with in-plane-oriented spin structures. The vertically expanded face-centered tetragonal ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{50}$ films alone could not induce PMA in Co/Ni films. By contrast, PMA was triggered through the application of Mn-rich NiMn alloy films or ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{50}/1$-monolayer Mn $(\mathrm{Mn}/1\text{−}\mathrm{ML} {\mathrm{Ni}}_{50}{\mathrm{Mn}}_{50})$ heterostructures. Detailed analyses of a series of samples indicated that two criteria must be met for a NiMn-based AFM film to trigger PMA in a neighboring FM film: (1) perpendicular interface crystalline anisotropy of interfacial uncompensated Mn moments must be established, and (2) strengthened lateral exchange coupling to volume AFM moments must be made. This paper clarifies how NiMn-based AFM films with in-plane-oriented AFM spin structures trigger the PMA of FM films, thus identifying a pathway to increase control over antiferromagnet-induced PMA through the interface/volume magnetic engineering of AFM layers.

We present a study of the magnetocaloric effect in metallic systems exhibiting first-order magnetic transitions and focus on consequences of magnetic phase separation. We account for ferrimagnetic, ferromagnetic, and Neel antiferromagnetic order. Based on the archetypal Hubbard model being treated within the mean-field approximation, we provide and explore its implications on the field-induced entropy change in metallic system with phase separation. Chosen framework allows us to properly analyze phase volumes' dependence on parameters of phase-separated (PS) system. Moreover, an account for phase separation boundaries as functions of magnetic field provides a natural splitting of the PS region, where each subregion corresponds to a different temperature dependence of entropy change: moving from one subregion to the other produces a kink, followed by a strong linear growth of entropy change. We encounter a second-order magnetic transition from paramagnetic to antiferromagnetic phase in PS region that occurs for particular parameter values. Despite the fact that both phases have zero total magnetization, the transition has a strong impact on entropy change.

We report the electronic and thermal properties of scandium monoantimonide ScSb by means of the Seebeck coefficient, thermal conductivity, specific heat, and nuclear magnetic resonance measurements. The experimental Seebeck coefficient exhibits a strong temperature dependence, and the theoretical calculation based on the two-band model provides a realistic description of the observed feature. The analysis of the thermal conductivity reveals that the lattice thermal conductivity dominates at low temperatures while electronic thermal conductivity makes a major contribution at high temperatures. A small value of the Sommerfeld coefficient of $0.38\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{\text{–}1}{\mathrm{K}}^{\text{–}2}$ was extracted from the low-temperature specific heat measurement, indicative of a low electronic Fermi-level density of states (DOS) in ScSb. Furthermore, we have deduced the Sc $3d$ and Sb $5s$ partial Fermi-level DOSs based on the Korringa behavior in the ${}^{45}\mathrm{Sc}$ and ${}^{121}\mathrm{Sb}$ NMR spin-lattice relaxation rates. The determined values of the DOS are quite low, giving strong evidence for the semimetallic character in ScSb.