An ab initio scheme is developed and used to investigate finite-temperature properties of a prototype of the complex rare-earth iron garnets, namely, $\mathrm{G}{\mathrm{d}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (that is, possessing gadolinium as its rare-earth ion), under applied magnetic fields. It is found that such a system undergoes a field-induced transition at a critical temperature, $T_{\mathrm{rev}}$, at which all magnetic ions reverse the direction of their spins. This $T_{\mathrm{rev}}$ critical temperature is well seen via a peak in the specific heat versus temperature curve under field and depends on different magnetic fields’ magnitude regimes: (1) for fields up to about 20 T, it is field independent, rather sudden, and coincides with the so-called compensation temperature, $T_M\sim 286$ K, at which the sum of all local magnetic moments is annihilated; (2) for fields larger than 70 T, there is no more full cancellation of these local magnetic moments for any temperature and $T_{\mathrm{rev}}$ is also field independent but now occurs via rotation of the local magnetic moments and at a temperature smaller than the zero-field $T_M$ by about 100 K; and (3) for intermediate fields ranging between 20 and 70 T, $T_{\mathrm{rev}}$ is now field dependent and decreases with the field's magnitude while also being associated with a rotation of these local magnetic moments. Our calculations also highlight the predominant role of the spins of the Gd ions in all these effects.

Recent experiments have shown that transition-metal oxide heterostructures, such as $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$-based interfaces, exhibit large gate-tunable spintronic responses. Our theoretical study showcases key factors controlling the magnitude of the conversion, measured by the inverse Edelstein and spin Hall effects, and their evolution with respect to an electrostatic doping. The origin of the response can be linked to spin-orbital textures. These stem from the broken inversion symmetry at the interface which produces an unusual form of the interfacial spin-orbit coupling, provided a bulk atomic spin-orbit contribution is present. The amplitudes and variations of these observables are direct consequences of the multiorbital subband structure of these materials, featuring avoided and topological crossings. Interband contributions to the coefficients lead to enhanced responses and nonmonotonic evolution with doping. We highlight these effects using analytical approaches and low-energy modeling.

Within the time-dependent Gutzwiller approximation applied to the negative-$U$ Hubbard model, we investigate the dynamics of a superconductor after an interaction quench for different values of the final interaction from weak to strong coupling. The equilibrium BCS-BEC crossover becomes a sharp transition in the out-of-equilibrium dynamics between weak- and strong-coupling dynamical phases. Two different frequencies (${\mathrm{\Omega }}_J<{\mathrm{\Omega }}_U$) dominate the order parameter dynamics. In the weak coupling phase, ${\mathrm{\Omega }}_J$ follows twice the asymptotic average value of the gap (${\mathrm{\Omega }}_J\approx 2{\mathrm{\Delta }}_{\infty }$) and ${\mathrm{\Omega }}_U$ is much larger but hardy visible in the anomalous density. At long times the BCS dynamics is recovered but surprisingly differences remain during the transient phase. In the strong coupling phase, the dynamics decouples from the asymptotic value of the order parameter except at exactly half filling and close to the dynamical transition where ${\mathrm{\Omega }}_U\approx 2{\mathrm{\Delta }}_{\infty }$. The out-of-equilibrium transient spectral density and optical conductivity are presented and discussed in relation to pump probe experiments. Both ${\mathrm{\Omega }}_J$ and ${\mathrm{\Omega }}_U$ give rise to a complex structure of self-driven slow Rabi oscillations which are visible in the nonequilibrium optical conductivity where also sidebands appear due to the modulation of the double occupancy by superconducting amplitude oscillations. Analogous results apply to CDW and SDW systems. Our results show that in systems with long coherence times, pump-probe experiments allow us to characterize the regime (BCS vs preformed pairs) through very specific out-of-equilibrium fingerprints.

We determine the fate of interacting fermions described by the Hamiltonian $H=\mathbf{p}·\mathbf{J}$ in three-dimensional topological semimetals with linear band crossing, where $\mathbf{p}$ is momentum and $\mathbf{J}$ are the spin-$j$ matrices for half-integer pseudospin $j\ge 3/2$. While weak short-range interactions are irrelevant at the crossing point due to the vanishing density of states, weak long-range Coulomb interactions lead to a renormalization of the band structure. Using a self-consistent perturbative renormalization group approach, we show that band crossings of the type $\mathbf{p}·\mathbf{J}$ are unstable for $j\le 7/2$. Instead, through an intriguing interplay between cubic crystal symmetry, band topology, and interaction effects, the system is attracted to a variety of infrared fixed points. We also unravel several other properties of higher-spin fermions for general $j$, such as the relation between fermion self-energy and free energy, or the vanishing of the renormalized charge. An $\text{O}\left(3\right)$ symmetric fixed point composed of equal chirality Weyl fermions is stable for $j\le 7/2$ and very likely so for all $j$. We then explore the rich fixed point structure for $j=5/2$ in detail. We find additional attractive fixed points with enhanced $\text{O}\left(3\right)$ symmetry that host both emergent Weyl or massless Dirac fermions, and identify a puzzling, infrared stable, anisotropic fixed point without enhanced symmetry in close analogy to the known case of $j=3/2$.

We use a quantum path-integral approach to describe the behavior of a microwave cavity coupled to a dissipative mesoscopic circuit. We integrate out the mesoscopic electronic degrees of freedom to obtain a cavity effective action at fourth order in the light/matter coupling. By studying the structure of this action, we establish sufficient conditions in which the cavity dynamics can be described with a Lindblad equation. This equation depends on effective parameters set by electronic correlation functions. It reveals that the mesoscopic circuit induces an effective Kerr interaction and two-photon dissipative processes. We use our method to study the effective dynamics of a cavity coupled to a double quantum dot with normal metal reservoirs. If the cavity is driven at twice its frequency, the double-dot circuit generates photonic squeezing and nonclassicalities visible in the cavity Wigner function. In particular, we find a counterintuitive situation where mesoscopic dissipation enables the production of photonic Schrödinger cats. These effects can occur for realistic circuit parameters. Our method can be generalized straightforwardly to more complex circuit geometries with, for instance, multiple quantum dots and other types of fermionic reservoirs such as superconductors and ferromagnets.

Recent studies indicated that helical organic molecules, such as DNA and the $\alpha$-helical protein, can behave as Thouless quantum pumps when a rotating electric field is applied perpendicularly to their helical axes. Here we investigate the influence of long-range hoppings on this topological pumping of electrons in single-helical organic molecules. Under variation of the long-range hoppings governed by a decay exponent $\mu$, we find an energy gap in the molecular band structure closes at a critical value ${\mu }_c$ of the decay exponent and reopens for $\mu$ deviating from ${\mu }_c$. The relevant bulk bands in a pumping cycle acquire different Chern numbers in the strong ($\mu <{\mu }_c$) and weak ($\mu >{\mu }_c$) long-range hopping regimes, with a sudden jump at criticality. This topological phase transition is also shown to separate two distinct behaviors of the midgap end states in the pumping process. The end states carry quantized current pumped by the rotating electric field, and the current forms a plateau by sweeping the Fermi energy over the gap. In the strong-hopping phase, the quantized current plateau is positive, which is reversed to a negative one with smaller amplitude in the weak-hopping phase. However, the reversal is a smooth crossover, not a sharp transition, due to the finite sizes of the molecules. We show that these transport characteristics of the topological phase transition could also be observed at finite temperatures.

We reveal how the vector field links are untied under the influence of anti-parity-time-symmetric couplings in a dissipative sublattice-symmetric topological photonic crystal lattice. The topology of the quasi-one-dimensional two-band system is encoded in the geometric topology of the vector fields associated with the Bloch Hamiltonian. The linked vector fields reflect the topology of the nontrivial phase. The topological phase transition occurs concomitantly with the untying of the vector field link at the exceptional points. Counterintuitively, more dissipation constructively creates a nontrivial topology. The linking number predicts the number of topological photonic zero modes.

We argue that edge electrons in monolayer $\mathrm{W}{\mathrm{Te}}_2$ can possess a “cooperative” orbital moment (COM) that critically impacts its edge magnetoresistance behavior. Arising from the cooperative action of both Rashba and Ising spin-orbit coupling, COM quickly achieves large magnitudes (of the order of a few Bohr magnetons) even for relatively small spin-orbit coupling strengths. As we explain, such large COM magnitudes arise from an unconventional cooperative spin canting of edge spins when Rashba and Ising spin-orbit coupling act together. Strikingly, COM can compete with spin moments to produce an unusual anisotropic edge magnetoresistance oriented at an oblique angle. In particular, this competition produces a direction along which $\mathbf{B}$ is ineffective at gapping out the edge spectrum, leaving it nearly gapless. As a result, large contrasts in gap sizes appear as $\mathbf{B}$ is rotated, granting a giant anisotropic magnetoresistance as large as several million percent at 10 T and low temperature.

Less common ligand coordination of transition-metal centers is often associated with peculiar valence-shell electron configurations and outstanding physical properties. One example is the ${\mathrm{Fe}}^{+}$ ion with linear coordination, actively investigated in the research area of single-molecule magnetism. Here we address the nature of $3d^9$ states for ${\mathrm{Cu}}^{2+}$ ions sitting in the center of trigonal bipyramidal ligand cages in the quasi-two-dimensional honeycomb compound $\mathrm{In}{\mathrm{Cu}}_{2/3}{\mathrm{V}}_{1/3}{\mathrm{O}}_3$, whose unusual magnetic properties were intensively studied in the recent past. In particular, we discuss the interplay of structural effects, electron correlations, and spin-orbit couplings in this material. A relevant computational finding is a different sequence of the Cu ($xz$, $yz$) and ($xy$, $x^2-y^2$) levels compared to existing electronic-structure models, which has implications for the interpretation of various excitation spectra. Spin-orbit interactions, both first- and second-order, turn out to be stronger than previously assumed, suggesting that rather rich single-ion magnetic properties can in principle be achieved also for the $3d^9$ configuration by properly adjusting the sequence of crystal-field states for such less usual ligand coordination.

We study the low-energy transport properties of a hybrid device composed by a native quantum dot coupled to both ends of a topological superconducting nanowire section hosting Majorana zero modes. The account of the coupling between the dot and the farthest Majorana zero mode allows one to introduce the topological quality factor, characterizing the level of topological protection in the system. We demonstrate that the Coulomb interaction between the dot and the topological superconducting section leads to the onset of the additional overlap of the wave functions describing the Majorana zero modes, leading to the formation of trivial Andreev bound states even for spatially well-separated Majoranas. This leads to the spoiling of the quality factor and introduces a constraint for the braiding process required to perform topological quantum computing operations.

It is demonstrated that shaped topological insulator (TI) nanowires, i.e., such that their cross-section radius varies along the wire length, can be tuned into a number of different transport regimes when immersed in a homogeneous coaxial magnetic field. This is in contrast with widely studied tubular nanowires with constant cross section, and is due to magnetic confinement of Dirac surface carriers. In flat two-dimensional systems, such a confinement requires inhomogeneous magnetic fields, while for shaped nanowires of standard size homogeneous fields of the order of $B\sim 1\mathrm{T}$ are sufficient. We put recent work [R. Kozlovsky et al., Phys. Rev. Lett. 124, 126804 (2020)] into broader context and extend it to deal with axially symmetric wire geometries with arbitrary radial profile. A dumbbell-shaped TI nanowire is used as a paradigmatic example for transport through a constriction and shown to be tunable into five different transport regimes: (i) conductance steps, (ii) resonant transmission, (iii) current suppression, (iv) Coulomb blockade, and (v) transport through a triple quantum dot. Switching between regimes is achieved by modulating the strength of a coaxial magnetic field and does not require strict axial symmetry of the wire cross section. As such, it should be observable in TI nanowires fabricated with available experimental techniques.

Epsilon-near-zero (ENZ) media are an emerging platform, embracing the great potential for novel nanophotonic phenomena. One method to obtain an ENZ medium is to operate at the cutoff wavelength of the fundamental mode of a plasmonic waveguide. Control over this mode is limited to the waveguide's material and size properties. Here, we demonstrate that a plasmonic nanostructure (nanorod) can be strongly coupled to the plasmonic waveguide, providing two new hybrid resonance modes exhibiting characteristics of both guided ENZ modes and localized surface plasmon modes. Strong coupling gives rise to a Rabi splitting of 300 meV, which is demonstrated by finite-difference time domain simulations where we calculate the decay rate enhancement of a dipole emitter located in the coupled system. The hybrid modes are retrieved using the analytic coupled harmonic oscillator model. This suggested method via hybridization of modes can be used to generate and manipulate ENZ media where the unique ENZ property of wavelength extension enables effectively shrinking spatially long distances down to optically short distances.

We present a first-principles study of pre-equilibrium stopping power and projectile charge capture in thin aluminum sheets irradiated by 6–60 keV protons. Our time-dependent density functional theory calculations reveal enhanced stopping power compared to bulk aluminum, particularly near the entrance layers. We propose the additional excitation channel of surface plasma oscillations as the most plausible explanation for this behavior. We also introduce a technique to compute the orbital-resolved charge state of a proton projectile after transmission through the sheet. Our results provide insight into the dynamics of orbital occupations after the projectile exits the aluminum sheet and have important implications for advancing radiation hardness and focused-ion beam techniques, especially for few-layer materials.

The transport spectral function of electron-phonon ($e$-ph) interaction in the double $\delta$-function approximation (DDFA) is extensively employed to calculate the intrinsic resistivity (arising from $e$-ph scattering) of metallic materials in recent works of first-principles calculations. In contrast, a more fundamental transport spectral function with the Fermi smearing effect due to finite temperature ($T$) and nonzero phonon frequency is less involved. In this work, we perform first-principles calculations of the intrinsic resistivity of ${\mathrm{Ti}}_2\mathrm{N}$ monolayer, a potential MXene material, by employing the two kinds of spectral function. We find that the spectral function with the DDFA fails to describe correctly the temperature dependence of the intrinsic resistivity of ${\mathrm{Ti}}_2\mathrm{N}$ monolayer at $T>250$ K, much lower than the Debye temperature. The underlying reason is that ${\mathrm{Ti}}_2\mathrm{N}$ monolayer has a multisheet Fermi surface formed by several bands, and some band edges are very close to the Fermi surface. Our results suggest that the transport spectral function with the Fermi smearing effect, instead of the one with the DDFA, is always adequate for studying the intrinsic resistivity of realistic materials on the level of first-principles calculations. In addition, we give a brief remark on the intrinsic resistivity of ${\mathrm{Ti}}_2\mathrm{N}$ monolayer, in contrast with other typical two-dimensional materials, which is significant from the viewpoint of application of such an MXene material.

We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of $4H$-silicon carbide. Au intercalation is achieved by deposition of an atomically thin Au layer on the Bu-L followed by annealing at 850 °C in an argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterization and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the Au-intercalated buffer layer displays all properties of monolayer graphene, namely gate-tunable ambipolar transport across the Dirac point, but we find no observable enhancement of spin-orbit effects in the graphene layer, despite its proximity to the intercalated Au layer.

Negative capacitance regime in ferroelectrics is associated with the negative slope in the dependence of the polarization on electric field and is prohibited by the laws of equilibrium thermodynamics. Very recently, it developed into a controversial but very productive area with a promise for next generation low-power transistors. At least, in part, the controversies arise from the lack of direct experimental insight into the mechanism for such phenomena. Although the experimental insight is, indeed, challenging at present, we probe the effect in ferroelectric $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ bulk and a thin film with realistic first-principles-based nonequilibrium molecular dynamics simulations which model most closely experimental conditions. The following insights emerge from the simulations: (i) the ferroelectric never enters the thermodynamically forbidden region even under extreme rates of electric-field application; (ii) the negative slope naturally arises when the compensating charge is either scarce or slowed down by the external circuitry; (iii) the negative slope is well controlled by the interplay between the applied step pulse field and the amount of the screening charge. The latter can be used to design the negative capacitance regime.

Using an effective Hamiltonian scheme within classical Monte Carlo simulations, we numerically investigate the effect of epitaxial strain on various physical quantities of the promising lead-free $(1-x)\mathrm{Ba}\left({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\right){\mathrm{O}}_3–x\left({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}\right){\mathrm{TiO}}_3$ solid solutions for different compositions. It is found that some combinations of strain and concentration lead to physical responses that are not only broad with temperature around 300 K but also large, including dielectric and piezoelectric coefficients. The origins of these useful and striking features are revealed.

We study the evolution of multiregion bipartite entanglement entropy under locally scrambled quantum dynamics. We show that the multiregion entanglement can significantly modify the growth of single-region entanglement, whose effect has been largely overlooked in the existing literature. We developed a theoretical framework, called the entanglement feature formalism, to organize all the multiregion entanglement systematically as a sign-free many-body state. We further propose a two-parameter matrix product state (MPS) ansatz to efficiently capture the exponentially many multiregion entanglement features. Using these tools, we are able to study the multiregion entanglement dynamics jointly and represent the evolution in the MPS parameter space. By comparing the dynamical constraints on the motion of entanglement cuts, we are able to identify different quantum dynamics models in a unifying entanglement feature Hamiltonian. Depending on the quantum dynamics model, we find that multiregion effects can dominate the single region entanglement growth and only vanish for Haar random circuits. We calculate the operator-averaged out-of-time-order correlator based on the entanglement feature Hamiltonian and extract the butterfly velocity from the result. We show that the previously conjectured bound between the entanglement velocity and the butterfly velocity holds true even under the influence of multiregion entanglement. These developments could enable more efficient numerical simulations and more systematic theoretical understandings of the multiregion entanglement dynamics in quantum many-body systems.

We investigate the spectral and transport properties of a two-arm tight-binding ladder perturbed by an external magnetic field following an Aubry-André-Harper profile. The varying magnetic flux trapped in consecutive ladder cells simulates an axial twist that enables us, in principle, to probe a wide variety of systems ranging from a ribbon Hofstadter geometry to helical DNA chains. We perform an in-depth numerical analysis, using a direct diagonalization of the lattice Hamiltonian to study the electronic spectra and transport properties of the model. We show that such a geometry creates a self-similar multifractal pattern in the energy landscape. The spectral properties are analyzed using the local density of states and a Green's function formalism is employed to obtain the two-terminal transmission probability. With the standard multifractal analysis and the evaluation of inverse participation ratio we show that the system hosts both critical and extended phase for a slowly varying aperiodic sequence of flux indicating a possible mobility edge. Finally, we report signatures of topological edge modes that are found to be robust against a correlated perturbation given to the nearest-neighbor hopping integrals. Our results can be of importance in experiments involving ladderlike quantum networks, realized with cold atoms in an optical trap setup.

We study one-dimensional chains of superconducting islands with a particular emphasis on the regime in which every second island is switched into its normal state, thus forming a superconductor-insulator-normal metal (S-I-N) repetition pattern. As is known since Giaever tunneling experiments, tunneling charge transport between a superconductor and a normal metal becomes exponentially suppressed, and zero-bias resistance diverges, as the temperature is reduced and the energy gap of the superconductor grows larger than the thermal energy. Here we demonstrate that this physical phenomenon strongly impacts transport properties of inhomogeneous superconductors made of weakly coupled islands with fluctuating values of the critical temperature. We observe a nonmonotonous dependence of the chain resistance on both temperature and magnetic field, with a pronounced resistance peak at temperatures at which some but not all islands are superconducting. We explain this phenomenon by the inhomogeneity of the chains, in which neighboring superconducting islands have slightly different critical temperatures. We argue that the Giaever's resistance divergence can also occur in the zero-temperature limit. Such quantum transition can occur if the magnetic field is tuned such that it suppresses superconductivity in the islands with the weaker critical field, while the islands with stronger energy gap remain superconducting. In such a field, the system acts as a chain of S-I-N junctions.