It is well known that cellular dynamical mean-field theory (CDMFT) leads to the artificial breaking of translation invariance. In spite of this, it is one of the most successful methods to treat strongly correlated electrons systems. Here, we investigate in more detail how this broken translation invariance manifests itself. This allows us to disentangle artificial broken translation invariance effects from the genuine strongly correlated effects captured by CDMFT. We report artificial density waves taking the shape of the cluster—cluster density waves—in all our zero temperature CDMFT solutions, including pair density waves in the superconducting state. We discuss the limitations of periodization regarding this phenomenon, and we present mean-field density-wave models that reproduce CDMFT results at low energy in the superconducting state. We then discuss how these artificial density waves help the agreement of CDMFT with high temperature superconducting cuprates regarding the low-energy spectrum, in particular for subgap structures observed in tunneling microscopy. We relate these subgap structures to nodal and antinodal gaps in our results, similar to those observed in photoemission experiments. This fortuitous agreement suggests that spatial inhomogeneity may be a key ingredient to explain some features of the low-energy underdoped spectrum of cuprates with strongly correlated methods. This work deepens our understanding of CDMFT and clearly identifies signatures of broken translation invariance in the presence of strong correlations.

A comprehensive study of the behavior of the Mott insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ under electrical current drive is performed by combining two experimental probes: the macroscopic electrical transport and the microscopic x-ray diffraction. The resistivity, $\rho$, versus electric current density, $J$, and temperature, $T,\rho (J,T)$, resistivity map is drawn. In particular, the metastable state, induced between the insulating and the metallic thermodynamic states by current biasing ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ single crystals, is investigated. Such an analysis, combined with the study of the resulting ${\mathrm{RuO}}_6$ octahedra energy levels, reveals that a metallic crystal phase emerges in the metastable regime. The peculiar properties of such a phase, coexisting with the well-established orthorhombic insulating and tetragonal metallic phases, allow one to explain some of the unconventional and puzzling behaviors observed in the experiments as a negative differential resistivity.

The number of electronic bands is usually considered invariant regardless of the electron density in a band picture. However, in interacting systems, the spectral-weight distribution generally changes depending on the electron density, and electronic states can even emerge or disappear as the electron density changes. Here, to clarify how electronic states emerge and become dominant as the electron density changes, the spectral function of the Hubbard ladder with strong repulsion and strong intrarung hopping is studied using the non-Abelian dynamical density-matrix renormalization-group method. A mode emerging in the low-electron-density limit gains spectral weight as the electron density increases and governs the dimer Mott physics at quarter-filling. In contrast, the antibonding band, which is dominant in the low-electron-density regime, loses spectral weight and disappears at the Mott transition at half-filling, exhibiting the momentum-shifted magnetic dispersion relation in the small-doping limit. This paper identifies the origin of the electronic states responsible for the Mott transition and brings a new perspective to electronic bands by revealing the overall nature of electronic states over a wide energy and electron-density regime.

We investigate strongly asymmetric self-assembled nanostructures with one of the dimensions reaching hundreds of nanometers. Close to the nanowirelike type of confinement, such objects are sometimes assigned as one dimensional in nature. Here, we directly observe the spectrum of exciton excited states corresponding to longitudinal quantization. This is based on probing the optical transitions via polarization-resolved microphotoluminescence excitation $\left(\mu \mathrm{PLE}\right)$ measurements performed on single nanostructures combined with theoretical calculations of the neutral and charged exciton optical properties. We successfully probe absorptionlike spectra for individual bright states forming the exciton ground-state fine structure, as well as for the negatively charged exciton. Confronting the calculated spectrum of excitonic absorption with $\mu \mathrm{PLE}$ traces, we identify optical transitions involving states that contain carriers at various excited levels related to the longest dimension. Based on a cross-polarized excitation-detection scheme, we show a very well-conserved spin configuration during orbital relaxation of the exciton from a number of excited states comparable to the quasiresonant pumping via the optical phonon, and no polarization memory for the trion, as theoretically expected.

We investigate the low-energy theory of a one-dimensional finite capacitance topological Josephson junction. Charge fluctuations across the junction couple to resonant microwave fields and can be used to probe microscopic excitations such as Majorana and Andreev bound states. This marriage between localized microscopic degrees of freedom and macroscopic dynamics of the superconducting phase, leads to unique spectroscopic patterns which allow us to reveal the presence of Majorana fermions among the low-lying excitations.

The generic model of two coupled multimode plasmonic nanospheres with gain and loss forming a parity-time $\left(\mathcal{PT}\right)$ symmetric dimer is considered. Analytical solutions based on a quasistatic approximation are used to explore the behavior of the eigenmodes in such a coupled nanoantenna system as a function of their separation distances and their gain-loss level. It is shown that the introduction of the same amount of gain and loss modifies the composition of eigenmodes and breaks their orthogonality. The resulting entanglement of different spatial order and symmetry modes can be advantageously exploited to actively control the scattering properties of such multimode $\mathcal{PT}$-symmetric nanoantennas.

Using the Bethe ansatz solution, we analytically study expansionary, magnetic, and interacting Grüneisen parameters (GPs) for one-dimensional (1D) Lieb-Liniger and Yang-Gaudin models. These different GPs elegantly quantify the dependence of characteristic energy scales of these quantum gases on the volume, the magnetic field, and the interaction strength, revealing the caloric effects resulting from the variations of these potentials. The obtained GPs further confirm an identity which is incurred by the symmetry of the thermal potential. We also present the universal scaling behavior of these GPs in the vicinities of the quantum critical points driven by different potentials. The divergence of the GPs not only provides an experimental identification of non-Fermi-liquid nature at quantum criticality but also elegantly determines low-temperature phases of the quantum gases. Moreover, the pairing and depairing features in the 1D attractive Fermi gases can be captured by the magnetic and interacting GPs, facilitating experimental observation of quantum phase transitions. Our results open the way to further study the interaction- and magnetic-field-driven quantum refrigeration and quantum heat engine in quantum gases of ultracold atoms.

The current scenario in multiferroics demands a breakthrough discovery of promising materials after ${\mathrm{BiFeO}}_3$. Recently, the controversial discovery of room temperature ferroelectricity (FE) in ${\mathrm{SmFeO}}_3$ [Lee et al., Phys. Rev. Lett. 107, 117201 (2011); Kuo et al., Phys. Rev. Lett. 113, 217203 (2014)] inspires the investigation of ${\mathrm{HoFeO}}_3$. Here we report a natural ferroelectric order around 210 K ($T_{\text{FE}}$) along the $c$ axis with a reasonably large polarization of $\sim 0.29\mu \mathrm{C}/{\mathrm{cm}}^2$ for a 10 kV/cm poling field. The strong magnetoelectric coupling of $\sim 22$% is observed at 210 K at 30 kOe, below which it decreases with a further increase in magnetic field. The synchrotron and neutron diffraction results confirm that a shift of O atoms along the $c$ axis, on the polar $P{b}^{\prime }{n}^{\prime }{2}_1$ magnetic space, causes the FE in ${\mathrm{HoFeO}}_3$. The potential results of reasonably high ferroelectric polarization associated with a strong magnetoelectric coupling at higher temperature are fascinating and create a renewed attention for searching promising candidates with a natural ferroelectric order and higher $T_{\text{FE}}$ in the rest of the $R{\mathrm{FeO}}_3$ series.

The Heusler ferromagnetic (FM) compound $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}$ interfaced with a high spin-orbit coupling nonmagnetic (NM) layer is a promising candidate for energy-efficient spin-logic circuits. The circuit potential depends on the strength of angular momentum transfer across the FM/NM interface, hence requiring low spin-memory loss and high spin-mixing conductance. To highlight this issue, spin pumping and spin torque ferromagnetic resonance measurements have been performed on $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}/\beta$-Ta heterostructures tailored with Cu interfacial layers. The interface tailored structure yields an enhancement of the effective spin-mixing conductance. The interface transparency and spin-memory loss corrected values of the spin-mixing conductance, spin Hall angle, and spin-diffusion length are found to be $3.40\pm 0.01\times {10}^{19}{\mathrm{m}}^{-2}$, $0.029\pm 0.003$, and $2.3\pm 0.5\mathrm{nm}$, respectively. Furthermore, a high current modulation of the effective damping of around $2.1\%$ has been achieved at an applied current density of $1\times {10}^9\mathrm{A}/{\mathrm{m}}^2$, which clearly indicates the potential of using this heterostructure for energy-efficient control in spin devices.

$\mathrm{L}{1}_0$ FePt is a technologically important material for a range of novel data storage applications. In the ordered FePt structure the normally nonmagnetic Pt ion acquires a magnetic moment, which depends on the local field originating from the neighboring Fe atoms. In this work a model of FePt is constructed in which the induced Pt moment is simulated by using combined longitudinal and rotational spin dynamics. The model is parameterized to include a linear variation of the moment with the exchange field, so that at the Pt site the magnetic moment depends on the Fe ordering. The Curie temperature of FePt is calculated and agrees well with similar models that incorporate the Pt dynamics through an effective Fe-only Hamiltonian. By computing the dynamic correlation function the anisotropy field and the Gilbert damping are extracted over a range of temperatures. The anisotropy exhibits a power-law dependence on the magnetization with exponent $n\approx 2.1$. This agrees well with what was observed experimentally, and it is obtained without including a two-ion anisotropy term as in other approaches. Our work shows that incorporating longitudinal fluctuations into spin dynamics calculations is crucial for understanding the properties of materials with induced moments.

We report the synthesis and magnetic properties of frustrated $\mathrm{N}{\mathrm{a}}_{0.22}\mathrm{Mn}{\mathrm{O}}_2·0.39{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{K}}_{0.6}\mathrm{Mn}{\mathrm{O}}_2·0.48{\mathrm{H}}_2\mathrm{O}$ with the birnessite structure. The structure, static, and dynamic magnetic properties of the compounds are investigated in detail. A combination of DC and AC magnetic susceptibility measurements and magnetization decay measurements reveal cluster glass behavior below the freezing temperature of 4 K for Na-birnessite and 6 K for K-birnessite. The frequency dependence of the freezing temperature is analyzed on the basis of dynamic scaling laws including the critical slowing-down formula and the Vogel-Fulcher law, which further confirm cluster glass formation in both compounds.

Antisite (B-site) disorder in double perovskite lattice is responsible for various magnetic phenomena such as exchange bias, spin glass, memory effect, colossal magnetoresistance, etc. By controlling the antisite disorder in the antiferromagnetic double perovskite ${\mathrm{LaSrCoFeO}}_6$, we achieve intrinsic exchange bias effect with a large exchange bias field ($\sim 1.2$ kOe) and giant coercive field ($\sim 12.8$ kOe). Further, we find that the effect of such antisite disorder induces a spin-glass state in ${\mathrm{LaSrCoFeO}}_6$. Multiple signatures of slow dynamics were confirmed by frequency-dependent peak shift, slow spin relaxation, and memory effect over a wide temperature regime ($5-80$ K). The AC susceptibility data near the spin-glass temperature ($\sim 72.1\pm 0.6$ K) are best fit by a critical slowing down model described by a dynamical exponent $\mathrm{z}\nu =7.5\pm 0.5$ and ${\tau }_0=1.05\times {10}^{-12}$ s. The origin of exchange bias and spin glass are briefly discussed.

First-principles calculations are conducted on a prototype of brownmillerite oxides, that is, ${\mathrm{Ca}}_2{\mathrm{FeAlO}}_5$, to explain and reveal the existence of many structural states, including ones with long periods, having close energy but rather different physical properties. Examples are polar states with different quantized-like electrical polarizations and significant linear magnetoelectric coefficients, as well as antipolar phases. These states can also differentiate themselves by the direction of their predominant antiferromagnetic vector and of their weak magnetization, which hint towards unusual magnetoelectric effects such as rotating the magnetization by applying an electric field. All these features can be traced back to the oxygen tetrahedral pattern that is inherent to brownmillerites, and the energetic easiness to alter such pattern. Furthermore, these features are also found in other brownmillerites, which emphasizes the generality.

We present an experimental and theoretical investigation of all-optical switching by single femtosecond laser pulses. Our experimental results demonstrate that, unlike rare-earth transition-metal ferrimagnetic alloys, $\mathrm{Pt}/\mathrm{Co}/{[\mathrm{Ni}/\mathrm{Co}]}_N/\mathrm{Gd}$ can be switched in the absence of a magnetization compensation temperature, indicative for strikingly different switching conditions. In order to understand the underlying mechanism, we model the laser-induced magnetization dynamics in Co/Gd bilayers and GdCo alloys on an equal footing, using an extension of the microscopic three-temperature model to multiple magnetic sublattices and including exchange scattering. In agreement with our experimental observations, the model shows that Co/Gd bilayers can be switched for a thickness of the Co layer far away from compensating the total Co and Gd magnetic moment. We identify the switching mechanism in Co/Gd bilayers as a front of reversed Co magnetization that nucleates near the Co/Gd interface and propagates through the Co layer driven by exchange scattering.

The Planckian relaxation rate $\hslash /t_{\mathrm{P}}=2\pi k_{\mathrm{B}}T$ sets a characteristic timescale for both the equilibration of quantum critical systems and maximal quantum chaos. In this Rapid Communication, we show that at the critical coupling between a superconducting dot and the complex Sachdev-Ye-Kitaev model, known to be maximally chaotic, the pairing gap $\mathrm{\Delta }$ behaves as $\eta \hslash /t_{\mathrm{P}}$ at low temperatures, where $\eta$ is an order one constant. The lower critical temperature emerges with a further increase of the coupling strength so that the finite $\mathrm{\Delta }$ domain is settled between the two critical temperatures.

Dynamical quantum phase transitions (DQPTs) represent a counterpart in nonequilibrium quantum time evolution of thermal phase transitions at equilibrium, where real time becomes analogous to a control parameter such as temperature. In quenched quantum systems, recently the occurrence of DQPTs has been demonstrated, both with theory and experiment, to be intimately connected to changes of topological properties. Here, we contribute to broadening the systematic understanding of this relation between topology and DQPTs to multiorbital and disordered systems. Specifically, we provide a detailed ergodicity analysis to derive criteria for DQPTs in all spatial dimensions and construct basic counterexamples to the occurrence of DQPTs in multiband topological insulator models. As a numerical case study illustrating our results, we report on microscopic simulations of the quench dynamics in the Harper-Hofstadter model. Furthermore, going gradually from multiband to disordered systems, we approach random disorder by increasing the (super)unit cell within which random perturbations are switched on adiabatically. This leads to an intriguing order of limits problem which we address by extensive numerical calculations on quenched one-dimensional topological insulators and superconductors with disorder.

A simple analytical method for the investigation of the magnetic properties of the finite-length biatomic chains proposed in the framework of the Heisenberg model with uniaxial magnetic anisotropy. The method allows to estimate the reversal time of magnetization of ferromagnetic and antiferromagnetic biatomic chains. Three cases have been considered: the spontaneous remagnetization, the remagnetization under the interaction with a scanning tunneling microscope, and the remagnetization under the external magnetic field. The applicability limits of the method have been discussed. Within its limits of applicability the method produces the results which are in a perfect agreement with those obtained with the use of the kinetic Monte Carlo simulations. As the examples, two physical systems have been considered: biatomic Fe chains on ${\mathrm{Cu}}_2\mathrm{N}/\mathrm{Cu}\left(001\right)$ surface and biatomic Co chains on Pt(997) surface. The presented method is incomparably less time-consuming than the commonly used kinetic Monte Carlo simulations, especially in the cases of low temperatures or long chains.

We study a generalized clock model on the simple cubic lattice. The parameter of the model can be tuned such that the amplitude of the leading correction to scaling vanishes. In the main part of the study, we simulate the model with $Z_8$ symmetry. At the transition, with increasing length scale, $O\left(2\right)$ symmetry emerges. We perform Monte Carlo simulations using a hybrid of local Metropolis and cluster algorithms of lattices with a linear size up to $L=512$. The field variable requires less memory and the updates are faster than for a model with $O\left(2\right)$ symmetry at the microscopic level. Our finite-size scaling analysis yields accurate estimates for the critical exponents of the three-dimensional $XY$-universality class. In particular, we get $\eta =0.03810\left(8\right),\nu =0.67169\left(7\right)$, and $\omega =0.789\left(4\right)$. Furthermore, we obtain estimates for fixed point values of phenomenological couplings and critical temperatures.

We use higher-dimensional bosonization and fermion decoration to construct exactly soluble interacting fermion models to realize fermionic symmetry-protected trivial (SPT) orders (which are also known as symmetry-protected topological orders) in any dimensions and for generic fermion symmetries $G_f$, which can be a nontrivial $Z_2^f$ extension $Z_2^f⋋G_b$ (where $Z_2^f$ is the fermion-number-parity symmetry and $G_b$ is the bosonic symmetry). This generalizes the previous results from group supercohomology of Gu and Wen (arXiv:1201.2648), where $G_f$ is assumed to be $Z_2^f\times G_b$. We find that the $(d+1)$-dimensional $\left[\right(d+1)\mathrm{D}]$ fermionic SPT phases with bosonic symmetry $G_b$ and from fermion decoration construction can be described in a compact way using higher group homomorphism: $\mathcal{B}{G}_b\stackrel{\phi }{\to }\mathcal{B}(Z_2,2;Z_2,d)$. In fact, the fermion symmetry is more precisely described by the structure $Z_2^f⋋G_b⋋S{O}_{\infty }$ (or $Z_2^f⋋G_b⋋O_{\infty }$ with time-reversal symmetry). In this case the $(d+1)\mathrm{D}$ fermionic SPT phases are better described by $\mathcal{B}(Z_2^f⋋G_b⋋S{O}_{\infty })\stackrel{\phi }{\to }\mathcal{B}(S{O}_{\infty },1;Z_2,d)$ [or $\mathcal{B}(Z_2^f⋋G_b⋋O_{\infty })\stackrel{\phi }{\to }\mathcal{B}(O_{\infty },1;Z_2,d)]$.