Recent experiments in superconducting qubit systems have shown an unexpectedly strong dependence of the qubit relaxation rate on the readout drive power. This phenomenon limits the maximum measurement strength and thus the achievable readout speed and fidelity. We address this problem here and provide a plausible mechanism for drive-power dependence of relaxation rates. To this end we introduce a two-parameter perturbative expansion in qubit anharmonicity and the drive amplitude through a unitary transformation technique introduced in Part I. This approach naturally reveals number-nonconserving terms in the Josephson potential as a fundamental mechanism through which applied microwave drives can activate additional relaxation mechanisms. We present our results in terms of an effective master equation with renormalized state- and drive-dependent transition frequency and relaxation rates. Comparison of numerical results from this effective master equation to those obtained from a Lindblad master equation which only includes number-conserving terms (i.e., Kerr interactions) shows that number-nonconserving terms can lead to significant drive-power dependence of the relaxation rates. The systematic expansion technique introduced here is of general applicability to obtaining effective master equations for driven-dissipative quantum systems that contain weakly nonlinear degrees of freedom.

Systematic benchmark calculations for elemental bulks are presented to validate the accuracy of density functional theory for superconductors. We developed a method to treat the spin-orbit interaction (SOI) together with the spin fluctuation (SF) and examine their effect on the superconducting transition temperature. We found the following results from the benchmark calculations: (1) The calculations, including SOI and SF, reproduce the experimental superconducting transition temperature ($T_c$) quantitatively. (2) The effect by SOI is small excepting a few elements, such as Pb, Tl, and Re. (3) SF reduces $T_c$'s, especially for the transition metals, whereas this reduction is too weak to reproduce the $T_c$'s of Zn and Cd. (4) We reproduced the absence of superconductivity for alkaline- (earth) and noble metals. These calculations confirm that our method can be applied to a wide range of materials and implies a direction for the further improvement of the methodology.

The development of long-range ferromagnetic (FM) order in dilute magnetic topological insulators can induce dissipationless electronic surface transport via the quantum anomalous Hall effect. We measure the magnetic excitations in a prototypical magnetic topological crystalline insulator, ${\mathrm{Sn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Te}$, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our ${\mathrm{Sn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Te}$ sample has no FM long-range order above a temperature of 2 K. However, we observe slow, collective FM fluctuations $\left(<70\mu \mathrm{eV}\right)$, indicating proximity to FM order. We also find a series of sharp peaks originating from local excitations of antiferromagnetically (AF) coupled and isolated Mn-Mn dimers with $J_{\mathrm{AF}}=460\mu \mathrm{eV}$. The simultaneous presence of collective and localized components in the magnetic spectra highlight different roles for substituted Mn ions, with competition between FM order and the formation of AF-coupled Mn-Mn dimers.

Color centers in diamond are considered as a platform for quantum computing and communications, biomedical markers, and nanosensors. Negatively charged split-vacancy centers have outstanding properties due to bright and almost monochromatic luminescence, but they have poor spin coherence and relaxation times. This drawback is believed to be absent in the neutral charge state of the defects. So far only the neutral silicon-vacancy center has been observed in luminescence and absorption spectra. Here we report the observation of its germanium-based analog in luminescence spectra with the zero-phonon line (ZPL) at 1.979 eV in samples containing negatively charged ${\mathrm{GeV}}^{-}$ centers. The relationship between the center and Ge dopant is unambiguously confirmed by studies of ${}^{12}\mathrm{C}$ diamonds containing ${}^{70}\mathrm{Ge}$, ${}^{73}\mathrm{Ge}$, or ${}^{76}\mathrm{Ge}$ isotopes. The intensity ratio of the ZPL of these centers varies with the crystal size, and the intensity of the ${\mathrm{GeV}}^0$ centers reaches its maximum in samples 150 nm in size. In the vibronic sideband of this center the local vibrational mode with an energy of 23 meV was identified. The density functional theory calculations yield a ZPL energy value of the ${\mathrm{GeV}}^0$ center which is 150 meV higher than the experimentally observed one and matches the values of its relaxational energy as well as the local phonon mode frequency.

We studied the magnetic properties of the garnet $\mathrm{C}{\mathrm{o}}_3\mathrm{A}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_{12}$ synthesized under 5.5 GPa and 1250 °C by means of dc magnetic susceptibility, neutron powder diffraction, and specific heat. Although the magnetic $\mathrm{C}{\mathrm{o}}^{2+}$ ions form a geometrically frustrated hyper-Kagome lattice, we find that $\mathrm{C}{\mathrm{o}}_3\mathrm{A}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_{12}$ exhibits a long-range antiferromagnetic order below $T_{\mathrm{N}}=11\mathrm{K}$ without showing magnetic frustration, i.e., the frustration index $f\equiv |{\theta }_{\mathrm{CW}}/T_N|$ is close to unity. Refinements from neutron-diffraction data show that $\mathrm{C}{\mathrm{o}}_3\mathrm{A}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_{12}$ forms a complex antiferromagnetic structure characterized by the coexistence of collinear and commensurate spiral magnetic ordering of $\mathrm{C}{\mathrm{o}}^{2+}$ sublattices. Magnetic field induced spin-flop transition at ${\mu }_0{H}_c\approx 4\mathrm{T}$ was also observed below $T_{\mathrm{N}}$.

Nonlinear magnetization dynamics are of great interest, being, for instance, leveraged for neuromorphic computing in spin-transfer torque nano-oscillators. Here, we demonstrate how to implement magnetoacoustics to reach this regime, using monochromatic $(f_{\text{saw}}=450$ MHz) surface acoustic waves traveling on a thin layer of (Ga,Mn)As. By careful tuning of the precession frequency to both $f_{\text{saw}}$ and $2f_{\text{saw}}$ using the magnetic field and temperature, we evidence clear signatures of a nonlinear magnetoacoustic response of the magnetic dynamics using the time- and space-resolved Kerr effect: (i) frequency and wave-vector doubling in time and space, respectively, (ii) quadratic (sublinear) evolution of the precession amplitude at $2f_{\text{saw}}\left(f_{\text{saw}}\right)$ with acoustic amplitude, and (iii) resonance field shift. While (i) can be well reproduced by a parametric resonance model where nonlinearities arise solely from the SAW, we show that features (ii) and (iii) also involve intrinsic magnetic nonlinearities. Understanding the conditions leading to these nonlinearities will mean better control of the acoustic-wave-driven magnetization dynamics, in order to implement optimally the wave properties enabled by this approach.

Using classical Monte Carlo simulation, we investigate conditions for the occurrence of Vollhardt invariance in chiral magnets. We consider a classical spin-lattice model with exchange and Dzyaloshinskii-Moriya (DM) interaction in a magnetic field. We demonstrate that the Vollhardt crossing point seen in the temperature dependence of the specific heat is pretty robust against variations of the magnetic field only in the limited range of applied magnetic fields and at sufficiently large DM interaction when chiral fluctuations dominate over the ferromagnetic ones. At the same time, the behavior of Bragg intensity profile does not demonstrate any particular singularity across the Vollhardt crossing point. The spin arrangement displays almost fully polarized pattern at this point. For smaller magnitude of DM interaction, the Vollhardt crossing point is not identified, the specific-heat curves cross at different temperatures, and the hump anomaly can be interpreted as a smeared second-order phase transition.

More than 30 years ago, Malozemoff [Phys. Rev. B 35, 3679 (1987)] hypothesized that exchange interaction at the interface between a ferromagnet (F) and an antiferromagnet (AF) can act as an effective random field, which can profoundly affect the magnetic properties of the system. However, until now this hypothesis has not been directly experimentally tested. We utilize magnetoelectronic measurements to analyze the effective exchange fields at permalloy/CoO interface. Our results cannot be explained in terms of quasiuniform effective exchange fields but are in agreement with the random-field hypothesis of Malozemoff. The presented approach opens a new route for the quantitative analysis of effective exchange fields and anisotropies in magnetic heterostructures for memory, sensing and computing applications.

We introduce a quantum spin-$1/2$ model hosting two exact plaquette singlet ground states in extended parameter regimes. There is an exact phase transition between both phases, at which the system has an extensive ground-state degeneracy. Further, the model exhibits an extensive number of conserved quantities, which allow the prediction of additional phases. We exploit this feature and explore the phase diagram in detail for two specific parameter regimes. The model is based solely on Heisenberg interactions and seems sufficiently simple to be realized in quantum materials. A general scheme to determine exact singlet product states is briefly discussed.

The transmission and reflection of a spin wave at an internal boundary created by the local variation of anisotropy (or a bias magnetic field) are studied taking into account not only the changes in the wave amplitude, but also the changes in the wave polarization. It is shown that the account of the changes in the spin-wave polarization before and after the boundary leads to (i) increase of the spin-wave amplitude reflection coefficient, (ii) appearance of an additional phase shift $\mathrm{\Delta }\varphi \ne 0,\pi$ in both transmitted and reflected waves, and (iii) creation of additional evanescent waves in the vicinity of the boundary. It is also shown that even when significant changes in the transmitted wave polarization take place at the boundary, a spin wave could pass a finite-width boundary without reflection, if a certain resonance condition is satisfied. The effect of the polarization change at an internal boundary is especially pronounced for the exchange-dominated spin waves, while in the case of the dipole-dominated spin waves, this effect can vanish completely for certain configurations of the static magnetization.

We theoretically study the profile of a supercurrent in two-dimensional Josephson junctions with Rashba-Dresselhaus spin-orbit interaction (RDSOI) in the presence of a Zeeman field. Two types of RDSOIs are considered that might be accessible in $\mathrm{GaAs}$ quantum wells and zinc-blende materials. Through investigating self-biased supercurrent (so called ${\phi }_0$-Josephson state), we obtain explicit expressions for the functionality of the ${\phi }_0$ state with respect to RDSOI parameters $\left(\alpha ,\beta \right)$ and in-plane Zeeman field components $\left(h_x,h_y\right)$. Our findings reveal that when the chemical potential $\left(\mu \right)$ is high enough compared to the energy gap $\left(\mathrm{\Delta }\right)$ in superconducting electrodes, i.e., $\mu \gg \mathrm{\Delta }$, RSOI and DSOI with equal strengths $\left(\left|\alpha \right|=\left|\beta \right|\right)$ cause vanishing ${\phi }_0$ states independent of magnetization and the type of RDSOI. A Zeeman field with unequal components, i.e., $|h_x|\ne |h_y|$, however, can counteract and nullify the destructive impact of equal-strength RDSOIs (for one type only), where $\mu \sim \mathrm{\Delta }$, although $|h_x|=|h_y|$ can still eliminate the ${\phi }_0$ state. Remarkably, in the $\mu \sim \mathrm{\Delta }$ limit, the ${\phi }_0$ state is proportional to the multiplication of both components of an in-plane Zeeman field, i.e., $h_x{h}_y$, which is absent in the $\mu \gg \mathrm{\Delta }$ limit. Furthermore, our results of critical supercurrents demonstrate that the persistent spin helices can be revealed in a high enough chemical potential regime $\mu \gg \mathrm{\Delta }$, while an opposite regime, i.e., $\mu \sim \mathrm{\Delta }$, introduces an adverse effect. In ballistic regime, the “maximum” of the critical supercurrent occurs at $\left|\alpha \right|=\left|\beta \right|$ and the Zeeman field can boost this feature. The presence of disorder and nonmagnetic impurities change this picture drastically so the “minimum” of the critical supercurrent occurs at and around the symmetry lines $\left|\alpha \right|=\left|\beta \right|$. We show that the signature of persistent spin helices explored in disordered systems originate from the competition of short-range spin-singlet and long-range spin-triplet supercurrent components. Our study uncovers delicate details of how the interplay of RDSOI and a Zeeman field manifests in the ${\phi }_0$ state and critical supercurrent. Relying on the fact that the ${\phi }_0$ state is accessible regardless of the amount of nonmagnetic impurities and disorder, our results can provide guidelines for future experiments to confirm the presence of persistent spin helices, determine the type of SOI, and reliably extract SOI parameters in a system, which might be helpful in devising spin-orbit-coupled spintronics devices and ultrasensitive spin-transistor technologies.

We construct an exactly solvable commuting projector model for a ($4+1$)-dimensional ${\mathbb{Z}}_2$-symmetry-protected topological phase (SPT) which is outside the cohomology classification of SPTs. The model is described by a decorated domain wall construction, with “three-fermion” Walker-Wang phases on the domain walls. We describe the anomalous nature of the phase in several ways. One interesting feature is that, in contrast to in-cohomology phases, the effective ${\mathbb{Z}}_2$ symmetry on a ($3+1$)-dimensional boundary cannot be described by a quantum circuit and instead is a nontrivial quantum cellular automaton. A related property is that a codimension-two defect (for example, the termination of a ${\mathbb{Z}}_2$ domain wall at a trivial boundary) will carry nontrivial chiral central charge 4 mod 8. We also construct a gapped symmetric topologically ordered boundary state for our model, which constitutes an anomalous symmetry-enriched topological phase outside of the classification of Chen and Hermele, and define a corresponding anomaly indicator.

We study the formation of chaos and strange attractors in the order parameter space of a system of two coupled, nonresonantly driven exciton-polariton condensates. The typical scenario of bifurcations experienced by the system with increasing external pumping consists of (i) formation of $\pi$-synchronized condensates at low pumping, (ii) symmetry-breaking pitchfork bifurcation leading to unequal occupations of the condensates with a nontrivial phase difference between them, (iii) loss of the stability of all fixed points in the system resulting in chaotic dynamics, (iv) limit cycle dynamics of the order parameter, which ends up in (v) in-phase synchronized condensates via the Hopf bifurcation from a limit cycle. The chaotic dynamics of the order parameter is evidenced by calculating the maximum Lyapunov exponent. The presence of a chaotic domain is studied as a function of polariton-polariton interaction and the Josephson coupling between the condensates. At some values of the parameters, the bifurcation route is more complex and the strange attractor can coexist with the stable fixed-point lasing. We also investigate how the chaotic dynamics is reflected in the light emission spectrum from the microcavity.

We present a theoretical scheme that allows one to perform a universal set of holonomic gates on a two-qubit register, formed by a ${}^{13}\mathrm{C}$ nuclear spin coupled to the electron spin of a nitrogen-vacancy center in diamond. Strong hyperfine interaction between the electron spin and the spins of the first three shells of ${}^{13}\mathrm{C}$ atoms allows one to operate the state of the register on the submicrosecond timescale using microwave pulses only. We describe the system and the operating regime analytically and numerically, as well as simulate the initialization protocols.

We numerically obtain the conformal spectrum of several classical spin models on a two-dimensional lattice with open boundaries for every boundary fixed point obtained by the Cardy's derivation [J. L. Cardy, Nucl. Phys. B 324, 581 (1989)]. In order to extract accurate conformal data, we implement the tensor network renormalization algorithm [G. Evenbly and G. Vidal, Phys. Rev. Lett. 115, 180405 (2015)] extended so as to be applicable to a square lattice with open boundaries. We successfully compute the boundary conformal spectrum consistent with the underlying boundary conformal field theories (BCFTs) for the Ising, tricritical Ising, and three-state Potts models on the lattice, which allows us to confirm the validity of the BCFT analyses for the surface critical behaviors of those lattice models.

We theoretically investigate concomitant topological properties of twisted bilayer graphene by a continuum model when gaps are opened in its two monolayers. An effective Hamiltonian is derived for the flat bands of a twisted bilayer in the vicinity of its Dirac points, then the topological characteristics of these flat bands can be identified for different valleys. Numerical calculations show that topological phases can be induced and modulated by the gaps in the two single layers. A phase diagram is obtained and is divided into three regions with the Chern number $C=0,\pm 1$, respectively, separated by two straight lines. These observed phenomena can be well explained using simplified analytical treatments. Moreover, we can distinguish the flat bands into four topologically different states, which will bring applications into electronics and valleytronics.

The results of the commented paper seem to disagree with the prediction of the paper by Safaei et al. [Phys. Rev. B 88, 045305 (2013)]. Some of the definitions of surface unit cells used in the commented article do not lead to the first surface Brillouin zone, defining instead a folded Brillouin zone. This is the source of the seeming discrepancies. To dispel the misunderstandings we present general rules for Brillouin zone shapes and $L$ points projections for $\left(ppq\right)$ surfaces of the rock salt crystals. We show that using the first Brillouin zones allows a simple, straightforward prediction of the topological properties of the surface states.

Adopting undoped ultraclean two-dimensional hole systems, we approach a strongly correlated limit by reducing the carrier density down to $1\times {10}^9{\mathrm{cm}}^{-2}$. The temperature dependence of the resistivity as a function of the carrier density reveals a characteristic energy scale displaying a benchmark critical behavior near a critical density of $p_c\sim 4\times {10}^9{\mathrm{cm}}^{-2}$. The insulating state below $p_c$ exhibits a sharp resistance discontinuity in response to heating across a critical temperature $T_{c1}\sim 30\mathrm{mK}$, consistent with a first-order transition. The dc response also identifies a second critical temperature $T_{c2}$ where linear IV behavior is recovered. Similar effects are also demonstrated by varying an external electric field. The results support a complex quantum phase transition with intermediate phases.

Quantum Hall stripe (QHS) phases, predicted by the Hartree-Fock theory, are manifested in GaAs-based two-dimensional electron gases as giant resistance anisotropies. Here, we predict a “hidden” QHS phase which exhibits isotropic resistivity whose value, determined by the density of states of QHS, is independent of the Landau index $N$ and is inversely proportional to the Drude conductivity at zero magnetic field. At high enough $N$, this phase yields to an Ando-Unemura/Coleridge-Zawadski-Sachrajda phase in which the resistivity is proportional to $1/N$ and to the ratio of quantum and transport lifetimes. Experimental observation of this border can provide an alternative way to obtain quantum relaxation time.

Nonsymmorphic symmetries protect Dirac line nodes in square-net materials. This phenomenon has been most prominently observed in ZrSiS. Here, we systematically study the symmetry-protected nodal fermions that result from different ways of embedding the square net into a larger unit cell. Surprisingly, we find that a nonsymmorphic space group is not a necessary condition for a filling enforced semimetal: symmorphic space groups can also host nodal fermions that are enforced by band folding and electron count, that is, a combination of a particular structural motif combined with electron filling. We apply the results of this symmetry analysis to define an algorithm, which we utilize to find square-net materials with nodal fermions in specific symmorphic space groups. We highlight one result of this search, the compound ThGeSe, which we discuss in the context of nodal fermions. Finally, we discuss how band folding can impose constraints on band connectivity beyond the connectivity of single elementary band representations.