Nitrogen-vacancy (NV) centers in diamond can be used for nanoscale sensing with atomic resolution and sensitivity; however, it has been observed that their properties degrade as they approach the diamond surface. Here we report that in addition to degraded spin coherence, NV centers within nanometers of the surface can also exhibit decreased fluorescence contrast for optically detected electron spin resonance (OD-ESR). We demonstrate that this decreased OD-ESR contrast arises from charge state dynamics of the NV center, and that it is strongly surface-dependent, indicating that surface engineering will be critical for nanoscale sensing applications based on color centers in diamond.

We investigate the stability of an Anderson localized chain to the inclusion of a single finite interacting thermal seed. This system models the effects of rare low-disorder regions on many-body localized chains. Above a threshold value of the mean localization length, the seed causes runaway thermalization in which a finite fraction of the orbitals are absorbed into a thermal bubble. This “partially avalanched” regime provides a simple example of a delocalized, nonergodic dynamical phase. We derive the hierarchy of length scales necessary for typical samples to exhibit the avalanche instability, and show that the required seed size diverges at the avalanche threshold. We introduce a dimensionless statistic that measures the effective size of the thermal bubble, and use it to numerically confirm the predictions of avalanche theory in the Anderson chain at infinite temperature.

We study the spin-1 honeycomb lattice magnets with frustrated exchange interactions. The proposed microscopic spin model contains first- and second-neighbor Heisenberg interactions as well as the single-ion anisotropy. We establish a rich phase diagram that includes a featureless quantum paramagnet and various spin spiral states induced by the mechanism of order by quantum disorder. Although the quantum paramagnet is dubbed featureless, it is shown that the magnetic excitations develop a contour degeneracy in the reciprocal space at the band minima. These contour degenerate excitations are responsible for the frustrated criticality from the quantum paramagnet to the ordered phases. This work illustrates the effects of magnetic frustration on both magnetic orderings and the magnetic excitations. We discuss the experimental relevance to various Ni-based honeycomb lattice magnets.

Motivated by recent experimental progress in the study of quantum systems far from equilibrium, we investigate the relation between several dynamical signatures of topology in the coherent time evolution after a quantum quench. Specifically, we study the conditions for the appearance of entanglement spectrum crossings, dynamical quantum phase transitions, and dynamical Chern numbers. For noninteracting models, we show that in general there is no direct relation between these three quantities. Instead, we relate the presence of level crossings in the entanglement spectrum to localized boundary modes that may not be of topological origin in the conventional sense. We exemplify our findings with explicit simulations of one-dimensional two-banded models. Finally, we investigate how interactions influence the presence of entanglement spectrum crossings and dynamical quantum phase transitions, by means of time-dependent density matrix renormalization group simulations.

Zeeman spectra of the R lines of ruby (${\mathrm{Cr}}^{3+}$: $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$) were studied in ultrahigh magnetic fields up to 230 T by magnetophotoluminescence measurements. The observed Zeeman patterns exhibit nonlinear behaviors above 100 T, evidencing the breakdown of the previously reported Paschen-Back effect for $\mathbf{B}\perp c$ geometry. We adopted the crystal-field multiplet theory including the cubic crystal field (${\mathcal{H}}_{\mathrm{cubic}}$), the trigonal crystal field (${\mathcal{H}}_{\mathrm{trig}}$), the spin-orbit interaction (${\mathcal{H}}_{\mathrm{SO}}$), and the Zeeman interaction (${\mathcal{H}}_{\mathrm{Z}}$). It is found that the nonlinear splitting of the R lines is owing to the hybridization between the ${}^{2}E$ and ${}^{2}T_1$ states, which leads to the quantization of these Zeeman levels with the orbital angular momentum. Our results suggest that the exquisite energy balance among ${\mathcal{H}}_{\mathrm{cubic}}$, ${\mathcal{H}}_{\mathrm{trig}}$, ${\mathcal{H}}_{\mathrm{SO}}$, and ${\mathcal{H}}_{\mathrm{Z}}$ realized in ruby offers a unique opportunity to observe the onset of the crystal-field Paschen-Back effect toward the high-field extreme.

This paper presents a theoretical description of both the valley Zeeman effect ($g$-factors) and Landau levels in two-dimensional H-phase transition metal dichalcogenides (TMDs) using the Luttinger-Kohn approximation with spin-orbit coupling. At the valley extrema in TMDs, energy bands split into Landau levels with a Zeeman shift in the presence of a uniform out-of-plane external magnetic field. The Landau level indices are symmetric in the $K$ and $K^{\prime }$ valleys. We develop a numerical approach to compute the single-band $g$-factors from first principles without the need for a sum over unoccupied bands. Many-body effects are included perturbatively within the GW approximation. Nonlocal exchange and correlation self-energy effects in the GW calculations increase the magnitude of single-band $g$-factors compared to those obtained from density functional theory. Our first-principles results give spin- and valley-split Landau levels, in agreement with recent optical experiments. The exciton $g$-factors deduced in this work are also in good agreement with experiment for the bright and dark excitons in monolayer ${\mathrm{WSe}}_2$, as well as the lowest-energy bright excitons in ${\mathrm{MoSe}}_2\text{−}{\mathrm{WSe}}_2$ heterobilayers with different twist angles.

Schemes for topological quantum computation with Majorana bound states rely heavily on the ability to projectively measure products of Majorana operators. Here, we employ Markovian quantum measurement theory, including the readout device, to analyze such measurements. Specifically, we focus on the readout of Majorana qubits via continuous charge sensing of a tunnel-coupled quantum dot by a quantum point contact. We show that projective measurements of Majorana products ${\prod }_i{\widehat{\gamma }}_i$ can be implemented by continuous charge sensing under quite general circumstances. Essential requirements are that a combined local parity $\widehat{\pi }$, involving the quantum dot charge along with the Majorana product of interest, be conserved, and that the two eigenspaces of the combined parity $\widehat{\pi }$ generate distinguishable measurement signals. We find that qubit readout may have to rely on measuring noise correlations of the quantum-point-contact current. The average current encodes the qubit readout only transiently for fine-tuned parameters or in the presence of relaxation processes. We also discuss the corresponding measurement and decoherence times and consider processes, such as residual Majorana hybridizations, which are detrimental to the measurement protocol. Finally, we emphasize that the underlying mechanism—which we term symmetry-protected readout—is quite general and has further implications for both Majorana and non-Majorana systems.

We study the time-dependent effect of Markovian readout processes on Majorana qubits whose parity degrees of freedom are converted into the charge of a tunnel-coupled quantum dot. By applying a recently established effective Lindbladian approximation [Kiršanskas, Franckié, and Wacker, Phys. Rev. B 97, 035432 (2018); Mozgunov and Lidar, Quantum 4, 227 (2020); Nathan and Rudner, arXiv:2004.01469], we obtain a completely positive and trace preserving Lindblad master equation for the combined dot-qubit dynamics, describing relaxation and decoherence processes beyond the rotating-wave approximation. This approach is applicable to a wide range of weakly coupled environments representing experimentally relevant readout devices. We study in detail the case of thermal decay in the presence of a generic Ohmic bosonic bath, in particular for potential fluctuations in an electromagnetic circuit. In addition, we consider the nonequilibrium measurement environment for a parity readout using a quantum point contact capacitively coupled to the dot charge.

We study the phase diagram of two-flavor massless two-color QCD (${\mathrm{QC}}_2\mathrm{D}$) under the presence of quark chemical potentials and imaginary isospin chemical potentials. At the special point of the imaginary isospin chemical potential, called the isospin Roberge-Weiss (RW) point, two-flavor ${\mathrm{QC}}_2\mathrm{D}$ enjoys the ${\mathbb{Z}}_2$ center symmetry that acts on both quark flavors and the Polyakov loop. We find a ${\mathbb{Z}}_2$ 't Hooft anomaly of this system, which involves the ${\mathbb{Z}}_2$ center symmetry, the baryon-number symmetry, and the isospin chiral symmetry. Anomaly matching, therefore, constrains the possible phase diagram at any temperatures and quark chemical potentials at the isospin RW point, and we compare it with previous results obtained by chiral effective field theory and lattice simulations. We also point out an interesting similarity of two-flavor massless ${\mathrm{QC}}_2\mathrm{D}$ with ($2+1$)-dimensional quantum antiferromagnetic systems.

At finite temperatures, the quantum critical region (QCR) emerges as a consequence of the interplay between thermal and quantum fluctuations. We seek suitable physical quantities, which during dynamics can give prominent response to QCR in the transverse field quantum $XY$ model. We report that the maximum energy absorbed, the nearest neighbor entanglement, and the quantum mutual information of the time-evolved state after a quench of the transverse magnetic field exhibit a faster drop with temperature when the initial magnetic field is taken from within the QCR, compared to the choice of the initial point from different phases. We propose a class of dynamical quantifiers, originating from the response of these physical quantities and show that they can faithfully mimic the equilibrium physics, namely detection of the QCR at finite temperatures.

We have synthesized a series of the Ruddlesden-Popper nickelate solid solution $A_{4–x}{B}_x{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($A$ and $B=\mathrm{La},\mathrm{Pr},\text{and} \mathrm{Nd};x=0,1,2,\text{and} 3$) via the citrate precursor method in different reacting atmospheres. Both the electronic-transport and magnetization measurements on these samples show well-defined phase transitions at temperatures between 135 and 165 K. These transition temperatures as well as the room-temperature resistivities strongly correlate with the Goldschmidt tolerance factor $t$, irrespective of the combination of the magnetic rare-earth ions with nonmagnetic $\mathrm{L}{\mathrm{a}}^{3+}$. We conclude that these changes of the electronic properties are mostly a consequence of the distortion of the $\mathrm{Ni}{\mathrm{O}}_6$ octahedra at the phase transition which is intimately related to the tolerance factor $t$, but appear to be rather insensitive to the average magnetic moment of the rare-earth ions upon exchanging La by Pr or Nd.

A central tenet in the theory of quantum phase transitions (QPTs) is that a nonanalyticity in a ground-state energy implies a QPT. Here we report on a finding that challenges this assertion. As a case study we take a phase diagram of a one-dimensional band insulator with spin-orbit coupled electrons, supporting trivial, and topological gapped phases separated by intersecting critical surfaces. The intersections define multicritical lines across which the ground-state energy becomes nonanalytical, concurrent with a closing of the band gap, but with no phase transition taking place.

Atomistic spin models have found enormous success in addressing the properties of magnetic materials, grounded on the identification of the relevant underlying magnetic interactions. For instance, the huge development in the field of magnetic skyrmions and other noncollinear magnetic structures is largely due to our understanding of the chiral Dzyaloshinskii-Moriya interaction. Recently, various works have proposed new types of chiral interactions, with seemingly different forms, but the big picture is still missing. Here, we present a systematic construction of a generalized spin model containing isotropic and chiral multisite interactions. These are motivated by a microscopic model that incorporates local spin moments and the spin-orbit interaction, and their symmetry properties are established. We show that the chiral interactions arise solely from the spin-orbit interaction and that the multisite interactions do not have to follow Moriya's rules, unlike the Dzyaloshinskii-Moriya and chiral biquadratic interactions. The chiral multisite interactions do not vanish as a result of inversion symmetry and comply with a generalized Moriya rule: If all sites connected by the interaction lie in the same mirror plane, the chiral interaction vector must be perpendicular to this plane. We then illustrate our theoretical considerations with density functional theory calculations for prototypical magnetic systems. These are triangular trimers built out of Cr, Mn, Fe, and Co adatoms on the Re(0001), Pt(111), and Au(111) surfaces, for which $C_3\mathrm{v}$ symmetry applies, and Cr and Fe square tetramers on Pt(001) with $C_4\mathrm{v}$ symmetry. The multisite interactions are substantial in magnitude and cannot be neglected when comparing the energy of different magnetic structures. Finally, we discuss the recent literature in light of our findings and clarify several unclear or confusing points.

We study bosonic symmetry-protected topological (SPT) phases with $C_2$ rotational symmetry in four spatial dimensions which is not captured by the group cohomology classification. By using the topological crystal approach, we show that the topological crystalline state of this SPT phase is given by placing an $E_8$ state on the two-dimensional rotational invariant plane, which provides a simple physical picture of this phase. Based on this understanding, we show that a variant of QED in four dimensions (${\mathrm{QED}}_4$) with charge-1 and charge-3 Dirac fermions is a field theoretical description of the three-dimensional boundary. We also discuss the connection to a symmetric gapped boundary with topological order and its anomalous signature.

Ultralow-lying magnon energy spectrum in yttrium iron garnet (YIG) has been studied by inelastic neutron scattering in an energy range from 10 to 45 $\mu \mathrm{eV}$. When a magnetic field of approximately 0.1 T was applied along [111] direction, ultralow-energy magnon anomaly was found at 10 K, suggesting the closure of the Zeeman energy gap. The anomaly was also observed in the temperature dependence of magnetization under a magnetic field along the [111] direction below 30 K. The specific heat capacity confirms the closure of the Zeeman energy gap at the magnetic field direction. All these anomalies strongly support the magnetic crossover below 30 K under the magnetic field along [111], in addition to a precursor anomaly below 150 K.

We show that dilute magnetic impurities in a conventional superconductor give origin to an odd-frequency component of superconductivity, manifesting itself in Yu-Shiba-Rusinov bands forming within the bulk superconducting gap. Our results are obtained in a general model solved within the dynamical mean field theory. By exploiting a disorder analysis and the limit to a single impurity, we are able to provide general expressions that can be used to extract explicitly the odd-frequency superconducting function from scanning tunneling measurements.

We demonstrate that the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in graphene can be strongly modified by a time-periodic driving field even in the weak drive regime. This effect is due to the opening of a dynamical band gap at the Dirac points when graphene is exposed to circularly polarized light. Using Keldysh-Floquet Green's functions, we develop a theoretical framework to calculate the time-averaged RKKY coupling under weak periodic drives, and we show that its magnitude in undoped graphene can be decreased controllably by increasing the driving strength, while mostly maintaining its ferromagnetic or antiferromagnetic character. In doped graphene, we find RKKY oscillations with a period that is tunable by the driving field. When a sufficiently strong drive is turned on that brings the Fermi level completely within the dynamically opened gap, the behavior of the RKKY coupling changes qualitatively from that of doped to undoped irradiated graphene.

In various topological phases, nontrivial states appear at the boundaries of the system. In this paper, we investigate anomalous dielectric response caused by such states caused by the $\pi$ Zak phase. First, by using the one-dimensional Su-Schrieffer-Heeger model, we show that, when the system is insulating and the Zak phase is $\pi$, the polarization suddenly rises to a large value close to $e/2$, by application of an external electric field. The $\pi$ Zak phase indicates the existence of half-filled edge states, and we attribute this phenomenon to charge transfer between the edge states at the two ends of the system. We extend this idea to two- and three-dimensional insulators with the $\pi$ Zak phase over the Brillouin zone and find similar anomalous dielectric response. We also show that diamond and silicon slabs with (111) surfaces have the $\pi$ Zak phase by ab initio calculations, and show that this anomalous response survives even surface reconstruction involving an odd number of original surface unit cells. Another material example with an anomalous dielectric response is polytetrafluoroethylene (PTFE), showing plateaus of polarization at $\pm e$ by ab initio calculation, in agreement with our theory.

Motivated by two independent experiments revealing a resistance minimum at the Landau level (LL) filling factor $\nu =2+4/9$, characteristic of the fractional quantum Hall effect (FQHE) and suggesting electron condensation into a yet unknown quantum liquid, we propose that this state likely belongs in a parton sequence, put forth recently to understand the emergence of FQHE at $\nu =2+6/13$. While the $\nu =2+4/9$ state proposed here directly follows three simpler parton states, all known to occur in the second LL, it is topologically distinct from the Jain composite fermion (CF) state which occurs at the same $\nu =4/9$ filling of the lowest LL. We predict experimentally measurable properties of the $4/9$ parton state that can reveal its underlying topological structure and definitively distinguish it from the $4/9$ Jain CF state.

We consider a non-Abelian candidate state at filling factor $\nu =3/7$ state belonging to the parton family. We find that, in the second Landau level of GaAs (i.e., at filling factor $\nu =2+3/7$), this state is energetically superior to the standard Jain composite-fermion state and also provides a very good representation of the ground state found in exact diagonalization studies of finite systems. This leads us to predict that $\mathit{if}$ a fractional quantum Hall effect is observed at $\nu =3/7$ in the second Landau level, it is likely to be described by this new non-Abelian state. We enumerate experimentally measurable properties that can verify the topological structure of this state.