We consider the behavior of quantum Hall edges away from the Luttinger liquid fixed point that occurs in the low-energy, large-system limit. Using the close links between quantum Hall wave functions and conformal field theories, we construct effective Hamiltonians from general principles and then constrain their forms by considering the effect of bulk symmetries on the properties of the edge. In examining the effect of bulk interactions on this edge, we find remarkable simplifications to these effective theories which allow for a very accurate description of the low-energy physics of quantum Hall edges relatively far away from the Luttinger liquid fixed point, and which apply to small systems and higher energies.

We report anomalous behavior in the energy dispersion of a three-electron double-quantum-dot hybrid qubit and argue that it is caused by atomic-scale disorder at the quantum-well interface. By employing tight-binding simulations, we identify potential disorder profiles that induce behavior consistent with the experiments. The results indicate that disorder can give rise to “sweet spots” where the decoherence caused by charge noise is suppressed, even in a parameter regime where true sweet spots are unexpected. Conversely, “hot spots” where the decoherence is enhanced can also occur. Our results suggest that interfacial atomic structure can be used in particular cases as a tool to enhance the fidelity of Si double-dot qubits.

We elucidate the magnetic interactions and the role of spin (electron) correlation in determining the ground state of the honeycomb compound $\mathrm{M}{\mathrm{n}}_4\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_9$, by neutron powder diffraction, inelastic neutron scattering (INS), specific-heat ($C_P$) measurements, and electronic-structure calculations. The antiferromagnetic long-range order with moments along $c$ occurs at 102 K with strong exchange striction and small anisotropy. It is described by the three-dimensional Ising model. Diffuse magnetic scattering has been observed above $T_N$, which is attributed to the two-dimensional spin correlations within the $\mathrm{M}{\mathrm{n}}^{2+}$ honeycombs. This is confirmed by the calculated exchange constants. INS experiments and spin-wave simulations together with $C_P$ measurements reveal two gapped modes on the $ab$ plane, originating from the rotation of the spins away from the easy axis $c$. The magnetic anisotropy is mainly determined by an electron-correlation-assisted dipole-dipole interaction. This work provides insight into the competing origins of the magnetic anisotropy, which leads to different magnetic ground states in the family of honeycomb compounds.

Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization survive longer barrier thicknesses when compared with singlet supercurrents. The distinctive feature of triplet supercurrents is the scaling of the characteristic voltage of the junction with increasing ferromagnetic barrier thickness - an algebraic decay in contrast to an exponential decay for singlet supercurrents. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynamics of junctions with triplet supercurrents exhibits long (in terms of proximity) junction behavior and moderately damped dynamics with renormalized capacitance and resistance. This unconventional behavior possibly provides a different way to dynamically detect triplets. Our results show that new theoretical models are required to fully understand the phase dynamics of triplet Josephson junctions for applications in superconducting spintronics.

Recent realization of a kinetically constrained chain of Rydberg atoms by Bernien et al., [Nature (London) 551, 579 (2017)] resulted in the observation of unusual revivals in the many-body quantum dynamics. In our previous work [C. J. Turner et al., Nat. Phys. 14, 745 (2018)], such dynamics was attributed to the existence of “quantum scarred” eigenstates in the many-body spectrum of the experimentally realized model. Here, we present a detailed study of the eigenstate properties of the same model. We find that the majority of the eigenstates exhibit anomalous thermalization: the observable expectation values converge to their Gibbs ensemble values, but parametrically slower compared to the predictions of the eigenstate thermalization hypothesis (ETH). Amidst the thermalizing spectrum, we identify nonergodic eigenstates that strongly violate the ETH, whose number grows polynomially with system size. Previously, the same eigenstates were identified via large overlaps with certain product states, and were used to explain the revivals observed in experiment. Here, we find that these eigenstates, in addition to highly atypical expectation values of local observables, also exhibit subthermal entanglement entropy that scales logarithmically with the system size. Moreover, we identify an additional class of quantum scarred eigenstates, and discuss their manifestations in the dynamics starting from initial product states. We use forward scattering approximation to describe the structure and physical properties of quantum scarred eigenstates. Finally, we discuss the stability of quantum scars to various perturbations. We observe that quantum scars remain robust when the introduced perturbation is compatible with the forward scattering approximation. In contrast, the perturbations which most efficiently destroy quantum scars also lead to the restoration of “canonical” thermalization.

We investigated third-order correlations between photons born in single quantum dot resonance fluorescence that were filtered with narrow-band tunable etalons. Three-time autocorrelation measurements in which photons were identically filtered resulted in correlation maps that are functions of two relative delays. A comparison with the correlation maps computed using the “sensors method” introduced by del Valle et al. [Phys. Rev. Lett. 109, 183601 (2012)] reveals faithful agreement with theory, with the strongest correlations obtained when filtering between Mollow triplet sidebands and the central peak. We characterized the correlations associated with these virtual transitions and compared them with correlations at other Mollow triplet frequency windows. Accentuated nonclassical characteristics are amongst the distinguishing features of three photon spectra.

Precision measurements of the magnetization and ac susceptibility of ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ are reported for magnetic fields along different crystallographic directions, focusing on the border between the conical and the field-polarized state for a magnetic field along the $\langle 100\rangle$ axis, complemented by selected specific heat data. Clear signatures of the emergence of a second skyrmion phase and a tilted conical phase are observed, as recently identified by means of small angle neutron scattering. The low-temperature skyrmion phase displays strongly hysteretic phase boundaries, but no dissipative effects. In contrast, the tilted conical phase is accompanied by strong dissipation and higher-harmonic contributions, while the transition fields are essentially nonhysteretic. The formation of the second skyrmion phase and tilted conical phase are found to be insensitive to a vanishing demagnetization factor. A quantitative estimate of the temperature dependence of the magnetocrystalline anisotropy may be consistently inferred from the magnetization and the upper critical field and agrees well with a stabilization of the low-temperature skyrmion phase and tilted conical state by conventional cubic magnetic anisotropies.

The evolution of the low temperature antiferromagnetic conical (characterized by two, commensurate and incommensurate propagation vectors), and the high temperature collinear spin arrangements of the 20% Co-doped $\mathrm{MnW}{\mathrm{O}}_4$ multiferroic has been studied in the presence of magnetic field up to 60 T by means of macroscopic magnetic and pyroelectric measurements, and by neutron diffraction experiments in fields up to 12 T on a single crystal. The complete magnetoelectric phase diagrams for magnetic fields along distinct magnetic directions with respect to the spin structure have been constructed up to magnetic field values exceeding those necessary to induce a spin-flip transition into the paramagnetic state. The differences in the topology of the diagrams are discussed. The obtained results might be common for other magnetic materials possessing conical antiferromagnetic structures.

We present results of low-temperature scanning tunneling microscopy/spectroscopy measurements on the one-atom-layer superconductor, $\mathrm{Si}\left(111\right)\text{−}\surd 3\times \surd 3\text{−}\left(\mathrm{Tl},\mathrm{Pb}\right)$ which has a spin-split band structure due to the Rashba-Bychkov effect. It was revealed that it has a multiple superconducting gap Δ with significant anisotropy and sizable magnitude of $2\mathrm{\Delta }/k_B{T}_C\sim 8.6(k_B$ is the Boltzmann constant and $T_C$ is the critical superconducting temperature). Under the magnetic field, a dip structure was observed even at the core of the vortex. The dip structure was like a “pseudogap” because it remained up to 2 T, well above the upper critical magnetic field determined in the transport measurements (∼0.7 T). This unconventional behavior suggests the possibility of spin-triplet Cooper pairs related to parity-broken superconductors.

The detailed evolution of the local atomic structure across the ($x$, $T$) phase diagram of transition metal dichalcogenide superconductor ${\mathrm{Ir}}_{1-x}{\mathrm{Rh}}_x{\mathrm{Te}}_2$ ($0\le x\le 0.3$, $10\mathrm{K}\le T\le 300\mathrm{K}$) is obtained from high-quality x-ray diffraction data using the atomic pair distribution function (PDF) method. The observed hysteretic thermal structural phase transition from a trigonal ($P\overline{3}m1$) to a triclinic ($P\overline{1}$) dimer phase for low Rh content emphasizes the intimate connection between the lattice and electronic properties. For superconducting samples away from the dimer/superconductor phase boundary, structural transition is absent and the local structure remains trigonal down to 10 K. In the narrow range of compositions close to the boundary, PDF analysis reveals structural phase separation, suggestive of weak first-order character of the Rh-doping induced dimer-superconductor quantum phase transition. Samples from this narrow range show weak anomalies in electronic transport and magnetization, hallmarks of the dimer phase, as well as superconductivity albeit with incomplete diamagnetic screening. The results suggest that the dimer and superconducting orders exist in the mutually exclusive spatial regions.

The exponential growth or decay with time of the out-of-time-order commutator (OTOC) is one widely used diagnostic of many-body chaos in spatially extended systems. In studies of many-body classical chaos, it has been noted that one can define a velocity-dependent Lyapunov exponent, $\lambda \left(\mathbf{v}\right)$, which is the growth or decay rate along rays at that velocity. We examine the behavior of $\lambda \left(\mathbf{v}\right)$ for a variety of many-body systems, both chaotic and integrable. The so-called light cone for the spreading of operators is defined by $\lambda \left(\widehat{\mathbf{n}}{v}_B\left(\widehat{\mathbf{n}}\right)\right)=0$, with a generally direction-dependent butterfly speed $v_B\left(\widehat{\mathbf{n}}\right)$. In spatially local systems, $\lambda \left(v\right)$ is negative outside the light cone where it takes the form $\lambda \left(v\right)\sim -{(v-v_B)}^{\alpha }$ near $v_B$, with the exponent $\alpha$ taking on various values over the range of systems we examine. The regime inside the light cone with positive Lyapunov exponents may only exist for classical, semiclassical, or large-$N$ systems, but not for “fully quantum” chaotic systems with strong short-range interactions and local Hilbert space dimensions of order one.

We present a paradoxical finding that, in the vicinity of a topological phase transition in a quantum anomalous Hall system (Chern insulator), topology nearly always (except when the system obeys charge-conjugation symmetry) results in a significant extension of the edge-state structure beyond the minimal one required to satisfy the Chern numbers. The effect arises from the universal gapless linear-in-momentum Hamiltonian of the nodal semimetal describing the system right at the phase transition, whose form is enforced by the change of the Chern number. Its emergent approximate chiral symmetry results in an edge-state band in the vicinity of the node, in the region of momenta where such a form is dominant. Upon opening the gap, this edge-state band is modified in the gap region, becoming “protected” (connected to the valence bulk band with one end and conduction band with the other) in the topologically nontrivial phase and “nonprotected” (connected to either the valence or conduction band with both ends) in the trivial phase. The edge-state band persists in the latter as long as the gap is small enough.

‘‘Pinch points,” singular features observed in (quasi-)elastic neutron scattering, are a widely discussed hallmark of spin liquids with an emergent gauge symmetry. Much less attention has been paid to “half-moons,” distinctive crescent patterns at finite energy, which have been observed in experiments on a number of pyrochlore magnets, and in a wide range of model calculations. Here we unify these two phenomena within a single framework, paying particular attention to the case of ordered, or field-saturated states, where pinch points and half moons can be found in bands of excitations above a gap. We find that half moons are nothing other than pinch points inscribed on a dispersing band. Molecular dynamics simulations of the kagome-lattice antiferromagnet are used to explore how these bands evolve into the ground state and excitations of a classical spin liquid. We explicitly demonstrate that this theory can reproduce the pinch points and half moons observed in ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$.

By measuring the spatial distribution of differential conductance near nonmagnetic impurities on Fe sites, we have obtained the quasiparticle interference (QPI) patterns in the $({\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x){\mathrm{OHFe}}_{1-y}{\mathrm{Zn}}_y\mathrm{Se}$ superconductor with only electron Fermi surfaces. By taking the Fourier transform on these patterns, we investigate the scattering features between the two circles of electron pockets formed by folding or hybridization. We treat the data by using the recent theoretical approach [Chi et al., arXiv:1710.09089], which is specially designed for the impurity-induced bound states. It is found that the opposite superconducting gap signs on the two electron pockets can be directly visualized by the phase-referenced QPI technique, indicating that the Cooper pairing is induced by the repulsive interaction. Our results show that this method is also applicable for data measured for multiple impurities, which provides an easy and feasible way for detecting the gap function of unconventional superconductors.

Employing the standard worldline-vortex mapping, we conclude that at the critical temperature, superconductors demonstrate the magnetohalon effect with respect to the quantized net magnetic flux generated by a solenoid inserted into the system. The effect is a direct counterpart of the recently revealed halon effect in terms of the quantized particle charge of a static impurity in the two-dimensional U(1) quantum-critical environment. The flux-loop model (a.k.a. frozen lattice superconductor) with a quasisolenoid perturbation proves to be the model of choice for qualitative and quantitative description of both the halon and the magnetohalon effects.

Magnetic helices and skyrmions in noncentrosymmetric magnets are representative examples of chiral spin textures in solids. Their spin swirling direction, often termed as the magnetic helicity and defined as either left handed or right handed, is uniquely determined by the Dzyaloshinskii-Moriya interaction (DMI) in fixed chirality host crystals. Thus far, there have been relatively few investigations of the DMI in metallic magnets as compared with insulating counterparts. Here, we focus on the metallic magnets ${\mathrm{Co}}_{8-x}{\mathrm{Fe}}_x{\mathrm{Zn}}_8{\mathrm{Mn}}_4$ ($0\le x\le 4.5$) with a $\beta$-Mn-type chiral structure and find that, as $x$ varies under a fixed crystal chirality, a reversal of magnetic helicity occurs at $x_{\mathrm{c}}\sim 2.7$. This experimental result is supported by a theory based on first-principles electronic structure calculations, demonstrating the DMI to depend critically on the electron band filling. Thus by composition tuning our work shows the sign change of the DMI with respect to a fixed crystal chirality to be a universal feature of metallic chiral magnets.

By combining electron energy-loss spectroscopy and state-of-the-art computational methods, we were able to provide an extensive picture of the excitonic processes in $1T-{\mathrm{HfS}}_2$. The results differ significantly from the properties of the more scrutinized group VI semiconducting transition metal dichalcogenides such as ${\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2$. The measurements revealed a parabolic exciton dispersion for finite momentum $q$ parallel to the $\mathrm{\Gamma }\mathrm{K}$ direction which allowed the determination of the effective exciton mass. The dispersion decreases monotonically for momentum exchanges parallel to the $\mathrm{\Gamma }\mathrm{M}$ high symmetry line. To gain further insight into the excitation mechanisms, we solved the ab initio Bethe-Salpeter equation for the system. The results matched the experimental loss spectra closely, thereby confirming the excitonic nature of the observed transitions, and produced the momentum-dependent binding energies. The simulations also demonstrated that the excitonic transitions for $\mathit{q}\left|\right|\mathrm{\Gamma }\mathrm{M}$ occur exactly along that particular high symmetry line. For $\mathit{q}\left|\right|\mathrm{\Gamma }\mathrm{K}$ on the other hand, the excitations traverse the Brillouin zone crossing various high symmetry lines. A particular interesting aspect of our findings was that the calculation of the electron probability density revealed that the exciton assumes a six-pointed star-like shape along the real space crystal planes indicating a mixed Frenkel-Wannier character.

In three-dimensional topological insulators, an effective closure of the bulk energy gap with increasing magnetic field expected at a critical point can yield a band crossing at a gapless Dirac node. Using high-field magneto-optical Landau-level spectroscopy on the topological crystalline insulator $\mathrm{P}{{\mathrm{b}}_1}_{-x}\mathrm{S}{\mathrm{n}}_x\mathrm{Se}$, we demonstrate that such a gap closure does not occur, and an avoided crossing is observed as the magnetic field is swept through the critical field. We attribute this anticrossing to orbital parity and spin mixing of the $N=0$ levels. Concurrently, we observe no gap closure at the topological phase transition versus temperature suggesting that both are due to a single, likely extrinsic, mechanism.

The lattice dynamics of ${\mathrm{Bi}}_2{\mathrm{SiO}}_5$ has been studied by x-ray thermal diffuse scattering (TDS), inelastic x-ray scattering (IXS), and density functional theory calculations across the ferroelectric transition at $T_C=663$ K. Rodlike diffuse features are observed along the [100] direction, perpendicular to the direction of the quasi-one-dimensional silicate chains. IXS measurements of the phonon dispersion along the [100] $\mathrm{\Gamma }-Y$ direction related these diffuse features to coupled unstable transverse optic and acoustic branches. A strong temperature-dependent TDS signal was also observed at the Brillouin zone edges, at the $Y$ and $S$ points, indicating a competition between ferroelectric and antiferroelectric distortions. We observed with IXS a substantial but finite phonon softening at the $Y$ point and a minor softening at the $S$ point, that are associated with the displacement of oxygen atoms. The good agreement between the experiment and the theoretical TDS and IXS allows us to provide a quantitative overview of the various lattice dynamical instabilities occurring at $T_C$. The direct evidence for competing ferroelectric and antiferroelectric orders opens the way towards tuning the properties of ${\mathrm{Bi}}_2{\mathrm{SiO}}_5$ by application of pressure or electrical field.

Four types of atomic-scale multipoles (electric, magnetic, magnetic toroidal, and electric toroidal multipoles) give a complete set to describe arbitrary degrees of freedom for coupled charge, spin, and orbital of electrons. We here present a systematic classification of these multipole degrees of freedom towards the application in condensed matter physics. Starting from the multipole description under the rotation group in real space, we generalize the concept of multipoles in momentum space with the spin degree of freedom. We show how multipoles affect the electronic band structure and linear responses, such as the magnetoelectric effect, magnetocurrent (magnetogyrotropic) effect, spin conductivity, piezoelectric effect, and so on. Moreover, we exhibit a complete table to represent the active multipoles under 32 crystallographic point groups. Our comprehensive and systematic analyses will give a foundation to identify enigmatic electronic order parameters and a guide to evaluate peculiar cross-correlated phenomena in condensed matter physics from the microscopic point of view.