Detection of diamond surface electric-field noise is of importance for studying the coherence properties of shallow nitrogen-vacancy (NV) centers. Using the double quantum relaxation method, we demonstrated that the noise has a power-law frequency dependence, and it increases slowly from 295 K to 420 K but reduces quickly at higher temperatures. As the Dutta-Horn model described, the noise is created by ensembles of two-level systems with activation energies ranging from 0.3 eV to 0.6 eV. For diamond surface passivation, we found that the solid covering material (PMMA) outperforms the liquid one (glycerol) in reducing the electric-field noise, which was attributed to the presence of surface-modified phononic coupling noise. We anticipate that our findings will contribute to a better understanding of the origins of surface 1/f electric-field noise, as well as identify optimal covering materials for enhancing the coherence time of near-surface NV centers.

Radiative heat transfer between bodies is often dominated by a narrow resonance in the transmission, e.g., due to a cavity mode or a surface excitation. However, this resonant character is not visible in the average heat current. Here, we show that the noise spectrum of heat current can serve as a direct probe of the heat-carrying excitations. Namely, the resonant mode produces a sharp peak in the noise spectrum with a width related to the mode lifetime. We demonstrate that heat transfer in realistic superconducting circuits or between two-dimensional metals can realize our predictions. We also arrive at a general conclusion that the Landauer-type description for the radiative heat transfer breaks down on the level of the noise, while it holds for the average.

All-dielectric photonic crystals or metasurfaces supporting optical bound states in the continuum (BIC) are emerging as a promising platform for the study of light-matter interactions in the strong coupling regime. However, coupling of BIC to other optical excitations is generally weak, with the coupling strength limited in the range of several tens of milli electronvolts with short decoherence time. This greatly hinders their applications in solid-base quantum information devices that require a fast information exchange rate and long coherence time. Herein, a cavity-coupling strategy is used to extend the BIC-based coupling platforms. We propose a hybrid system containing an all-dielectric metasurface embedded in an optical microcavity, demonstrating the formation of BIC-cavity polaritons in the strong coupling regime with a surprisingly large Rabi splitting over 220 meV, far beyond the strong coupling regime. A flexible tuning of BIC-cavity coupling from weak to strong coupling regime is realized by controlling the shape of dielectric element of the metasurface. Importantly, a full quantum model based on Heisenberg-Langevin formalism reveals the temporal population dynamics of the coupling system, demonstrating the ultrafast Rabi socillation within 19 fs with a long decoherence time over 200 fs. Utilizing the multipolar expansion approach combined with full wave simulations, we also investigate the coupling-induced polariton characteristics in both the weak and strong coupling regimes. We find that the electric dipole component of the BIC mode dominates in the polariton states in weak coupling regime, which dramatically changes as the system is tuned into strong coupling regime. Such a cavity-coupling strategy in the present hybrid system is expected to be of great use for enhancing BIC-based light-matter interactions for more complex hybrid nanostructures such as three-mode systems with mixing BIC-cavity-exciton interactions.

The Rice-Mele model has two topological and spatial-inversion-symmetric phases, namely, the Su-Schrieffer-Heeger (SSH) phase with alternating hopping only, and the charge-density-wave (CDW) phase with alternating energies only. The chiral symmetry of the SSH phase is robust in position space, so that it is preserved in the presence of the ends of a finite system and of textures in the alternating hopping. However, the chiral symmetry of the CDW phase is nonsymmorphic, resulting in a breaking of the bulk topology by an end or a texture in the alternating energies. We consider the presence of solitons (textures in position space separating two degenerate ground states) in finite systems with open boundary conditions. We identify the parameter range under which an atomically sharp soliton in the CDW phase supports a localized state which lies within the band gap, and we calculate the expectation value $p_y$ of the nonsymmorphic chiral operator for this state, and the soliton electric charge. As the spatial extent of the soliton increases beyond the atomic limit, the energy level approaches zero exponentially quickly or in a manner inversely proportional to the width, depending on microscopic details of the soliton texture. In both cases, the difference of $p_y$ from 1 is inversely proportional to the soliton width, while the charge is independent of the width. We investigate the robustness of the soliton level in the presence of disorder and sample-to-sample parameter variations, comparing it with a single soliton level in the SSH phase with an odd number of sites.

Optical plasmonic nanocircuits are a heuristic route inspired by electronics for light manipulation in deep-subwavelength scales at optical frequencies with lumped capacitors and inductors. Here, a class of negative capacitors and inductors is discovered in the paradigm of optical plasmonic nanocircuits based on a modified model of a plasmonic nanoparticle. In this model, the global electromagnetic response of a Drude-dispersive plasmonic nanoparticle is imitated by a parallel pair of a capacitor and an inductor by involving the host medium of the plasmonic nanoparticle. We theoretically and numerically demonstrated the performance of such negative optical elements for extremely wideband impedance matching. A comprehensive lumped-element map for optical plasmonic nanocircuits is thus constructed including positive/negative inductors/capacitors with series/parallel configurations, which will inspire more possibilities in nanocircuit design.

We demonstrate that in a class of disordered quantum systems the dynamical partition function is not an analytical function in a time window after certain quantum quenches. We related this behavior to rare and large regions with atypical inhomogeneity configurations. We also quantify the strength of the associated singularities and their signatures in experiments and numerical studies.

Spatial correlations between atoms can generate a depletion in the energy dispersion of acoustic phonons. Two well-known examples are rotons in superfluid helium and the Kohn anomaly in metals. Here we report on the observation of a large softening of the transverse acoustic mode in quantum paraelectric $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ by means of inelastic neutron scattering. In contrast to other known cases, this softening occurs at a tiny wave vector implying spatial correlation extending over a distance as long as 40 lattice parameters. We attribute this to the formation of mesoscopic fluctuating domains due to the coupling between local strain and ferroelectric fluctuations. Thus, a hallmark of the ground state of insulating $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ is the emergence of hybridized optical-acoustic phonons. Mesoscopic fluctuating domains may play a role in quantum tunneling, which impedes the emergence of a finite macroscopic polarization.

Due to the intertwining between electronic nematic and elastic degrees of freedom, lattice defects and structural inhomogeneities commonly found in crystals can have a significant impact on the electronic properties of nematic materials. Here, we show that defects commonly present at the surface of crystals generally shift the wave vector of the nematic instability to a nonzero value, resulting in an incommensurate electronic smectic phase. Such a smectic state onsets above the bulk nematic transition temperature and is localized near the surface of the sample. We argue that this effect may explain not only recent observations of a modulated nematic phase in iron-based superconductors, but also several previous puzzling experiments that reported signatures consistent with nematic order before the onset of a bulk structural distortion.

Kitaev quantum spin liquids have been the focus of intense research effort owing to the discovery of various materials (e.g., ${\mathrm{RuCl}}_3$) that approximate their intriguing physics. In this Letter we construct a mean-field approximation for a moiré superlattice emerging in twisted Kitaev bilayers in terms of solutions of commensurate bilayers. We show that the band structure of deconfined spinons, defined on the mini-Brillouin zone of the superlattice, is greatly modified. The system exhibits a hypermagic manifold: a series of nearly perfectly flat bands appear at energies above the lowest gap, exhibiting a very large local (spinon) density of states that could potentially be probed experimentally. Intriguingly, flat-band eigenstates exhibit a localization akin to wave functions of kagome lattices.

Efficient modulation of electrically injected spin signals that is suitable for modern-day transistor functionality is yet to be established. In this work, we demonstrate in detail the fabrication of a $\mathrm{Fe}/n\text{-GaAs}$ spin injection device and the experimental setup for an optical gating of the nonlocal spin transport signal. In situ scanning electron microscopy interface imaging reveals more uniform current distribution at the $\mathrm{Fe}/n\text{-GaAs}$ injector interface at bias voltages higher than the Schottky barrier height. Three- and four-terminal Hanle measurements confirm successful spin injection into $n$-GaAs, with strong interfacial spin dephasing at high magnetic fields. A time-resolved pump-probe Kerr rotation setup was used to illuminate circularly polarized light in the region of the pure spin current in $\mathrm{Fe}/n\text{-GaAs}$ lateral spin injection devices, where ($0.4\pm 0.3)\%$ modulation of the nonlocal signal depending on the light helicity was observed at 30 K.

The infinite Heisenberg XXZ spin-$\frac{1}{2}$ chain in the gapped antiferromagnetic regime has two degenerate vacua and kink topological excitations (which are also called spinons) interpolating between these vacua as elementary excitations. Application of an arbitrary weak staggered longitudinal magnetic field $h$ induces a long-range attractive potential between two adjacent spinons leading to their confinement into “meson” bound states. Exploiting the integrability of the XXZ model in the deconfined phase $h=0$, we perform perturbative calculations of the energy spectra of the two-spinon bound states in the weak confinement regime at $h\to +0$, using the strength of the staggered magnetic field $h$ as a small parameter. Both transverse and longitudinal dynamical structure factors of the local spin operators are calculated as well in the two-spinon approximation in the weak confinement regime to the leading order in $h$.

By solving the Bogoliubov-de Gennes equations, we study the quasiparticle spectrum of a magnetic atom chain placed inside a short constriction-type Josephson junction. A helical magnetic order of the atomic spins is assumed, so that a topologically nontrivial state with Majorana edge modes at the ends of the chain can appear. It is found that in the presence of a nonzero Josephson phase the subgap spectrum of an infinite chain consists of four branches (Shiba bands). This spectrum is almost certainly gapped if the atomic spins form a coplanar spiral. The Majorana number of the given system is calculated analytically. It is demonstrated that a Josephson phase shift can be used to drive the system into a topologically nontrivial state and to tune the size of the topological gap. The spatial structure of Majorana edge modes is studied as well. We generalize the effective model based on discrete Bogoliubov-de Gennes equations developed by Pientka et al. for a bulk superconductor to the case of a Josephson junction. Using these discrete equations, the wave functions of Majorana zero modes are calculated analytically for a coplanar atomic spin spiral with arbitrary pitch. The wave functions exhibit an intermediate asymptotic behavior with a nonexponential fall-off with distance from the edge of the atomic chain.

We present Raman spectroscopic as well as first-principles theoretical studies of the temperature dependence of phonon spectra in the pyrochlore ${\mathrm{Y}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Our experimental results show that the frequencies of several phonons display anomalous softening upon cooling. Our theoretical calculations of the temperature dependence, via phonon self-energy corrections arising from cubic anharmonic interactions, lead to such anomalies but only for a couple of high-frequency phonon modes. Our work strongly supports the suggestion that anharmonic phonon-phonon interactions are strong in the pyrochlore titanates and that they are responsible for the phonon anomalies observed. The strong anharmonic effects are likely due to the presence of the vacant Wyckoff sites and the shifts of the oxygen atoms from their actual positions in the pyrochlore lattice. However, the theoretical calculations fail to explain the anomalous behavior of low and intermediate frequency modes observed in our experiments. It seems likely that a much more sophisticated, perhaps nonperturbative, treatment of the anharmonic interactions will be required for a full explanation of the anomalies.

The atomistic resolution recently achieved by ultrafast spectroscopies demands corresponding theoretical advances. Real-time time-dependent density-functional theory (RT-TDDFT) with Ehrenfest dynamics offers an optimal trade-off between accuracy and computational costs to study electronic and vibrational dynamics of laser-excited materials in the subpicosecond regime. However, this approach is unable to account for thermal effects or zero-point energies, which are crucial in the physics involved. Herein, we adopt a quantum-semiclassical method based on RT-TDDFT $+$ Ehrenfest to simulate laser-induced electronic and vibrational coherences in condensed matter. With the example of carbon-conjugated molecules, we show that ensemble averaging with initial configurations from a nuclear quantum distribution remedies many shortcomings of single-trajectory RT-TDDFT $+$ Ehrenfest, damping electronic coherence and introducing ultrafast nonadiabatic coupling between excited states. As the number of sampled configurations decreases with size and rigidity of the compounds, computational costs remain moderate for large systems for which mean-field approaches shine. The explicit inclusion of a time-dependent pulse in the simulations makes this method a prime advance for first-principles studies of coherent nonlinear spectroscopy as an independent counterpart to experimental results.

Multiple unconventional superconducting phases are suspected to be driven by magnetic fluctuations in the heavy-fermion paramagnet ${\mathrm{UTe}}_2$, and a challenge is to identify the signatures of the electronic correlations, including the magnetic fluctuations, in the bulk physical quantities. Here, we investigate thoroughly the anisotropy of the electrical resistivity of ${\mathrm{UTe}}_2$ under intense magnetic fields up to 70 T, for different electrical-current and magnetic-field configurations. Two characteristic temperatures and an anisotropic low-temperature Fermi-liquid-like coefficient $A$, controlled by the electronic correlations, are extracted. Their critical behavior near the metamagnetic transition induced at ${\mu }_0{H}_m\simeq 35$ T for $\mathbf{H}\parallel \mathbf{b}$ is characterized. Anisotropic scattering processes are evidenced and magnetic fluctuations are proposed to contribute, via a Kondo hybridization, to the electrical resistivity. Our work appeals for a microscopic modeling of the anisotropic contributions to the electrical resistivity as a milestone for understanding magnetically mediated superconductivity in ${\mathrm{UTe}}_2$.

We suggest a possible origin of noncollinear magnetic textures in ferromagnets (FMs) with the $D_{3h}$ point group symmetry. The suggested mechanism is different from the Dzyaloshinskii-Moriya interaction (DMI) and its straightforward generalizations. The considered symmetry class is important because a large fraction of all single-layer intrinsic FMs should belong to it, including monolayer ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$. At the same time, the DMI vanishes identically in materials described by this point group, in the continuous limit. We use symmetry analysis to identify the only possible contribution to the free energy density in two dimensions that is of the fourth order with respect to the local magnetization direction and linear with respect to its spatial derivatives. This contribution predicts long-range conical magnetic spirals with both the average magnetization and the "average chirality" dependent on the spiral propagation direction. We relate the predicted spirals to a recent experiment on ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$. Finally, we demonstrate that, for easy-plane materials, the same mechanism may stabilize bimerons.

In a two-dimensional noncentrosymmetric Ising superconductor in a fluctuating regime under the action of a uniform external electromagnetic field, there emerge two contributions to the photogalvanic effect due to the trigonal warping of the valleys. The first contribution stems from the current of the electron gas in its normal state, while the second contribution is of an Aslamazov-Larkin nature: It originates from the presence of fluctuating Cooper pairs when the ambient temperature approaches (from above) the temperature of the superconducting transition in the sample. The way to lift the valley degeneracy is the application of a weak out-of-plane external magnetic field producing a Zeeman effect. The Boltzmann equations' approach for the electron gas in the normal state and the time-dependent Ginzburg-Landau equations for the fluctuating Cooper pairs allow for the study of the photogalvanic current in two-dimensional transition-metal dichalcogenide Ising superconductors.

Over the last few years, broken symmetry within crystals has attracted extensive attention since it can improve the control of light propagation. In particular, low-symmetry Bravais crystal can support shear polaritons, which has great potential in thermal photonics. In this work, we first use the fluctuation-dissipation theorem to investigate mechanisms of near-field thermal radiation (NFTR) in a low-symmetry Bravais crystal. The NFTR between such crystal slabs is nearly four orders of magnitude larger than the blackbody limit, demonstrating its remarkable potential for noncontact heat dissipation for nanoscale circuits or other devices. Moreover, we report a form of twist-induced near-field thermal control system employing the low-symmetry Bravais crystal medium $\left(\beta \text{-G}{\mathrm{a}}_2{\mathrm{O}}_3\right)$, showing that this crystal can serve as an excellent platform for twist-induced near-field thermal control. Due to the intrinsic shear effect, the twist-induced modulation supported by low-symmetry Bravais crystal exceeds that by high-symmetry crystal. We further clarify how the shear effect affects the twist-induced thermal-radiation modulation supported by hyperbolic and elliptical polaritons and show that the shear effect significantly enhances the twist-induced thermal control induced by the elliptical polariton mode. These results open directions for thermal-radiation control in low-symmetry materials, including geological minerals, common oxides, and organic crystals.