It is well known that nonlinear optical signals such as cross-phase modulation can be coherently enhanced in multilevel atomic gases under conditions of electromagnetically induced transparency, but analogous results in solids are challenging to obtain due to natural energetic disorder. We propose a solid-state cavity QED scheme to enable cross-phase modulation between two arbitrarily weak classical fields in the optical domain, using a highly disordered intracavity medium composed of organic molecular photoswitches. Even in the presence of strong energetic and orientational disorder, the unique spectral properties of organic photoswitches can be used to enhance the desired nonlinearity under conditions of vacuum-induced transparency, enabling cross-phase modulation signals that surpass the detection limit imposed by absorption losses. Possible applications of the scheme include integrated all-optical switching with low photon numbers.

Originating from image recognition, methods of machine learning allow for effective feature extraction and dimensionality reduction in multidimensional datasets, thereby providing an extraordinary tool to deal with classical and quantum models in many-body physics. In this study, we employ a specific unsupervised machine learning technique—self-organizing maps—to create a low-dimensional representation of microscopic states, relevant for macroscopic phase identification and detecting phase transitions. We explore the properties of spin Hamiltonians of two archetype model systems: a two-dimensional Heisenberg ferromagnet and a three-dimensional crystal, Fe in the body-centered-cubic structure. The method of self-organizing maps, which is known to conserve connectivity of the initial dataset, is compared to the cumulant method theory and is shown to be as accurate while being computationally more efficient in determining a phase transition temperature. We argue that the method proposed here can be applied to explore a broad class of second-order phase-transition systems, not only magnetic systems but also, for example, order-disorder transitions in alloys.

The super-Planckian features of radiative heat transfer in the near field are known to depend strongly on both material and geometric design properties. However, the relative importance and interplay of these two facets, and the degree to which they can be used to ultimately control energy flow, remains an open question. Recently derived bounds suggest that enhancements as large as ${\left|\chi \right|}^4{\lambda }^2/\left[{\left(4\pi \right)}^2\text{Im}{\left[\chi \right]}^2{d}^2\right]$ are possible between extended structures (compared to blackbody), but geometries reaching this bound, or designs revealing the predicted material ($\chi$) scaling, are lacking. Here, exploiting inverse techniques, in combination with fast computational approaches enabled by the low-rank properties of elliptic operators for disjoint bodies, we investigate this relation between material and geometry on a wide variety of periodic gratings. Crucially, we find that the material proportionality given above does indeed emerge in realistic structures, at least within the range of explored values of $\chi$. In reaching this result, we also show that (in two dimensions) lossy metals such as tungsten, typically considered to be poor candidate materials for strongly enhancing heat transfer in the near infrared, can be structured to selectively realize flux rates that come within $50\%$ of those exhibited by an ideal pair of resonant lossless metals for separations as small as $2\%$ of a tunable design wavelength.

Nonlinear optical responses are a crucial probe of physical systems including periodic solids. In the absence of electron-electron interactions, they are calculable with standard perturbation theory starting from the band structure of Bloch electrons, but the resulting formulas are often large and unwieldy, involving many energy denominators from intermediate states. This work gives a Feynman diagram approach to calculating nonlinear responses. This diagrammatic method is a systematic way to perform perturbation theory, which often offers shorter derivations and also provides a natural interpretation of nonlinear responses in terms of physical processes. Applying this method to second-order responses concisely reproduces formulas for the second-order-harmonic shift current. We then apply this method to third-order responses and derive formulas for third-order-harmonic generation and self-focusing of light, which can be directly applied to tight-binding models. Third-order responses in the semiclasscial regime include a Berry curvature quadrupole term, whose importance is discussed including symmetry considerations and when the Berry curvature quadrupole becomes the leading contribution. The method is applied to compute third-order optical responses for a model Weyl semimetal, where we find a new topological contribution that diverges in a clean material, as well as resonances with a peculiar linear character.

Nuclear spin coherence and relaxation dynamics of all constituent isotopes of an n-doped CdTe/(Cd,Mg)Te quantum well structure are studied employing optically detected nuclear magnetic resonance. Using time-resolved pump-probe Faraday ellipticity, we generate and detect the coherent spin dynamics of the resident electrons. The photogenerated electron spin polarization is transferred into the nuclear spin system, which becomes polarized and acts back on the electron spins as the Overhauser field. Under the influence of resonant radio frequency pulses, we trace the coherent spin dynamics of the nuclear isotopes ${}^{111}\mathrm{Cd}, {}^{113}\mathrm{Cd}$, and ${}^{125}\mathrm{Te}$. We measure nuclear Rabi oscillations, the inhomogeneous dephasing time $T_2^{*}$, the spin coherence time $T_2$, and the longitudinal relaxation time $T_1$. Furthermore, we investigate the influence of the laser excitation and the corresponding electron spin polarization on the nuclear spin relaxation time and find a weak extension of this time induced by interaction with the electron spins.

We revisit the weak-coupling limit of the hairy probes method for electronic open boundaries. In this limit, the electronic density matrix is approximately stationary. We exploit this fact to combine hairy probes with electron-phonon scattering at the level of low-order transitions between eigenstates of the electronic Hamiltonian. This provides a method that is computationally very efficient, and whose results can be interpreted quite straightforwardly. The resultant time-dependent hybrid method is illustrated through the numerical calibration of a thermoelectric nanoscale thermometer.

Polar metals counterintuitively bring two well-known phenomena into coexistence, namely, bulk polar displacements, and an electronic Fermi surface giving rise to metallic conduction. However, little is known about the polar domains or domain walls in such materials. Using atomic resolution electron microscopy imaging combined with first principles density functional theory, we show that uncharged head-to-tail walls, and “charged” head-to-head and tail-to-tail walls can exist in the bulk of such crystals of polar metals ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, where both structural changes at the wall as well as electrostatic considerations define the wall nature. Significant built-in potentials of 30–170 meV are predicted at such walls.

Reducing the thickness of three-dimensional samples on appropriate substrates is a promising way to control electron-electron interactions, responsible for so called electronic reconstruction phenomena. Although the electronic reconstruction has been investigated both extensively and intensively in oxide heterostructure interfaces, this paradigm is not well established in the van der Waals heterointerface system. In the present study, we examine the nature of a charge ordering transition in monolayer vanadium diselenide $\left({\text{VSe}}_2\right)$. This two-dimensional phase transition would be distinguished from that of ${\text{VSe}}_2$ bulk samples, driven by more enhanced electron-electron correlations. We recall that ${\text{VSe}}_2$ bulk samples show a charge-density-wave (CDW) transition around $T_{\text{CDW}}\sim 105\text{K}$. This bulk phase transition results from Fermi-surface nesting properties, where the low-temperature CDW state coexists with itinerant electrons of residual Fermi surfaces. Recently, angle-resolved photoemission spectroscopy measurements [Nano Lett. 18, 5432 (2018)] uncovered that the Fermi-surface nesting becomes perfect, where the dynamics of hot electrons is dispersionless along the orthogonal direction of the nesting wave vector. In addition, scanning tunneling microscopy measurements [Nano Lett. 18, 5432 (2018)] confirmed that the resulting CDW state shows essentially the same modulation pattern as the three-dimensional system of ${\text{VSe}}_2$. Here, we perform the renormalization group analysis based on an effective-field theory in terms of critical CDW fluctuations and hot electrons of imperfect Fermi-surface nesting. As a result, we reveal that the imperfect nesting universally flows into perfect nesting in two dimensions, where the Fermi velocity along the orthogonal direction of the nesting vector vanishes generically. We argue that this electronic reconstruction is responsible for the observation [Nano Lett. 18, 5432 (2018).] that the CDW transition temperature is much more enhanced to be around $T_{\text{CDW}}\sim 350\text{K}$ than that of the bulk sample.

Chaotic dynamics in closed local quantum systems scrambles quantum information, which is manifested quantitatively in the decay of the out-of-time-ordered correlators (OTOC) of local operators. How is information scrambling affected when the system is coupled to the environment and suffers from dissipation? In this paper, we address this question by defining a dissipative version of OTOC and numerically study its behavior in a prototypical chaotic quantum chain in the presence of dissipation. We find that dissipation leads to not only the overall decay of the scrambled information due to leaking but also structural changes so that the ‘information light cone’ can only reach a finite distance even when the effect of overall decay is removed. Based on this observation we conjecture a modified version of the Lieb-Robinson bound in dissipative systems.

Excitations of ordered insulating magnets gain renewed interest due to their potential topological properties and the natural realization of magnetic analogues of the celebrated topological models. Here, we study the topologically nontrivial multiplet excitations of the spin-$1/2$ kagome antiferromagnet with strong breathing anisotropy and Dzyaloshinskii-Moriya interaction. We show that in the chiral magnetic ground state the excitations can be characterized with a spin-$1/2$ doublet and a spin-$3/2$ quartet. Applying magnetic field, we can tune a band touching topological transition within the quartet band. At the transition point a spin-3/2 Dirac cone is formed by the touching of four bands. In the topologically nontrivial regime the spin-$3/2$ bands have large Chern numbers $-3,-1$, 1, 3. Furthermore, the chiral edge states appearing for open boundary condition naturally inherit the multipolar characters and we find novel quadrupolar edge modes.

We analyze the phase diagram of the exact ground state (GS) of spin-$s$ chains with ferromagnetic XXZ couplings under $n$-alternating field configurations, i.e., sparse alternating fields having nodes at $n-1$ contiguous sites. It is shown that such systems can exhibit a nontrivial magnetic behavior, which can differ significantly from that of the standard $\left(n=1\right)$ alternating case and enable mechanisms for controlling their magnetic and entanglement properties. The boundary in field space of the fully aligned phase can be determined analytically $\forall n$, and shows that it becomes reachable only above a threshold value of the coupling anisotropy $J_z/J$, which depends on $n$ but is independent of the system size. Below this value, the maximum attainable magnetization becomes much smaller. We then show that the GS can exhibit significant magnetization plateaus, persistent for large systems, at which the magnetization per site $m$ obeys the quantization rule $2n(s-m)=\text{integer}$, consistent with the Oshikawa, Yamanaka, and Affleck criterion. We also identify the emergence of field-induced spin polymerization, which explains the presence of such plateaus. Entanglement and field-induced frustration effects are also analyzed.

The coexisting magnetic and ferroelectric orders in multiferroic materials give rise to a handful of novel magnetoelectric phenomena, such as the absorption difference for the opposite propagation directions of light called the nonreciprocal directional dichroism (NDD). Usually, these effects are restricted to low temperature, where the multiferroic phase develops. In this paper, we report the observation of NDD in the paramagnetic phase of ${\mathrm{Sr}}_2{\mathrm{CoSi}}_2{\mathrm{O}}_7$ up to temperatures more than 10 times higher than its Néel temperature (7 K) and in fields up to 30 T. The magnetically induced polarization and NDD in the disordered paramagnetic phase is readily explained by the single-ion spin-dependent hybridization mechanism, which does not necessitate correlation effects between magnetic ions. The ${\mathrm{Sr}}_2{\mathrm{CoSi}}_2{\mathrm{O}}_7$ provides an ideal system for a theoretical case study, demonstrating the concept of magnetoelectric spin excitations in a paramagnet via analytical as well as numerical approaches. We applied exact diagonalization of a spin cluster to map out the temperature and field dependence of the spin excitations, as well as symmetry arguments of the single ion and lattice problem to get the spectrum and selection rules.

Color centers in hexagonal boron nitride have shown enormous promise as single-photon sources, but a clear understanding of the electron-phonon interaction dynamics is critical to developing the material for quantum communications or quantum simulations. We demonstrate photon antibunching in the filtered auto- and cross-correlations $g_{lm}^{\left(2\right)}\left(\tau \right)$ between zero-, one-, and two-phonon replicas of defect luminescence. With no background correction, we observe a single-photon purity of $g^{\left(2\right)}\left(0\right)=0.20$ in a phonon replica and cross-spectral correlations of $g_{lm}^{\left(2\right)}\left(0\right)=0.18$ between a phonon replica and the zero-phonon line. Moreover, we combine a violation of the Cauchy-Schwarz inequality and a phenomenological model of the phonon-mediated emission cross section to distinguish a low quantum-efficiency defect from phonon replicas of a bright defect.

Samarium hexaboride (${\mathrm{SmB}}_6$) is a purported topological Kondo insulator, with theory predicting that the experimentally observed metallic surface states manifest from a topologically nontrivial insulating bulk band structure. The insulating bulk itself is driven by strong correlations, and both bulk and surface are known to host compelling magnetic and electronic phenomena. We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Sm $M_{4,5}$ edges to probe the surface and bulk magnetic properties of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ within ${\mathrm{SmB}}_6$. We observed an unexpected antialignment to the applied field of the ${\mathrm{Sm}}^{3+}$ magnetic dipole moment below $T=75$ K and of the total orbital moment of samarium below 30 K. The total bulk magnetization at 2 K is, however, positive and driven by ${\mathrm{Sm}}^{2+}$ Van Vleck susceptibility as well as 1% paramagnetic impurities with ${\mu }_{\mathrm{eff}}=5.2\left(1\right){\mu }_{\mathrm{B}}$. This indicates the diamagneticlike ${\mathrm{Sm}}^{3+}$ magnetism is only a portion of the net magnetization, partially offsetting the response of paramagnetic impurities known within the bulk.

We report the observation of superconductivity in the pressurized type-II Weyl semimetal (WSM) candidate $\mathrm{TaIrT}{\mathrm{e}}_4$ by means of complementary high-pressure transport and synchrotron x-ray diffraction measurements. We find that $\mathrm{TaIrT}{\mathrm{e}}_4$ shows superconductivity with transition temperature $\left(T_C\right)$ of 0.57 K at the pressure of $\sim 23.8\mathrm{GPa}$. Then, the $T_C$ value increases with pressure and reaches $\sim 2.1\mathrm{K}$ at 65.7 GPa. In situ high-pressure Hall coefficient $\left(R_H\right)$ measurements at low temperatures demonstrate that the positive $R_H$ increases with pressure until the critical pressure of the superconducting transition is reached, but starts to decrease upon further increasing pressure. Above the critical pressure, the positive magnetoresistance effect disappears simultaneously. Our high-pressure x-ray diffraction measurements reveal that at around the critical pressure, the lattice of the $\mathrm{TaIrT}{\mathrm{e}}_4$ sample is distorted and its volume is reduced by $\sim 19.2\%$, the value of which is predicted to result in the change of the electronic structure significantly. We propose that the pressure-induced distortion in $\mathrm{TaIrT}{\mathrm{e}}_4$ is responsible for the change of topology of the Fermi surface, and such a change favors the emergence of superconductivity. Our results reveal the correlation among the lattice distortion, topological physics, and superconductivity in the WSM, a hot topic in condensed-matter physics.

We combine first-principles electronic structure calculated thermal conductivity data with a numerical solution of the one-dimensional heat equation to show that an asymmetric distribution of impurity scattering, if suitably designed, yields the conditions for a low-temperature thermal rectification. This happens as a result of the differences in the peaks of the temperature dependence of the thermal conductivity. We demonstrate the effectiveness of the method by probing the thermal rectification rendered by a silicon slab with a steplike position-dependent isotopic composition. The same conclusions are obtained by using experimentally measured values of the thermal conductivity of Si samples with different isotope distributions.

Oxygen, bismuth, and iron vacancies are studied by density-functional theory calculations in multiferroic bismuth ferrite in oxidative and reducing external conditions. We accurately describe the localized electronic states associated with isolated defects. Oxygen-rich conditions provide fully ionized oxygen vacancies, and ionized or partially ionized cationic vacancies, possibly yielding $p$-type conductivity mediated by oxygen-type holes. Oxygen-poor conditions provide fully ionized vacancies and a much smaller concentration of electronic carriers. Cationic and oxygen vacancies tend to pair (Bi-O, Fe-O) with an associated electric dipole being as close as possible to the polar axis, possibly leading to imprint effects. This clustering also results in an extended stability domain for the ionized defects. Furthermore, O and Fe vacancies are respectively associated with Fe-$3d$ and O-$2p$ type moderately deep defect states spatially localized close to the defect. The charge-neutral Fe vacancy possesses an electric dipole, despite being a point defect. Bi vacancies may be found in several metastable electronic states, with in the most stable one, a mid-gap band of defect states with Fe-$3d$ character, in addition to the three O-$2p$ defect states associated to the acceptor nature of this defect. Finally, electrons released in the lattice by oxygen vacancies, if not captured by acceptor defects, tend to localize as small polarons, while holes released by cationic vacancies, if not captured by donor defects, are associated with delocalized Bloch-like band states.

It is shown here that $1/f^{\alpha }$ flux noise in conventional low-$T_c$ SQUIDs is a result of low temperature superparamagnetic phase transitions in small clusters of strongly correlated color center defects. The spins in each cluster interact via long-range ferromagnetic interactions. Due to its small size, the cluster behaves like a random-telegraphic macrospin when transitioning to the superparamagnetic phase. This results in $1/f^{\alpha }$ noise when ensemble averaged over a random distribution of clusters. This model is self-consistent and explains all related experimental results which includes $\alpha \sim 0.8$ independent of system size. The experimental flux-inductance-noise spectrum is explained through three-point correlation calculations and time-reversal symmetry-breaking arguments. Also, unlike the flux noise, it is shown why the second-spectrum inductance noise is inherently temperature dependent due to the fluctuation-dissipation theorem. A correlation-function calculation methodology using Ising-Glauber dynamics was key for obtaining these results.

We study how time- and angle-resolved photoemission (tr-ARPES) reveals the dynamics of BCS-type, $s$-wave superconducting systems with time-varying order parameters. Approximate methods are discussed, based on previous approaches to either optical conductivity or quantum dot transport, to enable computationally efficient prediction of photoemission spectra. One use of such predictions is to enable extraction of the underlying order parameter dynamics from experimental data, which is topical given the rapidly growing use of tr-ARPES in studying unconventional superconductivity. The methods considered model the two-time lesser Green's functions with an approximated lesser self-energy that describes relaxation by coupling of the system to two types of baths. The approach primarily used here also takes into consideration the relaxation of the excited states into equilibrium by explicitly including the level-broadening of the retarded and advanced Green's functions. We present equilibrium and nonequilibrium calculations of tr-ARPES spectrum from our model and discuss the signatures of different types of superconducting dynamics.