We show that nitrogen-vacancy (NV) centers in diamond can produce a unique quantum hyperbolic metamaterial. We demonstrate that a hyperbolic dispersion relation in diamond with NV centers can be engineered and dynamically tuned by applying a magnetic field at 20 K. This quantum hyperbolic metamaterial with a tunable window for the negative refraction allows for the construction of a superlens beyond the diffraction limit. In addition to subwavelength imaging, this NV metamaterial can be used in spontaneous emission enhancement, heat transport, and acoustics, analog cosmology, and lifetime engineering. Therefore, our proposal interlinks the two hotspot fields, i.e., NV centers and metamaterials.

Formation of quantum scars in many-body systems provides a novel mechanism for enhancing coherence of weakly entangled states. At the same time, coherence of edge modes in certain symmetry-protected topological (SPT) phases can persist away from the ground state. In this work we show the existence of many-body scars and their implications on bulk coherence in such an SPT phase. To this end, we study the eigenstate properties and the dynamics of an interacting spin-$1/2$ chain with three-site “cluster” terms hosting a ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ SPT phase. Focusing on the weakly interacting regime, we find that eigenstates with volume-law entanglement coexist with area-law entangled eigenstates throughout the spectrum. We show that a subset of the latter can be constructed by virtue of repeated cluster excitations on the even or odd sublattice of the chain, resulting in an equidistant “tower” of states, analogous to the phenomenology of quantum many-body scars. We further demonstrate that these scarred eigenstates support nonthermal expectation values of local cluster operators in the bulk and exhibit signatures of topological order even at finite energy densities. Studying the dynamics for out-of-equilibrium states drawn from the noninteracting “cluster basis,” we unveil that nonthermalizing bulk dynamics can be observed on long timescales if clusters on odd and even sites are energetically detuned. In this case, cluster excitations remain essentially confined to one of the two sublattices such that inhomogeneous cluster configurations cannot equilibrate and thermalization of the full system is impeded. Our work sheds light on the role of quantum many-body scars in preserving SPT order at finite temperature and the possibility of coherent bulk dynamics in models with SPT order beyond the existence of long-lived edge modes.

Five transport coefficients of the cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ were measured in the normal state down to low temperature, reached by applying a magnetic field (up to 66 T) large enough to suppress superconductivity. The electrical resistivity, Hall coefficient, thermal conductivity, Seebeck coefficient, and thermal Hall conductivity were measured in two overdoped single crystals, with La concentration $x=0.2$ ($T_{\mathrm{c}} =18$ K) and $x=0.0$ ($T_{\mathrm{c}} =10$ K). The samples have dopings $p$ very close to the critical doping $p^{★}$ where the pseudogap phase ends. The resistivity displays a linear dependence on temperature whose slope is consistent with Planckian dissipation. The Hall number $n_{\mathrm{H}}$ decreases with reduced $p$, consistent with a drop in carrier density from $n=1+p$ above $p^{★}$ to $n=p$ below $p^{★}$. This drop in $n_{\mathrm{H}}$ is concomitant with a sharp drop in the density of states inferred from prior NMR Knight shift measurements. The thermal conductivity satisfies the Wiedemann-Franz law, showing that the pseudogap phase at $T=0$ is a metal whose fermionic excitations carry heat and charge as do conventional electrons. The Seebeck coefficient diverges logarithmically at low temperature, a signature of quantum criticality. The thermal Hall conductivity becomes negative at low temperature, showing that phonons are chiral in the pseudogap phase. Given the observation of these same properties in other, very different cuprates, our study provides strong evidence for the universality of these five signatures of the pseudogap phase and its critical point.

Attaining an energy-efficient pathway to switch the rotation of polarization vortex with the use of trivial homogeneous electric field is essentially important for low-power consumption electronic nanodevices, yet still challenging. In this study, we systematically investigate the switching behaviors of polarization vortex in triangular ferroelectric nanodots with different crystal orientations. The obtained results exhibit diversity pathways for the vortex switching, including both indirect and direct switching, which depend on the crystal orientation. In the indirect pathway, the switching process requires a destruction of initial vortex domain structure to form a rectilinear domain at a large electric field. Otherwise, in the direct switching pathway, the initial vortex is switched to an opposite one without the stabilization of an intermediate state. Importantly, the direct pathway requires a lower switching field compared to that in the indirect ones. We further elucidate the temporal evolution of domain structure during the direct switching of polarization vortex in the triangular nanodot. A comprehensive viewpoint on the effect of crystal orientation is expressed, from which the advantages of direct switching are clarified. The present study suggests an efficient route on the practical control of vortex rotation in ferroelectric nanostructures under homogeneous electric field.

We investigated the effect of pressure on the crystal structure and transport properties of the Magnéli phase η-${\mathrm{Mo}}_4{\mathrm{O}}_{11}$, which at ambient pressure undergoes two successive charge-density-wave (CDW) transitions at $T_{\mathrm{CDW}\text{−}1}\approx 105\mathrm{K}$ and $T_{\mathrm{CDW}\text{−}2}\approx 30\mathrm{K}$, respectively. We find that η-${\mathrm{Mo}}_4{\mathrm{O}}_{11}$ exhibits a structural phase transition from the low-pressure monoclinic $P{2}_1/a$ phase to a high-pressure $P{2}_1$ phase at $P_c\approx 3.5\mathrm{GPa}$. Around $P_{\mathrm{c}}$, the lattice parameters experience a sudden change with a large volume collapse of $\mathrm{\Delta }V/V=-8.1\%$, while the room-temperature resistivity exhibits a sudden jump by two orders of magnitude, signaling a pressure-induced metal-to-insulator transition. For $P<P_c$, the high-pressure resistivity measurements revealed opposite pressure dependences of these two CDW transitions, i.e., $T_{\mathrm{CDW}\text{−}1}$ is enhanced gradually to ∼130 K while $T_{\mathrm{CDW}\text{−}2}$ is almost suppressed completely by the application of 2.6 GPa pressure. For $P\ge P_c$, the temperature dependence of resistivity changes to an insulating-like behavior, but the activation energy is reduced gradually upon further increasing pressure. We have rationalized the insulating ground state of the high-pressure phase in terms of the structural modifications and charge redistribution based on the refinement of single-crystal x-ray diffraction data at 8.9 GPa.

Despite many years of research, the full complexity of the electron-lattice interactions in ${\mathrm{UO}}_2$ is not fully understood. We present x-ray inelastic scattering at low temperature showing that the interaction between electronic degrees of freedom and transverse acoustic phonons is strong only along the reciprocal space direction [100]. The anisotropy is reflected in the phonon-linewidth broadening, which persists also well above the Néel temperature. This intrinsic effect infers an anisotropy in the thermal conductivity, which has been observed, but which is formally forbidden in a cubic material. We have no model capable of connecting our experimental observations with the low thermal conductivity of ${\mathrm{UO}}_2$ below room temperature.

We study Majorana zero-mode properties in cylindrical cross-section semiconductor quantum wires based on the effective-mass theory and a discretized lattice model. Within this model the influence of disordered potentials in the quantum wire as well as amplitude and phase fluctuations of the superconducting order parameter are discussed. We find that for typical wire geometries, pairing potentials, and spin-orbit coupling strengths, coupling between quasi-one-dimensional subbands is weak, low-energy quasiparticles near the Fermi energy are nearly completely spin polarized, and the number of electrons in the active subbands of topological states is less than 10.

We report complex magnetotransport patterns of the $\nu =1$ integer quantum Hall state in a GaAs/AlGaAs sample from the newest generation with a record high electron mobility. The reentrant integer quantum Hall effect in the flanks of the $\nu =1$ plateau indicates the formation of the integer quantum Hall Wigner solid, a collective insulator. Moreover, at a fixed filling factor, the longitudinal resistance versus temperature in the region of the integer quantum Hall Wigner solid exhibits a sharp peak. Such sharp peaks in the longitudinal resistance versus temperature so far were only detected for bubble phases forming in high Landau levels but were absent in the region of the Anderson insulator. We suggest that in samples of sufficiently low disorder, sharp peaks in the longitudinal resistance versus temperature traces are universal transport signatures of all isotropic electron solids that form in the flanks of integer quantum Hall plateaus. We discuss possible origins of these sharp resistance peaks and we draw a stability diagram for the insulating phases in the $\nu -T$ phase space.

We study the interaction of graphene quantum dots (GQDs) with ultrashort and strong optical pulses theoretically. An important characteristic of such interaction is the energy accumulated by GQDs after the pulse. We show that the GQD absorbance has a highly nonlinear dependence on the field amplitude. At small-field amplitudes, the absorbance strongly depends on the frequency of the pulse and the band gap of a QD, i.e., its size. At large-field amplitudes, $\sim 1$ V/Å, the absorbance has a weak nonmonotonic dependence on the size of the dot with its maximum value of $\approx 4.5$% realized for GQD consisting of $\approx 60$ atoms. As a function of the field amplitude, the absorbance also has a maximum, the position of which depends on the size of the dot.

We measure the charge periodicity of Coulomb blockade conductance oscillations of a hybrid InSb-Al island as a function of gate voltage and parallel magnetic field. The periodicity changes from $2e$ to $1e$ at a gate-dependent value of the magnetic field, $B^{*}$, decreasing from a high to a low limit upon increasing the gate voltage. In the gate voltage region between the two limits, which our numerical simulations indicate to be the most promising for locating Majorana zero modes, we observe correlated oscillations of peak spacings and heights. For positive gate voltages, the $2e\text{−}1e$ transition with low $B^{*}$ is due to the presence of nontopological states whose energy quickly disperses below the charging energy due to the orbital effect of the magnetic field. Our measurements highlight the importance of a careful exploration of the entire available phase space of a proximitized nanowire as a prerequisite to define future topological qubits.

Semiconductor nanowires (NWs) often present different structural and opto-electronic properties than their thin film counterparts. The thinner they are, the larger these differences are. Here, we present femtosecond transient absorbance measurements on ${\mathrm{GaAs}}_{0.8}{\mathrm{P}}_{0.2}$ NWs of two different diameters, 36 and 51 nm. The results show that thinner NWs sustain a higher carrier temperature for longer times than thicker NWs. This observation suggests that, in thinner NWs, the buildup of a hot-phonon bottleneck is easier than in thicker NWs because of the increased phonon scattering at the NW sidewalls, which facilitates the buildup of a large phonon density. Moreover, the important observation that the carrier temperature in thin NWs is higher than in thick NWs already at the beginning of the hot carrier regime suggests that the phonon-mediated scattering processes in the nonthermal regime play a major role at least for the carrier densities investigated here $(8\times {10}^{18} \mathrm{to} 4\times {10}^{19}\mathrm{c}{\mathrm{m}}^{\text{–}3}$). Our results also suggest that the phonon scattering at crystal defects is negligible compared with the phonon scattering at the NW sidewalls.

We consider a problem of superconductivity coexistence with the spin-density-wave order in disordered multiband metals. It is assumed that random variations of the disorder potential on short length scales render the interactions between electrons to become spatially correlated. As a consequence, both superconducting and magnetic order parameters become spatially inhomogeneous and are described by the universal phenomenological quantities, whereas all the microscopic details are encoded in the correlation function of the coupling strength fluctuations. We consider a minimal model with two nested two-dimensional Fermi surfaces and disorder potentials which include both intra- and interband scattering. The model is analyzed using the quasiclassical approach to show that short-scale pairing-potential disorder leads to a broadening of the coexistence region.

The recently discovered heavy fermion ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ compound crystallizes in nonsymmorphic $P4/nmm$ symmetry, which enables the occurrence of topological protection. Experimental results show that this material exhibits unusual behavior, which is manifested by the appearance of two superconducting phases. In this Letter, we uncover and discuss the role of ${\mathrm{Rh}}_2{\mathrm{As}}_2$ layers and their impact on the electronic and dynamical properties of the system. The location of Ce atoms between two nonequivalent layers allows for the realization of orbital order. We point out that the electronic band structure around the Fermi level is associated mostly with Ce $4f$ and Rh $4d$ orbitals and suggest the occurrence of the Lifshitz transition induced by an external magnetic field. We discuss also the role played by the $f-d$ orbital hybridization in the electronic band structure.

We experimentally investigated the magnetic properties of ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magnetotorque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature ($T_{\mathrm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although $T_{\mathrm{SR}}$ is dependent on the growth method as well as the conditions. The cone angles measured from the $c$ axis increased successively at $T_{\mathrm{SR}}$ and approached a maximum of 45–50 degrees at the lowest measurement temperature of 5 K. Calculation with the cluster model suggests that the ${\mathrm{Ni}}^{3+}$ ions occupying the $T_d$ site could be the origin of the ECMA. Both the magnetic properties and the results of the calculation based on the cluster model indicate that the ECMA is attributable to the cation antisite distribution of ${\mathrm{Ni}}^{3+}$, which is possibly formed during the growth process of the thin films.

The nonlocal spin valve (NLSV) enables unambiguous study of spin transport, owing to its ability to isolate pure spin currents. A key principle of NLSV operation is that the “spin signal” is invariant under application of in-plane magnetic fields (above the ferromagnetic contact saturation field). Yet, for certain ferromagnet/normal metal pairings in NLSVs, an unexpected field enhancement of the spin signal occurs, presenting a challenge that has, thus far, been difficult to resolve with existing models. By correlating the extracted spin transport parameters with material, temperature, and field dependencies, in this work we identify field quenching of magnetic impurity scattering as the origin of this effect, confirmed by excellent agreement between our results and field-dependent Kondo theory. In addition to addressing this long-standing mystery, our findings highlight a potential systematic underestimation of spin transport parameters. By identifying signature field and temperature dependencies, we provide here a relatively simple means to isolate and quantify this additional relaxation mechanism.

The Eliashberg theory of superconductivity is based on a dynamical electron-phonon interaction as opposed to a static interaction present in BCS theory. The standard derivation of Eliashberg theory is based on an equation of motion approach, which incorporates certain assumptions such as Migdal's approximation for the pairing vertex. In this paper we provide a functional-integral-based derivation of Eliashberg theory and we also consider its Gaussian-fluctuation extension. The functional approach enables a self-consistent method of computing the mean-field equations, which arise as saddle-point conditions, and here we observe that the conventional Eliashberg self-energy and pairing function both appear as Hubbard-Stratonovich auxiliary fields. An important consequence of this fact is that it provides a systematic derivation of the Cooper and density-channel interactions in the Gaussian-fluctuation response. We also investigate the fluctuation contribution to the diamagnetic susceptibility near the critical temperature.

We report on a theoretical study on ferromagnetic superconductors URhGe and UCoGe and identify the pairing state as a nonunitary spin-triplet one with time-reversal symmetry broken, analogous to the superfluid ${}^3\mathrm{He}-A$ phase. A recently found superconductor ${\mathrm{UTe}}_2$ with almost ferromagnetism is analyzed by the same manner. Through investigating their peculiar upper critical field $H_{\mathrm{c}2}$ shapes, it is shown that the pairing symmetry realized in all three compounds can be tuned by their magnetization curves under applied fields. This leads to the reentrant $H_{\mathrm{c}2}$ in URhGe, an S-shaped in UCoGe and an L-shaped $H_{\mathrm{c}2}$ in ${\mathrm{UTe}}_2$ observed for the magnetic hard $b$-axis direction. The identification with double chiral form $\mathbf{d}\left(k\right)=(\widehat{b}+i\widehat{c})(k_b+i{k}_c)$ in ${\mathrm{UTe}}_2$ naturally enables us to understand (1) multiple phases with $A_1, A_2$, and $A_0$ phases observed under pressure, (2) the enhanced reentrant $H_{\mathrm{c}2}$ for the off-axis direction associated with first-order metamagnetic transition, and (3) Weyl point nodes oriented along the magnetic easy $a$ axis. All three compounds are found to be topologically rich solid-state materials worth further investigation.

${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ is a novel spin-$1/2$ magnet exhibiting spin liquid behavior, which sets it apart from any previously studied model or material. However, understanding ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ presents a significant challenge, because the low symmetry of the crystal structure leads to very complex interactions, with up to seven inequivalent coupling parameters in the unit cell. Here we explore the origin of the spin-liquid behavior in ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$, starting from the simplest microscopic model consistent with experiment—a Heisenberg model on a single bilayer of the breathing-kagome (BBK) lattice. We use a combination of classical Monte Carlo (MC) simulation and (semi)classical molecular dynamics (MD) simulation to explore the thermodynamic and dynamic properties of this model and compare these with experimental results for ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$. We uncover qualitatively different behaviors on different timescales, and argue that the ground state of ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ is born out of a slowly fluctuating “spiral spin liquid”, while faster fluctuations echo the U(1) spin liquid found in the kagome antiferromagnet. We also identify key differences between longitudinal and transverse spin excitations in applied magnetic field, and argue that these are a distinguishing feature of the spin liquid in the BBK model.

We report an investigation of the superconducting properties of the hexagonal noncentrosymmetric compound LaPdIn. Electrical resistivity, specific heat, and ac susceptibility measurements demonstrate the presence of bulk superconductivity below $T_c=1.6$ K. The specific heat, together with the penetration depth measured using transverse-field muon spin rotation and the tunnel-diode-oscillator-based method, are well described by single-gap $s$-wave superconductivity, with a gap magnitude of $1.8k_B{T}_c$. From zero-field muon spin relaxation results no evidence is found for the spontaneous emergence of magnetic fields in the superconducting state, indicating that time-reversal symmetry is preserved. Band structure calculations reveal that there is a relatively weak effect of antisymmetric spin-orbit coupling on the electronic bands near the Fermi level, which is consistent with there being negligible singlet-triplet mixing due to broken inversion symmetry. On the other hand, isostructural LuPdIn and LaPtIn do not exhibit superconductivity down to 0.4 K, which may be due to these systems having a smaller density of states at the Fermi level.

We investigate polariton quantum correlations in a coherently driven box cavity in the low driving regime, with a particular focus on accounting for the polarization degree of freedom. The possibility of having different interaction strengths between co- and cross-circularly polarized polaritons as well as a realistic linear-polarization splitting allows one to model the system as two coupled nonlinear resonators with both self- and cross-Kerr-like nonlinearities, thus making our results potentially relevant for other experimental platforms. Within an effective wave-function approach, we obtain analytical expressions for the steady-state polarization-resolved polariton populations and second-order correlation functions, which agree very well with our numerical results obtained from a Lindblad master equation. Notably, we highlight that depending on the excitation polarization (circular or linear), both the unconventional (interference-mediated) and conventional (mediated by nonlinearities) antibunchings can be investigated in a single cavity. Moreover, using our results, we argue that recent experiments on confined fiber-cavity polaritons are likely to have probed a regime where the dominant interaction is between cross-polarized polaritons, which is characteristic of the polariton Feshbach resonance. We furthermore investigate the regime close to resonance using a two-channel model and we show that systems with large biexciton binding energies, such as atomically thin semiconductors, are promising platforms for realizing strong polariton antibunching.