Extracting information from quantum many-body systems remains a key challenge in quantum technologies due to experimental limitations. In this work, we employ a single spin qubit to probe a strongly interacting system, creating an environment conducive to qubit decoherence. By focusing on the $XXZ$ spin chain, we observe diverse dynamics in the qubit evolution, reflecting different parameters of the chain. This demonstrates that a spin qubit can probe both quantitative properties of the spin chain and qualitative characteristics, such as the bipartite entanglement entropy, phase transitions, and perturbation propagation velocity within the system. This approach reveals the power of small quantum systems to probe the properties of large, strongly correlated quantum systems.

The exploration of neural network quantum states has become widespread in studies of complicated quantum many-body systems. However, achieving high precision remains challenging due to the exponential growth of Hilbert space size and the intricate sign structures. Utilizing symmetries of the physical system, we propose a method to evaluate and sample the variational Ansatz within a symmetric subspace. This approach isolates different symmetry sectors, reducing the relevant Hilbert space size by a factor approximately proportional to the size of the symmetry group. It is inspired by exact diagonalization techniques and the work of Choo et al., [Phys. Rev. Lett. 121, 167204 (2018)]. We validate our method using the frustrated spin-$\frac{1}{2}{J}_1\text{−}{J}_2$ antiferromagnetic Heisenberg chain, and we compare its performance to the case without symmetrization. The results indicate that our symmetric subspace approach achieves a substantial improvement over the full Hilbert space on optimizing the Ansatz, reducing the energy error by orders of magnitude. We also compare the results on degenerate eigenstates with different quantum numbers, highlighting the advantage of operating within a smaller Hilbert subspace.

X-ray diffraction study of black phosphorus (BP) shows forbidden reflections, which imply a broken inversion symmetry with atomic distortion in the orthorhombic lattice. The unexpected distortion occurs throughout the entire bulk crystal, rather than being confined to just the top-surface layer. Thus our observation suggests that this distortion is an intrinsic bulk property, in contrast to earlier studies which attributed the distortion to surface effects or defects. Our diffusion quantum Monte Carlo calculation predicts the same asymmetric structure. Density functional theory calculation incorporating Hubbard correction attributes the symmetry breaking to intrinsic electronic interaction in $sp$ hybridized orbitals. We propose that the asymmetric lattice distortion in BP could enable stable two-dimensional ferroelectricity.

We investigate the probable delocalization-localization transition in open quantum systems with disorder. The disorder can induce localization in isolated quantum systems, and it is generally recognized that localization is fragile under the action of dissipations from the external environment due to its interfering nature. Recent work [Liu et al., Phys. Rev. Lett. 132, 216301 (2024)] found that a one-dimensional quasiperiodic system can be driven into the localization phase by a tailored local dissipation where a dissipation-induced delocalized-localized transition is proposed. Based on this, we consider a more realistic system and show that a dissipation-induced transition between delocalization and localization appears in the three-dimensional (3D) Anderson model. By tuning local dissipative operators acting on nearest-neighbor sites, we find that the system can relax to a steady state dominated by localized states instead of the choice of initial conditions and dissipation strengths. Moreover, we can also realize a steady state dominated by delocalized states from a localized initial state by using a kind of dissipation operator acting on next-nearest-neighbor sites. Our results enrich the applicability of dissipation-induced localization and identify the transition between delocalized and localized phases in 3D disordered systems.

In pure nonmetallic crystals, the resistance to heat conduction at temperatures around and above room temperature is conventionally accepted to originate from anharmonic phonon-phonon interactions. Herein, we reveal a different mechanism of heat transfer, which cannot be described by the typical phonon gas picture, where local two-dimensional correlations of the octahedral motions lead to a temperature-independent thermal conductivity (above room temperature and approaching the melting temperature) in ${\mathrm{CsPbI}}_3$. Our systematic atomistic simulations allow us to relate the unique lattice dynamics to the thermal transport in ${\mathrm{CsPbI}}_3$, providing evidence for the indirect (but crucial) role of the ${\mathrm{Cs}}^{+}$ ions in facilitating heat transfer at relatively higher temperatures. This contrasts with the conventionally accepted notion of the “rattling” scenario leading to the reduced thermal conductivities in metal halide perovskites, where the decoupled motion of the ${\mathrm{Cs}}^{+}$ ions scatters the acoustic modes of the framework. Rather, we show that the interaction between the ${\mathrm{Cs}}^{+}$ ions and the ${\mathrm{PbI}}_6$ framework enables the local two-dimensional spatiotemporal correlations of the octahedra leading to better heat conduction at higher temperatures in these solids. Our work reveals that local spatiotemporal correlations between atoms, beyond the typical phonon gas picture, can dictate thermal transport in the highly anharmonic metal halide perovskites, and could help guide their further advancements in photovoltaic and thermoelectric applications.

Magnetic skyrmions are considered to be potential information carriers in racetrack memory. However, the skyrmion Hall effect limits the maximum speed of skyrmions along the current direction, which hampers their further application. Here, we investigate the fundamental properties of an elongated skyrmion induced by an anisotropic Dzyaloshinskii-Moriya interaction (DMI). Spin-orbit torque-driven dynamics of the elongated skyrmion under various conditions, including anisotropic DMI, Gilbert damping, and current, are also performed by a combination of micromagnetic simulations and solutions of the Thiele equation. It is found that elongated skyrmions can realize a zero skyrmion Hall angle, and the gradient of Gilbert damping is an effective method to significantly control the skyrmion Hall angle. It should be noted that the rotational symmetry breaking of the elongated skyrmions results in lower thermal stability than the circular skyrmions. Our results offer the possibility of zero skyrmion Hall angle motion, which may help guide the potential application of elongated skyrmions in racetrack memory.

Using the nuclear norm regularization techniques on tensor network renormalization algorithm, we study the phase diagram, the critical behavior, and the duality property of the antiferromagnetic 6-state clock model on the Union Jack lattice. We find that this model undergoes multiple phase transitions; there is the Berezinskii-Kosterlitz-Thouless, $Z_6$ symmetry breaking, and chiral transition with decreasing temperature. Furthermore, we provide convincing numerical evidence that its quasi-long-range order is well explained by the compactified boson conformal field theory (CFT) and the chiral transition is in perfect agreement with the Ising CFT, including central charge, scaling dimension spectrum, and operator product expansion coefficients.

Magnons have inspired potential applications in modern quantum technologies and hybrid quantum systems due to their intrinsic nonlinearity, nanoscale scalability, and a unique set of experimentally accessible parameters for manipulating their dispersion. Such magnon-based quantum technologies demand long decoherence times, millikelvin temperatures, and minimal dissipation. Due to its low magnetic damping, the ferrimagnet yttrium iron garnet (YIG), grown on gadolinium gallium garnet (GGG), is the most promising material for this objective. To comprehend the magnetic losses of propagating magnons in such YIG-GGG heterostructures at cryogenic temperatures, we investigate magnon transport in a micrometer-thick YIG sample via propagating spin-wave spectroscopy measurements for temperatures between $4\mathrm{K}$ to $26\mathrm{m}\mathrm{K}$. We demonstrate an increase in the dissipation rate with wavenumber at cryogenic temperatures, caused by dipolar coupling to the partially magnetized GGG substrate. Additionally, we observe a temperature-dependent decrease in spin-wave transmission, attributed to rare earth ion relaxations. The critical role of the additional dissipation channels at cryogenic temperatures is underpinned by the comparison of the experimental results with theoretical calculations and micromagnetic simulations. Our findings strengthen the understanding of magnon losses at millikelvin temperatures, which is essential for the future detection of individual propagating magnons.

Clarifying the physical origin of valley polarization and exploring promising ferrovalley materials are conducive to the application of valley degrees of freedom in the field of information storage. Here, we explore two altermagnetic semiconductors (monolayers $\mathrm{N}{\mathrm{b}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ and $\mathrm{N}{\mathrm{b}}_2\mathrm{SeTeO}$) with Néel temperature above room temperature based on first-principles calculations. We find that uniaxial strain induces valley polarization without spin-orbital coupling (SOC) in altermagnets owing to the piezovalley effect, while uniaxial compressive strain transforms the intrinsic ferrovalley semiconductor into a semimetal, half metal, and metal. Moreover, moderate biaxial compressive strain renders Janus monolayer $\mathrm{N}{\mathrm{b}}_2\mathrm{SeTeO}$ to robust Dirac-conelike band dispersion. The SOC and intrinsic in-plane magnetocrystalline anisotropy energy induce Dirac-conelike altermagnets to generate apparent valley polarization through magnetovalley coupling. In terms of SOC perturbation, we elucidate the physical mechanism behind in-plane magnetization-induced valley polarization and demonstrate that valley polarization is negatively correlated with the band gap. The present work reveals the physical origin of valley polarization in altermagnets and expands the application of ferrovalley at room temperature in valleytronics.

The relation between superconductivity and time-reversal symmetry is one of the most fascinating problems in condensed-matter physics. Here we report an anomalous transport behavior in superconducting Josephson junctions induced by surface disorder. We fabricate vertical Josephson junctions using metallic superconductor (Al) and ion-bombarded ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. Below the superconducting transition temperature, we observe persistent magnetoresistive hysteresis behavior dependent on the disorder deposition time. Field- and temperature-dependent measurements suggest that the observed effects arise from disorder-induced anomalous flux in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ which can be sensitively detected by superconducting Al. To account for the observed findings, we consider different physical scenarios such as interband surface pairing reconstruction, magnetic surface states, and vortex phases.

Refractory materials play an important role in lunar formation as they are some of the first components to coalesce from vaporized material that resulted from a giant impact. CaO is chemically and structurally similar to MgO and together they make up over 40% of both pyrolite and lunar composition models. In this work, we explore the liquid-vapor coexistence region of CaO. We determine the critical point to be located between 6000 and 6500 K and between 0.3 and $0.7\mathrm{g}/{\mathrm{cm}}^3$, which puts it slightly below the one previously obtained for MgO. Structurally, both CaO and MgO consist mostly of sixfold and sevenfold coordinated cations at the higher density and threefold and fourfold ones at the lower end of the density studied. The vapor consists mainly of Ca, CaO, O, ${\mathrm{O}}_2$, ${\mathrm{Ca}}_2{\mathrm{O}}_2$, ${\mathrm{Ca}}_2\mathrm{O}$, and ${\mathrm{CaO}}_2$. The main difference with MgO is that the relative abundance of molecular oxygen is much lower in the vapor of CaO. Similarly to the MgO system, we find that the vapor appears to be close to neutral in charge, indicating an atomic vapor as opposed to an ionic one.

This paper addresses the first-principles description of coherent phonons in systems subjected to optical excitations and/or doping. We reformulate the first-principles Ehrenfest equation (fpEE) [Phys. Rev. X 13, 031026 (2023)] in terms of Born-Oppenheimer phonon frequencies and dynamical Born effective charges. We demonstrate that nonadiabatic effects renormalize the Born-Oppenheimer frequencies and introduce a damping term responsible for the finite lifetime of coherent phonons. Notably, both the frequency renormalization and the lifetime are identical to those of quantum phonons. Furthermore, we show that electrons exert a force driven by an unconventional dynamically screened electron-phonon coupling. This coupling is smaller than the bare one even in the adiabatic limit, highlighting the need to revise current models. The fpEE is also used to develop a first-principles polaron theory that describes lattice distortions induced by doping.

In search of substrates for the adsorption of He atoms allowing for novel quantum phases in restricted geometry, we study the case of borophane, a hydrogenated compound of borophene. We consider two allotropes of borophane, ${\alpha }^{\prime }\text{−}4H$ and rect-$2H$. With a suitable density functional theory we characterize the adsorption potential of a He atom on such crystalline substrates, finding its corrugation, the preferential adsorption sites, and the energy barrier between sites. In the case of ${\alpha }^{\prime }\text{−}4H$ borophane the adsorption potential has some similarity to that of graphite but with much larger energy barriers between adsorption sites, so that the first adsorbed layer of ${}^4\mathrm{He}$ should be localized in a registered triangular crystal similar to the $\sqrt{3}\times \sqrt{3}\mathrm{R}{30}^{\circ }$ phase of graphite. Rect-$2H$ borophane appears more interesting due to the presence of ridges in the adsorption potential with modest energy barriers in one direction of the basal plane and a much higher barrier in the orthogonal direction, thus forming channels for motion of the adsorbed atoms. Using path integral Monte Carlo (PIMC) simulations, we find that in the first adsorbed layer the ${}^4\mathrm{He}$ atoms are rather delocalized along a channel with no exchanges between channels. This strong anisotropy is present also in the first few additional adsorption layers with the presence of ordered and disordered regions. In the second, fourth, and fifth layers we find superfluidity on the length scale of the simulated systems. In the second layer, the superfluidity is one-dimensional (1D) along the grooves. In a narrow low-coverage region of the fourth layer we observe an intriguing state with one unidirectional structure across the grooves which supports a tiny superfluid signal. In the fifth layer we find a two-dimensional (2D) superfluid, with a crossover from strongly anisotropic at low coverage to isotropic at layer completion. Starting from the sixth layer, the adsorbed ${}^4\mathrm{He}$ film evolves towards a three-dimensional (3D) superfluid. Our main prediction is that adsorption of ${}^4\mathrm{He}$ on rect-$2H$ borophane will allow us to probe 1D superfluidity in the second and possibly the fourth layers, the evolution from a 2D anisotropic superfluid to an isotropic one in the fifth layer, and, eventually, the onset of 3D superfluidity for higher coverages.

We investigate the second-order Floquet topological (SOFT) phase transitions, from the perspective of spatial symmetries. In this respect, we consider a generic honeycomb lattice Floquet topological insulator (FTI), realized by circularly polarized light irradiation, in the presence of spin-orbit coupling (Kane-Mele model). We find that our studied FTI presents chiral symmetry on a preferential direction in Fourier space, the same property that protects the topological phases in the Su-Schrieffer-Heeger (SSH) model. Thus we were allowed to characterize the SOFT phases in terms of mirror-graded winding numbers (Zak phase). Moreover, our model exhibits $C_2$ and $C_3$ symmetry in Fourier space, a property which lead us to investigate two finite structures having the aforementioned symmetries, namely, rhombic and triangular shapes. Indeed, we find that both of them undergo SOFT phase transitions, characterized by the appearance of 0D corner states symmetrically localized over the whole sheet. Finally, we investigate the $C_2$ and $C_3$ symmetry breaking. Interestingly, we reveal that the corner states are not destroyed, but localize at preferential corners instead, giving rise to a corner polarization.

Drawing inspiration from the recent breakthroughs in the ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$ quantum magnet, renowned for its spin supersolidity phase and its potential for solid-state cooling applications, our study delves into the interplay among lattice, spin, and orbital degrees of freedom within this compound. Using temperature-, field-, and pressure-dependent Raman scattering techniques, we present experimental evidence revealing crystal-electric-field (CEF) excitations, alongside the interplay of CEF-phonon interactions. We performed density functional theory calculations for the phonon frequencies and compared them with the experimentally observed modes. In addition, our experiments elucidated electronic transitions from $j_{1/2}$ to $j_{3/2}$ and from $j_{1/2}$ to $j_{5/2}$, with energy levels closely aligned with theoretical predictions based on point-charge models. Moreover, the application of a magnetic field and pressure revealed Zeeman splittings characterized by Landé-$g$ factors as well as the CEF-phonon resonances. The anomalous shift in the coupled peak at low temperatures originated from the hybridization of CEF and phonon excitations due to their close energy proximity and shared symmetry. These findings constitute a significant step towards unraveling the fundamental properties of this exotic quantum material for future research in fundamental physics or engineering application.

In this work, we develop a symmetry-based classification of Chern phases in honeycomb photonic crystals, considering arbitrary nonreciprocal couplings compatible with energy conservation. Our analysis focuses on crystals formed through nonreciprocal perturbations of photonic graphene. These perturbations, which can have arbitrary spatial variations, are generally described by scalar and vector fields. Using a tight-binding model, we consider the most general nonreciprocal interactions, including gyromagnetic, pseudo-Tellegen, and moving medium responses, and examine how the corresponding nonreciprocal fields influence the crystal's topology. Our findings reveal that nonreciprocal interactions alone are insufficient to induce a topologically nontrivial phase. Instead, a nontrivial $p6m$ component in the nonreciprocal fields is required to open a band gap and achieve a nonzero Chern number. These results provide a symmetry-based roadmap for engineering photonic topological phases via nonreciprocal perturbations of photonic graphene, offering practical guidelines for designing topological phases in graphenelike photonic crystals.

We study the quantum transport in double quantum dots (DQD) sandwiched between surfaces of topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, which possess strong spin-orbit coupling (SOC) and ${}^d{\mathrm{C}}_{3v}$ double group symmetry. Different from the spin-conserved case with two-dimensional electron gas (2DEG) electrodes, the conductance displays a universal scaling relation for different Fermi energy associated with the topological nature/linear dispersion of topological surface states. The interplay between direct interdot tunneling and surface state mediated interaction leads to tunable Dicke and Fano effects with changing the interdot distance. We propose nano-rulers with different measurement range and resolution based on the Fano $q$-factor. Furthermore, when applying an in-plane Zeeman field, a crossover from a double-peak shape to a quad-peak shape in conductance curve appears. Moreover, the rotational symmetry of the system could also be revealed from the conductance pattern. Our findings contribute to a better understanding of the quantum transport in the presence of electrode's SOC topological states.

Systems with $\mathrm{P}{2}_{1}3$ symmetry are characterized by the realization of chiral edge modes, propagating in one direction along closed loops around some high symmetry points of the Brillouin zone. We study the phononic and electronic properties of EuPtSi, which crystallizes with $\mathrm{P}{2}_{1}3$ symmetry. EuPtSi is also characterized by intriguing magnetic properties, such as the realization of the skyrmion lattice. Here, using ab initio techniques, we study bulk and slab properties of EuPtSi. The bulk phononic and electronic band structures exhibit a spin-1 Weyl point and a charge-2 Dirac point at the $\mathrm{\Gamma }$ and $\mathrm{R}$ points, respectively. Consequently, the surface states exhibit chiral edge modes. Such features are present in both the phononic and electronic surface spectra. The chiral phononic edge mode is associated with the vibration of the atoms in close vicinity, while the chiral electronic surface states correspond to carrier accumulation at the edge of chiral atomic chains.