Recent advances in delicate topology have expanded the classification of topological bands, but its presence in solid-state materials remains elusive. Here we show that delicate topology naturally emerges in the Luttinger-Kohn model that describes many semiconductors and semimetals. In particular, the Luttinger semimetal (LSM) is found to be a quantum critical point leading to a quantized jump of an integer-valued delicate topological invariant, i.e., the returning Thouless pump (RTP) index. Away from this criticality, we have identified new types of electronic insulators and semimetals with intertwined stable and delicate topologies. They all carry gapless surface states that transform anomalously under rotation symmetry. We further unveil that RTP physics widely exists in a large class of LSM-based Weyl materials that respect a rotoinversion symmetry, and we discuss their unconventional surface physics. Our work provides a starting point for exploring delicate topological phenomena in quantum materials.

The calculation of material phonon thermal conductivity from density functional theory calculations requires the computationally expensive evaluation of anharmonic interatomic force constants and has remained a computational bottleneck in the high-throughput discovery of materials. In this paper, we present a machine learning assisted approach for the extraction of anharmonic force constants through local learning of the potential energy surface. We demonstrate our approach on a diverse collection of 220 ternary materials for which the total computational time for anharmonic force constant evaluation is reduced by more than an order of magnitude from 480 000 CPU hours to less than 12 000 CPU hours while preserving the thermal conductivity prediction accuracy to within 10%. Our approach removes a major hurdle in computational thermal conductivity evaluation and will pave the way forward for the high-throughput discovery of materials.

Single-photon emitters integrated in optical microcavities are key elements in quantum com munication applications. However, optimizing their emission properties and achieving efficient cavity coupling remain significant challenges. In this study, we investigate semiconductor quantum dots (QDs) emitting in the telecom O-band and integrate them in an open fiber-cavity. Such cavities offer spatial and spectral tunability and intrinsic fiber-coupling. The design promises high collection efficiency and enables the investigation of multiple emitters in heterogeneous samples. We observe a reduction of the decay times of several individual emitters by up to a factor of $2.46\left(2\right)$ due to the Purcell effect. We comprehensively analyze the current limitations of the system, including cavity and emitter properties, the impact of the observed regime where cavity and emitter linewidths are comparable, as well as the mechanical fluctuations of the cavity length. The results elucidate the path towards efficient telecom quantum light sources.

Magnetoelectric coupling in insulating multiferroic materials is invaluable for both fundamental research and multifunctional device applications. However, material realization remains a significant challenge. We employ first-principles calculations to predict an enhanced and stacking-robust magnetoelectric coupling in a multiferroic van der Waals heterostructure consisting of ${\mathrm{CrCl}}_3$ and ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$. Within this heterostructure, the intralayer ferroic orders exhibit characteristics of a type-I multiferroic, while the interlayer ferroic orders resemble those of a type-II multiferroic, thus creating a hybrid-type multiferroic. Notably, the interlayer magnetic configuration can be switched upon electric-polarization reversal, while maintaining the insulating property without any nontrivial phase transitions. Our analysis reveals a strongly interfacial spin local-field effect and proximity polarization effect on band alignments. These factors work synergistically to decide the interlayer exchange interaction and drive enhanced magnetoelectric coupling. Finally, we show that this magnetoelectric effect is robust in multilayer structures, exhibiting a dependence on the layer number's parity. This research offers promising prospects for discovering enhanced magnetoelectric couplings in hybrid multiferroic van der Waals heterostructures.

Two-dimensional van der Waals heterostructures (vdWHs) are promising candidates for multi-functional electronic and optoelectronic devices. Here, based on reconfigurable band alignment, we design $n\text{−}\mathrm{InSe}/m\text{−}\mathrm{SnS}$ (GeSe) multilayer vdWHs with different layer combinations to explore their ultrafast dynamic process and optoelectronic device performance. When $n>1\left(3\right)\mathrm{L}$, the band structure transforms from staggered- into broken-gap band alignment, and increasing the layer number can effectively widen the band-to-band tunneling window to achieve adjustable tunneling field-effect transistors. For the type-II InSe/GeSe vdWHs, the ultrafast electron transfer timescale reduces from 95 to 33 fs when the InSe layer number increases from 1L to 2L. Moreover, the photocurrent density of $n\text{−}\mathrm{InSe}/m\text{−}\mathrm{SnS}$ (GeSe) multilayer vdWHs photodetectors can be significantly improved with layer-combination modulation, and the value of 2-InSe/1-GeSe vdWHs can be approximately increased by 12 times compared with that of 1-InSe/1-GeSe vdWHs. This research provides an effective way to design tunneling field-effect transistors and photodetectors in low-power consumption and high-speed switching.

We propose a valley filter in the few-layer black-phosphorus-based n-p-n (BP-npn) junction, by applying linearly polarized light (LPL) in the scattering region. It is found that the considered BP can separate two good valleys in the presence of band gap inversion. Moreover, the LPL has been found to selectively match the pseudospin of the scattering region with the pseudospin of incident electrons in opposite valleys. This is more apparent than the valley polarization controlled by electric potential barriers. As LPL is strengthened, the null-polarization region decreases, leading to the widening of the regions of valley polarization. In addition, the width of the scattering region can cause substantial Fabry-Pérot oscillations in tunneling, leading to a difference in $D-$ and $D+$ valley-dependent conductance, thereby improving valley polarization efficiency. Therefore, this junction can be a promising candidate for the valley filter.

Quantum spin Hall (QSH) insulators, with their unique helical edge states where counterpropagating edge channels possess opposite spins, have attracted broad interest across various fields. While the exact quantization of spin Hall conductance (SHC) is elusive in realistic materials due to intrinsic spin mixing effects, the near quantization, as a compromised definition of the QSH effect, cannot be captured by rigorous topological invariants. In this Letter, we present a universal symmetry indicator for diagnosing the QSH effect in realistic materials, termed spin U(1) quasisymmetry. Such a symmetry eliminates the first-order spin-mixing perturbation and thus protects the near-quantization of SHC, applicable to time-reversal-preserved cases with either $Z_2=1$ or $Z_2=0$, as well as time-reversal-broken scenarios. We propose that spin U(1) quasisymmetry is hidden in the subspace spanned by the doublets with unquenched orbital momentum and emerges when SOC is present, which can be realized in 19 crystallographic point groups. Our theory is applied to identify previously overlooked QSH phases such as time-reversal-preserved even spin Chern phase and time-reversal-broken phase, as exemplified by twisted bilayer transition metal dichalcogenides, monolayer ${\mathrm{RuBr}}_3$, and monolayer FeSe. Our work provides a comprehensive symmetry-based framework for understanding the QSH effect and significantly expands the material pool for the screening of exemplary material candidates.

We present laser driven many electron dynamics within the time-dependent configuration interaction (TD-CI) approach for the simulation of exciton-exciton fusion in perylene dimers. We use a selected active orbital window and a small distortion of the ground state geometry to construct a reasonably sized model system based on ab initio quantum chemical wave functions, which shows a large exciton-exciton fusion probability. For the TD-CI simulations, we adopt a CI variant, i.e., the time-dependent double configuration interaction singles (TD-dCIS), which allows a direct mapping of excited monomer and charge transfer states to single and double exciton states in aggregates. We use semicanonical orbitals, which are localized on one of the perylene molecules in the dimer and are very similar to canonical Hartree-Fock orbitals of the isolated perylene molecule. We compare the TD-dCIS results to few-state and many-state molecular exciton Hamiltonian (MEH) models employing the point dipole approximation for exactly the same systems. We find that the usual few-state MEH models perform quite well compared with TD-dCIS, while including many states in MEH models leads to large deviations if the usual point dipole approximation is used. Further, we prove that even in strongly coupled dimers the double exciton state can be efficiently excited using ultrashort laser pulses.

We report a detailed study of the magnetic field switching of ferrotoroidal/multiferroic domains in the Y-type hexaferrite compound ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{Mg}}_2{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$. By combining data from superconducting quantum interference device (SQUID) magnetometry, magnetocurrent measurements, and resonant x-ray scattering experiments, we arrive at a complete description of the deterministic switching, which involves the formation of a temperature-dependent mixed state in low magnetic fields. This mechanism is likely to be shared by other members of the hexaferrite family, and presents a challenge for the development of high-speed read-write memory devices based on these materials.

Owing to the robust spin-polarized topological conducting surface states, topological insulators (TIs) offer huge promise of establishing ultrafast operation and low power consumption in spin-based applications like quantum communication and magnetic memory devices. In this paper, high spin-to-charge conversion (SCC) is evidenced at room temperature in the TI/ferromagnet (FM) interface engineered ${\mathrm{Al}}_2{\mathrm{O}}_3\text{(0001)}\text{/}{\mathrm{Sb}}_2{\mathrm{Te}}_3$ (10 nm)/Ru(0–12 nm)/${\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$(20 nm)/Ru (4 nm) heterostructure system (with ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ as TI and ${\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$ as FM) grown via DC magnetron cosputtering. The generation of a significantly high charge current density is observed for a particular Ru interlayer (IL) thickness of 4 nm, above which the SCC efficiency of these heterostructures is shown to be inversely governed with the increase in the thickness of the Ru IL. The SCC efficiency in these heterostructure is quantitatively estimated through inverse spin Hall effect (ISHE) measurements, where ∼500% enhancement in the SCC is witnessed corresponding to insertion of a 4-nm-thin Ru IL. The inverse Edelstein effect length (${\lambda }_{\mathrm{IEE}}$) for the heterostructure with the largest SCC is determined to be ∼0.19 ± 0.01 nm, which is comparable with the reported values of other TIs/FM heterostructures. Longitudinal spin Seebeck effect (LSSE) measurements, which corroborate the ISHE findings, also demonstrated the significant power-generation potential in these heterostructures. When the heterostructure is considered as a parallel load resistance model, the analysis of LSSE results disentangles the pure SSE contribution of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ from the anomalous Nernst effect contribution of the ${\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$ FM. Similar trends are confirmed in other test heterostructures fabricated, wherein either the TI ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ layer is replaced with other chalcogenide TI ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, or the Ru HM layer is replaced with Nb, thereby confirming the generalization of the approach of controlling the SCC efficiency. These results highlight the potential of TIs and interface engineering for the realization of ultralow power devices and high-current spin Seebeck thermoelectric devices for future spin-based technologies and quantum communication.

Collective behavior of spins, frustration-induced strong quantum fluctuations, and subtle interplay between competing degrees of freedom in quantum materials can lead to correlated quantum states with exotic excitations that are essential ingredients for establishing paradigmatic models and have immense potential for quantum technologies. Disorder is ubiquitous in real materials, and the detailed insights into the role of disorder on the intriguing ground state borne out of quenched randomness provide a route towards the design and discovery of functional quantum materials. Herein we report magnetization, specific heat, electron spin resonance, and muon spin resonance studies on a $3d$-electron-based antiferromagnet ${\mathrm{Sr}}_3{\mathrm{CuTa}}_2{\mathrm{O}}_9$. The negative value of Curie-Weiss temperature, obtained from the Curie-Weiss fit of high-temperature magnetic susceptibility data indicates the presence of antiferromagnetic interaction between ${\mathrm{Cu}}^{2+}$ moments. Specific heat data show the absence of long-range magnetic ordering down to 64 mK despite a reasonably strong exchange interaction between ${\mathrm{Cu}}^{2+}$ ($S=1/2$) spins as reflected from a Curie-Weiss temperature of $-27\pm 1\mathrm{K}$. The power-law behavior and the data collapse of specific heat and magnetization data evince the emergence of a random-singlet state in ${\mathrm{Sr}}_3{\mathrm{CuTa}}_2{\mathrm{O}}_9$. The power-law-like spin autocorrelation function and the data collapse of muon polarization asymmetry with longitudinal field dependence of $t/{\left({\mu }_{0}H\right)}^{\gamma }$ further support credence to the presence of a randomness-induced quantum disordered state. Our results suggest that randomness induced by disorder is an alternate route to realize quantum spin disordered state in this antiferromagnet.

This paper systematically investigates the phase diagram of a parallel double-quantum-dot Andreev molecule, where the two quantum dots are coupled to a common superconducting lead. Using the numerical renormalization group method, we map out the evolution of the ground state across a wide parameter space of level detunings, size of the superconducting gap, lead couplings, and interdot coupling strength. The intricate phase diagrams feature singlet, doublet, and a relatively uncommon triplet ground states, with the latter being a distinct signature of strong lead-mediated interactions between the quantum dots. We benchmark the applicability of simplified effective models, including the atomic limit and zero-bandwidth approximations, in capturing the complex behavior of this parallel configuration. Our analysis reveals severe limitations of these models, underscoring the necessity for maximal caution when extrapolating beyond their tested validity. In particular, all effective models except for the extended version of the zero-bandwidth approximation failed in reproducing the triplet ground state and made several false predictions. These findings provide crucial insights for interpreting experimental observations and designing superconducting devices based on quantum-dot architectures.

As an emergent van der Waals multiferroic semiconductor of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ (CCPS), its high-temperature and high-pressure structural, ferroelectric, and electrical transport properties were comprehensively studied at 298–873 K and 0.8–45.0 GPa using an externally heated diamond-anvil cell joined with Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy observations under different hydrostatic environments. For nonhydrostatic compression of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$, two sequential structural transitions at 5.6 and 13.6 GPa took place, followed by a semiconductor-to-metal crossover at 28.6 GPa. Nevertheless, the pressure-driven structural transition and metallization of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ were delayed by $\sim 2.0\mathrm{GPa}$ under hydrostatic condition owing to the influence of defects and dislocation. Upon depressurization, the phase transformation of CCPS was demonstrated to be reversible under different hydrostatic environments. Additionally, the linearity between electrical current and sinusoidal voltage with the nonlinearity factors of $\sim 1.00$ manifested the Ohmic response of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ under high pressure. Simultaneously, the noticeable discontinuities in temperature-dependent Raman shifts, full width at half maximums Raman peak intensity ratios, and electrical conductivity at 423 K provided robust evidence of the ferroelectric transition of CCPS at ∼0.4 GPa. These findings on ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ offer insight into the interplay of crystalline structure, electric polarization, and electrical transport behavior for two-dimensional multiferroic materials under high-temperature and high-pressure conditions and stimulate the potential development of advanced multifunctional devices.

The nonequilibrium ultrafast carrier dynamics of the photoinduced phase transition in chalcogenide phase-change material (rocksalt Ge-Sb-Te) has been investigated via real-time time-dependent density-functional theory, reproducing experimental observations of subpicosecond timescales and electronic diffraction. We develop a picture of the photocarrier distribution in momentum and real space dependent on the laser external field, revealing that thermal and coherent phonons contend to modulate the crystalline order. Localized coherent Peierls suppression intensifies with electronic excitation, and ultrafast dissipation of antibonding electrons near the Fermi surface incubates Lindemann particles. Full-domain hot-carrier excitation upon intense photon scattering cools and participates in the self-amplified growth of molten particles, followed by a sustained nonradiative recombination of photoelectron and photogenerated hole pairs. These findings present a comprehensive and definitive energy landscape for the nonequilibrium phase engineering in chalcogenide phase-change materials.

The altermagnet (AM) is a magnetic state with collinear antiferromagnetic ground state but it presents some transport properties that were only believed to exist in ferromagnets or noncollinear antiferromagnets. To have a comprehensive understanding of the transport properties of AMs, especially from the experimental point of view, a promising altermagnetic metal is crucial. In all the proposed altermagnetic metals, ${\mathrm{RuO}}_2$ has a special position, since it is the AM with the largest spin splitting and several important altermagnetism featured experiments were performed based on it. However, a very recent report based on sensitive muon-spin measurements suggests a supersmall local magnetization from Ru, i.e., a nonmagnetic ground state in ${\mathrm{RuO}}_2$. Therefore, a determination of the existence of the altermagnetic ground state is the basic starting point for all the previously altermagnetic transport properties in ${\mathrm{RuO}}_2$. In this work, we propose to identify its magnetic ground state from the Fermi surface (FS) via the electronic transport property of quantum oscillation (QO). We systematically analyzed the FSs of ${\mathrm{RuO}}_2$ in both nonmagnetic and altermagnetic states via first principles calculations. Our work should be helpful for future experiments on QO measurements to confirm its ground state by the interplay between transport measurements and computations.

The magnetic properties of $\mathrm{NdP}{\mathrm{t}}_6\mathrm{A}{\mathrm{l}}_3$ with a honeycomb structure of Nd ions have been studied by magnetic susceptibility $M$($T$)/$B$, isothermal magnetization $M$($B$), specific heat $C$($T$), and electrical resistivity ρ($T$) measurements using single-crystalline samples as well as by neutron diffraction and inelastic scattering using polycrystalline samples. A λ-type anomaly in $C$($T$), a bend in ρ($T$), and a jump in $M$($T$)/$B_{\left|\right|a}$ occur concomitantly at 1.2 K, revealing a long-range magnetic order. A spontaneous moment of 0.2 ${\mu }_{\mathrm{B}}$/f.u. (formula units) in the $M(B\parallel a)$ curve at 0.3 K suggests a canted antiferromagnetic (AFM) order with the $\mathrm{N}{\mathrm{d}}^{3+}$ moments lying in the honeycomb plane. The inelastic neutron-scattering measurements disclose crystalline electric-field (CEF) excitations at 0.6 and 2.9 meV. The energy of the CEF excitations measured at 1.5 and 100 K, temperature dependences of $B/M$($T$), and two Schottky anomalies in $C$($T$) are simultaneously analyzed using a trigonal CEF model for the $J$ = 9/2 multiplet of the $\mathrm{N}{\mathrm{d}}^{3+}$ ion. The CEF doublet ground state is described as $\left|\psi \rangle =-0.082\right|\pm \frac{7}{2}\rangle +0.555|\mp \frac{5}{2}\rangle \pm 0.827|\pm \frac{1}{2}\rangle$. The neutron powder-diffraction experiment determines the canted AFM structure defined with a propagation vector k = [0, 0, 0] and ordered moments of 2.62 ${\mu }_{\mathrm{B}}$ canting by an angle of 15 ° from antiparallel alignment in the honeycomb plane. We propose that the canting of AFM moments results from the Dzyaloshinskii-Moriya interaction between the nearest-neighbor $\mathrm{N}{\mathrm{d}}^{3+}$ moments at the midpoint of which the inversion symmetry is broken by the presence of the Pt triangle inside the Nd hexagon.

Three-dimensional Dirac semimetals can be driven into an insulating state by coupling to a charge density wave (CDW) order. Here, we consider the quantized crystalline responses of such charge-ordered Dirac semimetals, which we dub Dirac-CDW insulators, in which charge is bound to disclination defects of the lattice. Using analytic and numeric methods we show the following. First, when the CDW is lattice commensurate, disclination-line defects of the lattice have a quantized charge per length. Second, when the CDW is inversion symmetric, disclinations of the lattice have a quantized electric polarization. Third, when the CDW is lattice commensurate and inversion symmetric, disclinations are characterized by a “disclination filling anomaly,” a quantized difference in the total charge bound to disclination lines of Dirac-CDW with open and periodic boundaries. We construct an effective response theory that captures the topological responses of the Dirac-CDW insulators in terms of a total derivative term, denoted the $R\wedge F$ term. The $R\wedge F$ term describes the crystalline analog of the axion electrodynamics that are found in Weyl semimetal-CDW insulators. We also use the crystalline responses and corresponding response theories to classify the strongly correlated topological phases of three-dimensional Dirac semimetals.

The ground state of the bipartite $t\text{−}J$ model must satisfy a specific sign structure, based on which the single-hole and two-hole ground state $Ans\ddot{a}tze$ on honeycomb lattice are constructed and studied by a variational Monte Carlo (VMC) method. The VMC results are in good agreement with the exact diagonalization (ED) calculation. For the single-hole case, the degenerate ground states are characterized by quantum numbers of a spin-1/2 and an orbital angular momentum $L_z=\pm 2$. The latter is associated with the emergent chiral spin/hole currents mutually surrounding the hole/spin-1/2 as a composite object or “twisted hole”. A vanishing quasiparticle spectral weight is shown in the large-sample limit. In the two-hole ground state, the holes form a spin-singlet pairing with $d+id$ symmetry in the Cooper channel, but are of $s$-wave symmetry as a tightly bound pair of the “twisted holes”. Such a pairing mechanism of dichotomy can be attributed to eliminating the local spin currents which has nothing to do with the long-range antiferromagnetic correlation. Superconducting ground state at finite doping is briefly discussed in terms of the tightly bound hole pairs as the building blocks.

We present a scheme for achieving the clone of dark solitons in a nonresonant, incoherently pumped exciton-polariton condensate. By employing a composite pumping technique that combines homogeneous pumping and multiple Gaussian pumpings, and initially injecting a single dark soliton into one Gaussian pumping field, soliton cloning of high fidelity can be realized within the other Gaussian pumping fields due to the gain and loss mechanism. The “periodical copying” can be achieved for dark solitons, where we find the cloned breathing dark soliton groups. The cloning time of solitons can be manipulated by adjusting the pumping and system parameters. Furthermore, by controlling Gaussian pumping, the breathing dark soliton group can be transformed into a single dark soliton, which can be used to stabilize the dark soliton. In addition, in the case of the isometric arrangement of multiple Gaussian pumpings, the cloned breathing dark solitons occur at the outermost area. Our proposal offers a promising avenue for generating and controlling soliton cloning, discovering an alternate approach to find dynamically stable solutions in nonequilibrium systems. The soliton cloning also has potential applications including the synthesis of molecules and multisoliton clusters, and quantum communication technologies.