Photons and nuclear spins are two well-known building blocks in quantum information science and technology. Establishing an efficient interface between optical photons and nuclear spins, while highly desirable for hybridizing these two quantum systems, is challenging, because the interactions between nuclear spins and the environment are usually weak in magnitude, and there is also a formidable gap between nuclear spin frequencies and optical frequencies. In this work, we propose an optonuclear quadrupolar (ONQ) effect, whereby optical photons can be efficiently coupled to nuclear spins, similar to Raman scattering. Compared to previous works, ancilla electron spins are not required for the ONQ effect. This leads to advantages such as applicability in defect-free nonmagnetic crystals and longer nuclear spin coherence time. In addition, the frequency of the optical photons can be arbitrary, so they can be fine-tuned to minimize the material heating and to match telecom wavelengths for long-distance communications. Using perturbation theory and first-principles calculations, we demonstrate that the ONQ effect is stronger by several orders of magnitude than other nonlinear optical effects that could couple to nuclear spins. Based on this rationale, we propose promising applications of the ONQ effect, including quantum memory, quantum transduction, and materials isotope spectroscopy. We also discuss issues relevant to the experimental demonstration of the ONQ effect.

Understanding the spin dynamics in antiferromagnetic (AFM) thin films is fundamental for designing novel devices based on AFM magnon transport. Here, we study the magnon dynamics in thin films of AFM $S=\frac{5}{2}$ $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ by combining resonant inelastic x-ray scattering, Anderson impurity model plus dynamical mean-field theory, and Heisenberg spin model. Below 100 meV, we observe the thickness-independent (down to 15 nm) acoustic single-magnon mode. At higher energies (100–500 meV), an unexpected sequence of equally spaced, optical modes is resolved and ascribed to $\mathrm{\Delta }{S}_z=1$, 2, 3, 4, and 5 magnetic excitations corresponding to multiple, noninteracting magnons. Our study unveils the energy, character, and momentum dependence of single and multimagnons in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ thin films, with impact on AFM magnon transport and its related phenomena. From a broader perspective, we generalize the use of $L$-edge resonant inelastic x-ray scattering as a multispin-excitation probe up to $\mathrm{\Delta }{S}_z=2S$. Our analysis identifies the spin-orbital mixing in the valence shell as the key element for accessing excitations beyond $\mathrm{\Delta }{S}_z=1$, and up to, e.g., $\mathrm{\Delta }{S}_z=5$. At the same time, we elucidate the novel origin of the spin excitations beyond the $\mathrm{\Delta }{S}_z=2$, emphasizing the key role played by the crystal lattice as a reservoir of angular momentum that complements the quanta carried by the absorbed and emitted photons.

Electron-lattice interactions play a prominent role in quantum materials, making a deeper understanding of direct routes to phonon-mediated high-transition-temperature ($T_c$) superconductivity desirable. However, it has been known for decades that weak electron-phonon coupling gives rise to low values of $T_c$, while strong electron-phonon coupling leads to lattice instability or formation of bipolarons, generally assumed to be detrimental to superconductivity. Thus, the route to high-$T_c$ materials from phonon-mediated mechanisms has heretofore appeared to be limited to raising the phonon frequency as in the hydrogen sulfides. Here we present a simple model for phonon-mediated high-$T_c$ superconductivity based on superfluidity of light bipolarons. In contrast to the widely studied Holstein model where lattice distortions modulate the electron’s potential energy, we investigate the situation where lattice distortions modulate the electron hopping. This physics gives rise to small-size, yet light bipolarons, which we study using an exact sign-problem-free quantum Monte Carlo approach demonstrating a new route to phonon-mediated high-$T_c$ superconductivity. We find that $T_c$ in our model generically and significantly exceeds typical upper bounds based on Migdal-Eliashberg theory or superfluidity of Holstein bipolarons. The key ingredient in this bipolaronic mechanism that gives rise to high $T_c$ is the combination of light mass and small size of bipolarons. Our work establishes principles for the design of high-$T_c$ superconductors via functional material engineering.

Competing and intertwined orders including inhomogeneous patterns of spin and charge are observed in many correlated electron materials, such as high-temperature superconductors. Introducing a new development of the constrained-path auxiliary-field quantum Monte Carlo method, we study the interplay between thermal and quantum fluctuations in the two-dimensional Hubbard model. We obtain an accurate and systematic characterization of the evolution of the spin and charge correlations as a function of temperature $T$ and how it connects to the ground state, at three representative hole doping levels $\delta =1/5$, $1/8$, and $1/10$. We find increasing short-range commensurate antiferromagnetic correlations as $T$ is lowered. As the correlation length grows sufficiently large, a modulated spin-density wave (SDW) appears. At $\delta =1/5$ and $U/t=6$, the SDW saturates and remains short-ranged as $T\to 0$. In contrast, at $\delta =1/8$, $1/10$ and $U/t=8$, this evolves into a ground-state stripe phase. We study the relation between spin and charge orders and find that formation of charge order appears to be driven by that of the spin order. We identify a finite-temperature phase transition below which charge ordering sets in and discuss the implications of our results for the nature of this transition.

Color centers are point defects in crystals that can provide an optical interface to a long-lived spin state for distributed quantum information processing applications. An outstanding challenge for color center quantum technologies is the integration of optically coherent emitters into scalable thin-film photonics, a prerequisite for large-scale photonics integration of color centers within a commercial foundry process. Here, we report on the integration of near-transform-limited silicon vacancy (${\mathrm{V}}_{\mathrm{Si}}$) defects into microdisk resonators fabricated in a CMOS-compatible $4H$-silicon carbide-on-insulator platform. We demonstrate a single-emitter cooperativity of up to 0.8 as well as optical superradiance from a pair of color centers coupled to the same cavity mode. We investigate the effect of multimode interference on the photon scattering dynamics from this multiemitter cavity quantum electrodynamics system. These results are crucial for the development of quantum networks in silicon carbide and bridge the classical-quantum photonics gap by uniting optically coherent spin defects with wafer-scalable, state-of-the-art photonics.

With recent advances in time-resolved device control, the dynamical engineering of novel quantum phases is becoming reality. One of the striking new options is the dynamical generation of space-synthetic dimensions, transcending the confines of static crystalline solid-state physics. We apply this principle to propose protocols allowing for the engineered realization of topological surface states in isolation. As a concrete example, we consider $3\mathrm{D}$ topological surface states of a $4\mathrm{D}$ quantum Hall insulator via a $(1+2_{\mathrm{syn}})$-dimensional protocol. We present first-principle analytical calculations demonstrating that no supporting $4\mathrm{D}$ bulk phase is required for a $3\mathrm{D}$ topological surface phase. We back the analytical approach by numerical simulations and present a detailed blueprint for the realization of the synthetic surface phase with existing quantum linear optical network device technology. We then discuss generalizations, including a proposal for a quantum simulator of the $(1+1_{\mathrm{syn}})$-dimensional surface of the common $3\mathrm{D}$ topological insulator.

Intense debate has recently arisen regarding the photoinduced changes to the iron-chalcogenide superconductors, including the enhancement of superconductivity and a metastable state. Here, by employing high-energy resolution, we directly observe the melting of superconductivity on ultrafast timescales. We demonstrate a distinctly nonequilibrium response on short timescales, where the gap fills in prior to the destruction of the superconducting peak, followed by a metastable response. We propose that the former is due to pair phase decoherence and speculate that the latter is due to the increase in double-stripe correlations that are known to compete with superconductivity. Our results add to exciting new developments on the iron-based superconductors, indicating that the photoinduced metastable state possibly competes with superconductivity.

Extreme ultraviolet light delivering radiation with a wavelength shorter than approximately 100 nm is now available from solid-state sources. However, despite exceptional progress, efficient focusing of extreme ultraviolet photons to their ultimate diffraction limit remains a formidable challenge because of the precision of the focusing by curved, optical surfaces. Here we integrate coherent short-wavelength high-order harmonics from a MgO crystal, with a high-numerical-aperture, nanostructured, focusing element etched onto the surface of the crystal itself. We focus extreme ultraviolet light, seventh harmonic of an 800-nm laser, with a zone plate of numerical aperture 0.35, down to a waist radius of 150 nm, with 18% focusing efficiency. The estimated intensity approaches ${10}^7\mathrm{W}/{\mathrm{cm}}^2$. Future developments may demonstrate nanoscale laser ablation and miniaturization of extreme ultraviolet coherent sources on a chip.

Electronic properties of superatomic crystals have not been sufficiently explored due to the versatility of their building units; moreover, their interunit couplings are even poorly understood. Here, we present a joint experiment-theory investigation of a rationally designed layered superatomic crystal of ${\mathrm{Au}}_6{\mathrm{Te}}_{12}{\mathrm{Se}}_8$ (ATS) cubes stacked by noncovalent intercube quasibonds. We find a sequential-emerged anisotropic triple-cube charge density wave (TCCDW) and polarized metallic states below 120 K, as revealed via scanning tunneling microscopy and spectroscopy, angle-resolved photoemission spectroscopy, transport measurement, Raman spectra, and density-functional theory. The polarized states are locked in an antiparallel configuration, which is required for maintaining the inversion symmetry of the center cube in the TCCDW. The antipolar metallic states are thus interweaved by the CDW and the polarized metallic states, and primarily ascribed to electronic effects via theoretical calculations. This work not only demonstrates a microscopic picture of the interweaved CDW and polarized charge orders in the superatomic crystal of ATS, but also sheds light on expanding the existing category of quantum materials to noncovalent solids.

The relationship between many-body interactions and dimensionality is integral to numerous emergent quantum phenomena. A striking example is the Bose gas, which upon confinement to one dimension (1D) obeys an infinite set of conservation laws, prohibiting thermalization and constraining dynamics. In our experiment, we demonstrate that such a 1D behavior can extend much further into the dimensional crossover toward 3D than expected. Starting from a weakly interacting Bose gas trapped in a highly elongated potential, we perform a quench to instigate the dynamics of a single density mode. Employing the theory of generalized hydrodynamics, we identify the dominant relaxation mechanism as the 1D dephasing of the relevant collective excitations of the system, the rapidities. Surprisingly, the dephasing remains dominant even for temperatures far exceeding conventional limits of one dimensionality where thermalization should occur. We attribute our observations to an emergent Pauli blocking of transverse excitations caused by the rapidities assuming fermionic statistics, despite the gas being purely bosonic. Thus, our study suggests that 1D physics is less fragile than previously thought, as it can persist even in the presence of significant perturbations. More broadly, by employing the exact Bethe ansatz solutions of the many-body system, we facilitate an interpretation of how the emergent macroscopic behavior arises from the microscopic interactions.

Because electrons and ions form a coupled system, it is a priori clear that the dynamics of the lattice should reflect symmetry breaking within the electronic degrees of freedom. Recently, this has been clearly evidenced for the case of time-reversal and mirror symmetry breaking by observations of a large phononic thermal Hall effect in many strongly correlated electronic materials. However, the mechanism by which time-reversal breaking and chirality is communicated to the lattice is far from evident. In this paper, we discuss how this occurs via many-body scattering of phonons by collective modes: a consequence of non-Gaussian correlations of the latter modes. We derive fundamental new results for such skew (i.e., chiral) scattering and the consequent thermal Hall conductivity. We emphasize that these results apply to any collective variables in any phase of matter: electronic, magnetic, or neither; highly fluctuating and correlated, or not. As a proof of principle, we compute general formulas for the above quantities for ordered antiferromagnets. From the latter, we obtain the scaling behavior of the phonon thermal Hall effect in clean antiferromagnets. The calculations show several different regimes and give quantitative estimates of similar order to that seen in recent experiments.

Exotic magnetic textures emerging from the subtle interplay between thermodynamic and topological fluctuation have attracted intense interest due to their potential applications in spintronic devices. Recent advances in electron microscopy enable the imaging of random photogenerated individual skyrmions. However, their deterministic and dynamical manipulation is hampered by the chaotic nature of such fluctuations and the intrinsically irreversible switching between different minima in the magnetic energy landscape. Here, we demonstrate a method to coherently control the rotation of a skyrmion crystal by discrete amounts at speeds which are much faster than previously observed. By employing circularly polarized femtosecond laser pulses with an energy below the band gap of the Mott insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, we excite a collective magnon mode via the inverse Faraday effect. This triggers coherent magnetic oscillations that directly control the rotation of a skyrmion crystal imaged by cryo-Lorentz transmission electron microscopy. The manipulation of topological order via ultrafast laser pulses shown here can be used to engineer fast spin-based logical devices.

The exploration of quantum spin liquids (QSLs) has been guided by different approaches including the resonating valence bond (RVB) picture, deconfined lattice gauge theories, and the Kitaev model. More recently, a spin-liquid ground state was numerically established on the ruby lattice, inspired by the Rydberg blockade mechanism. Here, we unify these varied approaches in a single parent Hamiltonian, in which local fluctuations of anyons stabilize deconfinement. The parent Hamiltonian is defined on kagomé triangles—each hosting four RVB-like states—and includes only Ising interactions and single-site transverse fields. In the weak-field limit, the ruby spin liquid and exactly soluble kagomé dimer models are recovered, while the strong-field limit reduces to the Kitaev honeycomb model, thereby unifying three seemingly different approaches to QSLs. We similarly obtain the chiral Yao-Kivelson model, the honeycomb toric code, and a new spin-1 quadrupolar Kitaev model. The last is shown to be in a QSL phase by a nonlocal mapping to the kagomé Ising antiferromagnet. We demonstrate various applications of our framework, including (a) an adiabatic deformation of the ruby lattice model to the exactly soluble kagomé dimer model, conclusively establishing the QSL phase in the former and (b) demystifying the dynamical protocol for measuring off-diagonal strings in the Rydberg implementation of the ruby lattice spin liquid. More generally, we find an intimate connection between Kitaev couplings and the repulsive interactions used for emergent dimer models. For instance, we show how a spin-$1/2$ $XXZ$ model on the ruby lattice encodes a Kitaev honeycomb model, providing a new route toward realizing the latter in cold-atom or solid-state systems.

Magnetism is one of the largest, most fundamental, and technologically most relevant fields of condensed-matter physics. Traditionally, two basic magnetic phases have been distinguished ferromagnetism and antiferromagnetism. The spin polarization in the electronic band structure reflecting the magnetization in ferromagnetic crystals underpins the broad range of time-reversal symmetry-breaking responses in this extensively explored and exploited type of magnets. By comparison, antiferromagnets have vanishing net magnetization. Recently, there have been observations of materials in which strong time-reversal symmetry-breaking responses and spin-polarization phenomena, typical of ferromagnets, are accompanied by antiparallel magnetic crystal order with vanishing net magnetization, typical of antiferromagnets. A classification and description based on spin-symmetry principles offers a resolution of this apparent contradiction by establishing a third distinct magnetic phase, dubbed altermagnetism. Our perspective starts with an overview of the still emerging unique phenomenology of this unconventional $d$-wave (or higher even-parity wave) magnetic phase, and of the wide array of altermagnetic material candidates. We illustrate how altermagnetism can enrich our understanding of overarching condensed-matter physics concepts and how it can have impact on prominent condensed-matter research areas.

We report magnetometry data obtained on twin-free single crystals of ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$, which is considered a candidate material for realizing the Kitaev honeycomb model for quantum spin liquids. Contrary to a common belief that such materials can be modeled with the symmetries of an ideal honeycomb lattice, our data reveal a pronounced twofold symmetry and in-plane anisotropy of over 200%, despite the honeycomb layer’s tiny orthorhombic distortion of less than 0.2%. We further use magnetic neutron diffraction to elucidate a rich variety of field-induced phases observed in the magnetometry. These phases manifest themselves in the paramagnetic state as diffuse scattering signals associated with competing ferromagnetic and antiferromagnetic instabilities, consistent with a theory that also predicts a quantum spin liquid phase nearby. Our results call for theoretical understanding of the observed in-plane anisotropy and render ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ a promising ground for finding exotic quantum phases by targeted external tuning.

Colloidal gels are an important class of materials with mesoscale building blocks, and they have wide-ranging applications, from water purification to cement to biotechnology. However, the formation of colloidal gels is beset by inadequate control over phase behavior and slow aging kinetics. In this work, we report on experiments that examine structure, structural relaxation, and dynamics in colloid-polymer suspensions, with fine, tunable control: the concentration of nonadsorbing polymer controls the strength of a depletion attraction, and an external electric field induces dipolar interactions that are instantly switchable and tunable in strength. With these switchable interactions, we have studied the “dipolar-depletion” phase diagram in real space via fluorescence confocal laser scanning microscopy. At lower polymer concentrations, combining depletion with dipolar interactions lowers the field threshold for observing ordered sheetlike dipolar structures. At intermediate polymer concentrations, depletion-induced clusters suppress field-induced ordering. At high depletion strengths, we can create partially ordered gel states. We have quantitatively characterized the transition from reversible to irreversible structures and used the cycling of the external field to accelerate aging in a gel-forming system. For processes that take months or years to study, such as the collapse of certain gels, such accelerated aging would prove extremely useful.

Epitaxial superconductor-semiconductor heterostructures combine superconductivity with strong spin-orbit interaction resulting in synthetic Rashba superconductors. The theoretical description of such superconductors involves Lifshitz invariants that are predicted to feature numerous exotic effects with so far sparse experimental evidence. Using a new observable—vortex inductance—we investigate the pinning properties of epitaxial $\mathrm{Al}/\mathrm{InAs}$-based heterostructures. We find a pronounced decrease of the vortex inductance with increasing in-plane field which corresponds to a counterintuitive increase of the pinning force. When rotating the in-plane component of the field with respect to the current direction, the pinning interaction turns out to be highly anisotropic. We analytically demonstrate that both the pinning enhancement and its anisotropy are consequences of the presence of Lifshitz invariant terms in the Ginzburg-Landau free energy. Hence, our experiment provides access to a fundamental property of Rashba superconductors and offers an entirely new approach to vortex manipulation.

Phononic frequency combs have attracted increasing attention both as a qualitatively new type of nonlinear phenomena in vibrational systems and from the point of view of applications. It is commonly believed that at least two modes must be involved in generating a comb. We demonstrate that a comb can be generated by a single nanomechanical mode driven by a resonant monochromatic drive. The comb emerges where the drive is still weak, so the anharmonic part of the mode potential energy remains small. We relate the experimental observation to a negative nonlinear friction induced by the resonant drive, which makes the vibrations at the drive frequency unstable. We directly map the measured trajectories of the emerging oscillations in the rotating frame and show how these oscillations lead to the frequency comb in the laboratory frame. The results go beyond nanomechanics and suggest a qualitatively new approach to generating tunable frequency combs in single-mode vibrational systems. They demonstrate new sides of the interplay of conservative and dissipative nonlinearities in driven systems.

We use tensor network techniques to obtain high-order perturbative diagrammatic expansions for the quantum many-body problem at very high precision. The approach is based on a tensor train parsimonious representation of the sum of all Feynman diagrams, obtained in a controlled and accurate way with the tensor cross interpolation algorithm. It yields the full time evolution of physical quantities in the presence of any arbitrary time-dependent interaction. Our benchmarks on the Anderson quantum impurity problem, within the real-time nonequilibrium Schwinger-Keldysh formalism, demonstrate that this technique supersedes diagrammatic quantum Monte Carlo by orders of magnitude in precision and speed, with convergence rates $1/N^2$ or faster, where $N$ is the number of function evaluations. The method also works in parameter regimes characterized by strongly oscillatory integrals in high dimension, which suffer from a catastrophic sign problem in quantum Monte Carlo calculations. Finally, we also present two exploratory studies showing that the technique generalizes to more complex situations: a double quantum dot and a single impurity embedded in a two-dimensional lattice.

Many sought-after exotic states of matter are known to emerge close to quantum phase transitions, such as quantum spin liquids and unconventional superconductivity. It is, thus, desirable to experimentally explore systems that can be continuously tuned across these transitions. Here, we demonstrate such tunability and the electronic correlation effects in triangular moiré superlattices formed between trilayer ${\mathrm{MoTe}}_2$ and monolayer ${\mathrm{WSe}}_2$ ($3\mathrm{L}\text{−}{\mathrm{MoTe}}_2/{\mathrm{WSe}}_2$). Through transport measurements, we observe an electronic analog of the Pomeranchuk effect at half filling of the first moiré subband, where increasing temperature paradoxically enhances charge localization. At low temperatures the system exhibits the characteristic of a Fermi liquid with strongly renormalized effective mass, suggesting a correlated metal state. The state is highly susceptible to out-of-plane electric and magnetic fields, which induce a Lifshitz transition and a metal-insulator transition (MIT), respectively. It enables identification of a tricritical point in the quantum phase diagram at the base temperature. We explain the Lifshitz transition in terms of interlayer charge transfer under the vertical electric field, which leads to the emergence of a new Fermi surface and immediate suppression of the Pomeranchuk effect. The existence of quantum criticality in the magnetic-field-induced MIT is supported by scaling behaviors of the resistance. Our work shows the $3\mathrm{L}\text{−}{\mathrm{MoTe}}_2/{\mathrm{WSe}}_2$ lies in the vicinity to the MIT point of the triangular lattice Hubbard model, rendering it a unique system to manifest the rich correlation effects at an intermediate interaction strength.