It has been experimentally established that the occurrence of charge density waves is a common feature of various underdoped cuprate superconducting compounds. The observed states, which are often found in the form of bond density waves (BDWs), usually occur in a temperature regime immediately above the superconducting transition temperature. Motivated by recent optical experiments on superconducting materials, where it has been shown that optical irradiation can transiently improve the superconducting features, here we propose an approach for the enhancement of superconductivity by the targeted destruction of the BDW order which we expect to be more efficient than the previous methods. Since BDW states are usually found in competition with superconductivity, suppression of the BDW order enhances the tendency of electrons to form Cooper pairs after reaching a steady state. By investigating the optical coupling of gapless, collective fluctuations of the BDW modes, we argue that the resonant excitation of these modes can melt the underlying BDW order parameter. We propose an experimental setup to implement such an optical coupling using two-dimensional plasmon-polariton hybrid systems.

Quantum spin fluctuations provide a unique way to study spin dynamics without system perturbation. Here we put forward an optical resonance shift spin noise spectroscopy as a powerful tool to measure the spin noise of various systems from magnetic impurities in solids to free atoms and molecules. The quantum spin fluctuations in these systems can shift the optical resonances by more than the homogeneous linewidth and produce huge Faraday rotation noise. We demonstrate that the resonance shift spin noise spectroscopy gives access to the high-order spin correlators, which contain complete information about the spin dynamics in contrast with the second-order correlator measured by conventional Pauli-blocking spin noise spectroscopy. The high-order quantum spin correlators manifest themselves as a comb of peaks in the Faraday rotation noise spectra in a transverse magnetic field. This effect is closely related to the multispin flip Raman scattering observed in the Mn-doped nanostructures.

We develop a systematic approach for constructing symmetry-based indicators of a topological classification for superconducting systems. The topological invariants constructed in this work form a complete set of symmetry-based indicators that can be computed from knowledge of the Bogoliubov–de Gennes Hamiltonian on high-symmetry points in the Brillouin zone. After excluding topological invariants corresponding to the phases without boundary signatures, we arrive at a natural generalization of symmetry-based indicators [H. C. Po, A. Vishwanath, and H. Watanabe, Nat. Commun. 8, 50 (2017)] to Hamiltonians of Bogoliubov–de Gennes type.

Continuous quantum phase transition involving all-in–all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled $5d$ compounds could give rise to various exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally trace the AIAO spin order in $\mathrm{S}{\mathrm{m}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ using direct resonant x-ray magnetic diffraction techniques under high pressure. The magnetic order is suppressed at a critical pressure $P_{\mathrm{c}}=6.30\mathrm{GPa}$, while the lattice symmetry remains in the cubic $Fd\text{−}3m$ space group across the quantum critical point. Comparing pressure tuning and the chemical series $R_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ reveals that the approach to the AIAO quantum phase transition is characterized by contrasting evolutions of the pyrochlore lattice constant $a$ and the trigonal distortion surrounding individual Ir moments, which affects the $5d$ bandwidth and the Ising anisotropy, respectively. We posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, the observed low pressure scale of the AIAO quantum phase transition in $\mathrm{S}{\mathrm{m}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ identifies a circumscribed region of P-T space for investigating the putative magnetic Weyl semimetal state.

We construct and solve a two-dimensional, chirally symmetric model of Dirac cones subjected to a quasiperiodic modulation. In real space, this is realized with a quasiperiodic hopping term. This hopping model, as we show, at the Dirac node energy has a rich phase diagram with a semimetal-to-metal phase transition at intermediate amplitude of the quasiperiodic modulation, and a transition to a phase with a diverging density of states (DOS) and subdiffusive transport when the quasiperiodic hopping is strongest. We further demonstrate that the semimetal-to-metal phase transition can be characterized by the multifractal structure of eigenstates in momentum space and can be considered as a unique “unfreezing” transition. This unfreezing transition in momentum space generates flat bands with a dramatically renormalized bandwidth in the metallic phase similar to the phenomena of the band structure of twisted bilayer graphene at the magic angle. We characterize the nature of this transition numerically as well as analytically in terms of the formation of a band of topological zero modes. For pure quasiperiodic hopping, we provide strong numerical evidence that the low-energy DOS develops a divergence and the eigenstates exhibit Chalker (quantum-critical) scaling despite the model not being random. At particular commensurate limits the model realizes higher-order topological insulating phases. We discuss how these systems can be realized in experiments on ultracold atoms and metamaterials.

We study the spin dynamics in a high-mobility two-dimensional electron gas confined in a GaAs/AlGaAs quantum well. An unusual magnetic field dependence of the spin relaxation is found: As the magnetic field becomes stronger, the spin relaxation time first increases quadratically but then changes to a linear dependence before it eventually becomes oscillatory, whereby the longitudinal and transverse times reach maximal values at even and odd filling Landau level factors, respectively. We show that the suppression of spin relaxation is due to the effect of electron gyration on the spin-orbit field, while the oscillations correspond to oscillations of the density of states appearing at low temperatures and high magnetic fields. The transition from quadratic to linear dependence can be related to a transition from classical to Bohm diffusion and reflects an anomalous behavior of the two-dimensional electron gas analogous to that observed in magnetized plasmas.

The Jaynes-Cummings (JC) model represents one of the simplest ways in which single qubits can interact with single photon modes, leading to profound quantum phenomena like superpositions of light and matter states. One system, that can be described with the JC model, is a single quantum dot embedded in a micropillar cavity. In this joint experimental and theoretical study we investigate such a system using four-wave mixing (FWM) microspectroscopy. Special emphasis is laid on the dependence of the FWM signals on the number of photons injected into the microcavity. By comparing simulation and experiment, which are in excellent agreement with each other, we infer that up to $\sim 20$ photons take part in the observed FWM dynamics. Thus we verify the validity of the JC model for the system under consideration in this nontrivial regime. We find that the inevitable coupling between the quantum dot exciton and longitudinal acoustic phonons of the host lattice influences the real time FWM dynamics and has to be taken into account for a sufficient description of the quantum-dot–microcavity system. Performing additional simulations in an idealized dissipationless regime, we observe that the FWM signal exhibits quasiperiodic dynamics, analog to the collapse and revival phenomenon of the JC model. In these simulations we also see that the FWM spectrum has a triplet structure, if a large number of photons is injected into the cavity.

We have performed an extensive pressure-dependent structural, spectroscopic, and electrical transport study of ${\mathrm{LaCrSb}}_3$. The ferromagnetic phase ($T_C=120$ K at p = 0 GPa) is fully suppressed by p = 26.5 GPa and the Cr moment decreases steadily with increasing pressure. The unit-cell volume decreases smoothly up to p = 55 GPa. We find that the bulk modulus and suppression of the magnetism are in good agreement with theoretical predictions, but the Cr moment decreases smoothly with pressure, in contrast to steplike drops predicted by theory. The ferromagnetic ordering temperature appears to be driven by the Cr moment.

External fields are a powerful tool to probe optical excitations in a material. The linear energy shift of an excitation in a magnetic field is quantified by its effective $g$ factor. Here we show how exciton $g$ factors and their sign can be determined by converged first-principles calculations. We apply the method to monolayer excitons in semiconducting transition metal dichalcogenides and to interlayer excitons in ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayers and obtain good agreement with recent experimental data. The precision of our method allows us to assign measured $g$ factors of optical peaks to specific transitions in the band structure and also to specific regions of the samples. This revealed the nature of various, previously measured interlayer exciton peaks. We further show that, due to specific optical selection rules, $g$ factors in van der Waals heterostructures are strongly spin- and stacking-dependent. The calculation of orbital angular momenta requires the summation over hundreds of bands, indicating that for the considered two-dimensional materials the basis set size is a critical numerical issue. The presented approach can potentially be applied to a wide variety of semiconductors.

Precision measurements of optical phases have many applications in science and technology. Entangled multiphoton states have been suggested for performing such measurements with precision that significantly surpasses the shot-noise limit. Until recently, such states have been generated mainly using spontaneous parametric down-conversion—a process which is intrinsically probabilistic, counteracting the advantages that the entangled photon states might have. Here we use a semiconductor quantum dot to generate entangled multiphoton states in a deterministic manner, using periodic timed excitation of a confined spin. This way we entangle photons one by one at a rate which exceeds 300 MHz. We use the resulting multiphoton state to demonstrate super-resolved optical phase measurement. Our results open up a scalable way for realizing genuine quantum enhanced supersensitive measurements in the near future.

Ultrasmall ferroelectrics with nontrivial topological field textures such as polar vortices, skyrmions, and merons hold promise in technological paradigms. Such nontrivial ferroic orders and their functionalities, however, inevitably disappear below a critical size of several nanometers. Here, we propose a strategy to overcome this limitation and design atomically small ferroelectrics with topological polarization vortices by engineering excess-electron polarons. Our first-principles calculations demonstrate that excess-electron polarons formed in antiferrodistortive $\mathrm{SrTi}{\mathrm{O}}_3$ induce localized ferroelectric polarization with a topological vortex form due to local symmetry breaking and demonstrate the possibility of an atomic-scale “ferrotoroidic” materials. We further show that the electron polaron carries a magnetic moment coupled with ferrotoroidicity, i.e., the magnetoelectric effect. We also discuss possible methods to switch the toroidal moment via the magnetoelectric effect. Our result, thus, provides insight into the ultimate miniaturization of ferrotoroic materials and a class of functional polaron families.

The decahedral cage is the theoretically established ground state of the hydrogen saturated ${\mathrm{Si}}_{20}{\mathrm{H}}_{20}$ fullerene. However it has never been observed experimentally. Based on an extensive exploration of the potential energy surface and by constructing theoretical reaction pathways from possible initial structures to the ground state of ${\mathrm{Si}}_{20}{\mathrm{H}}_{20}$, we show that there is no driving force towards the global minimum. There exists a huge number of intermediate structures that consist mainly of collapsed cages. Visiting all these intermediate states to find the ground state is not possible on experimentally relevant time scales. In this way the ground state becomes kinetically inaccessible. We contrast the features of the potential energy landscape of ${\mathrm{Si}}_{20}{\mathrm{H}}_{20}$ with that of ${\mathrm{C}}_{60}$ which spontaneously forms by condensation.

Controlling magnetism and spin structures in strongly correlated systems by using electric fields is of fundamental importance but challenging. Here, a high-spin ruthenate phase is achieved via a solid ionic chemical junction at the $\mathrm{SrRu}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface with distinct formation energies and diffusion barriers of oxygen vacancies, an analog to electronic band alignment in the semiconductor heterojunction. Oxygen vacancies trapped within this interfacial $\mathrm{SrRu}{\mathrm{O}}_3$ reconstruct the Ru-$4d$ electronic structure and orbital occupancy, leading to an enhanced magnetic moment. Furthermore, this emergent interfacial magnetic phase can be switched reversibly by electric-field-rectifying oxygen migration in a solid-state ionic gating device, providing a framework for the atomic design of functionalities in strongly correlated oxides using a method of solid chemistry.

We present a robust algorithm that computes (maximally localized) Wannier functions without the need of providing an initial guess. Instead, a suitable starting point is constructed automatically from so-called local orbitals which are fundamental building blocks of the basis set within (linearized) augmented plane-wave methods. Our approach is applied to a vast variety of materials such as (semi)metals, bulk and low-dimensional semiconductors, and complex inorganic-organic hybrid interfaces. For the interpolation of electronic single-particle energies, an accuracy in the meV range can be easily achieved. We exemplify the capabilities of our method by the calculation of the joint density of states in aluminum, (generalized) Kohn-Sham and quasiparticle band structures in various semiconductors, and the electronic structure of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, including electron and hole effective masses.

In the quantum Hall regime of graphene, antiferromagnetic and spin-polarized ferromagnetic states at the zeroth Landau level compete, leading to a canted antiferromagnetic state depending on the direction and magnitude of an applied magnetic field. Here, we investigate this transition at 2.7 K in graphene Hall bars that are proximity coupled to the ferrimagnetic insulator ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$. From nonlocal transport measurements, we demonstrate an induced magnetic exchange field in graphene, which lowers the magnetic field required to modulate the magnetic state in graphene. These results show that a magnetic proximity effect in graphene is an important ingredient for the development of two-dimensional materials in which it is desirable for ordered states of matter to be tunable with relatively small applied magnetic fields (>6 T).

Layered van der Waals (vdW) magnetic materials have attracted significant research interest to date. In this work, we employ the first-principles many-body perturbation theory to calculate excited-state properties of a prototype vdW magnet, chromium trichloride ($\mathrm{CrC}{\mathrm{l}}_3$), covering monolayer, bilayer, and bulk structures. Unlike usual nonmagnetic vdW semiconductors, in which many-electron interactions and excited states are sensitive to dimensionality, many-electron interactions are always enhanced and dominate quasiparticle energies and optical responses of both two-dimensional and bulk $\mathrm{CrC}{\mathrm{l}}_3$. The electron-hole (e-h) binding energy can reach 3 eV in monolayer and remains as high as 2 eV in bulk. Because of the cancellation effect between self-energy corrections and e-h binding energies, the lowest-energy exciton (“optical gap”) is almost not affected by the change of dimensionality. In addition, for the excitons with similar e-h binding energies, their dipole oscillator strength can differ by a few orders of magnitude. Our analysis shows that such a big difference is from a unique interference effect between complex exciton wave functions and interband transitions. Finally, we find that the interlayer stacking sequence and magnetic coupling barely change quasiparticle band gaps and optical absorption spectra of $\mathrm{CrC}{\mathrm{l}}_3$. Our calculated low-energy exciton peak positions agree with available measurements. These findings give insight into the understanding of many-electron interactions and the interplay between magnetic orders and optical excitations in vdW magnetic materials.

The Sachdev-Ye-Kitaev (SYK) model describes electrons with random and all-to-all interactions and realizes a many-body state without quasiparticle excitations and a nonvanishing extensive entropy $S_0$ in the zero-temperature limit. Its low-energy theory coincides with the low-energy theory of near-extremal charged black holes with Bekenstein-Hawking entropy $S_0$. Several mesoscopic experimental configurations realizing SYK quantum dynamics over a significant intermediate temperature scale have been proposed. We investigate quantum thermoelectric transport in such configurations and describe low-temperature crossovers out of SYK criticality into regimes either with Fermi liquid behavior, with a Coulomb blockade, or criticality associated with Schwarzian quantum gravity fluctuations. Our results show that thermopower measurements can serve as a direct probe for $S_0$.

Spins in solids are an ideal candidate to act as a memory and interface with superconducting qubits due to their long coherence times. We spectroscopically investigate erbium-167-doped yttrium orthosilicate as a possible microwave-addressed memory employing its microwave frequency transitions that occur without applying an external magnetic field. We obtain coherence times of 380 $\mu \mathrm{s}$ in a ground state spin transition and 1.48 ms in an excited state spin transition. This is 28 times longer compared to previous zero field measurements, as well as 200 times longer than a previous microwave memory demonstration in the same material. These long coherence times show that erbium-167-doped yttrium orthosilicate has potential as a microwave-addressed quantum memory.

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ as a putative U(1) QSL but a new class of ultrapure single crystals make it abundantly clear that the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for $\mathit{B}\parallel \langle 100\rangle$, while a field parallel to $\langle 110\rangle$ or $\langle 111\rangle$ enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of $T_{\mathrm{C}}$ and a reentrant phase boundary under applied magnetic fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.

We discuss the construction of low-energy tight-binding Hamiltonians for condensed-matter systems with a strong coupling to the quantum electromagnetic field. Such Hamiltonians can be obtained by projecting the continuum theory on a given set of Wannier orbitals. However, different representations of the continuum theory lead to different low-energy formulations because different representations may entangle light and matter, transforming orbitals into light-matter hybrid states before the projection. In particular, a multicenter Power-Zienau-Woolley transformation yields a dipolar Hamiltonian which incorporates the light-matter coupling via both Peierls phases and a polarization density. We compare this dipolar gauge Hamiltonian and the straightforward Coulomb gauge Hamiltonian for a one-dimensional solid to describe subcycle light-driven electronic motion in the semiclassical limit and a coupling of the solid to a quantized cavity mode which renormalizes the band-structure into electron-polariton bands. Both descriptions yield the same result when many bands are taken into account but the dipolar Hamiltonian is more accurate when the model is restricted to few electronic bands, while the Coulomb Hamiltonian requires fewer electromagnetic modes.