Pressure evolution of the crystal structure and magnetism of the honeycomb $\alpha \text{−}{\mathrm{RuBr}}_3$ is studied using high-pressure x-ray diffraction, magnetometry, and density-functional band-structure calculations. Hydrostatic compression transforms antiferromagnetic $\alpha \text{−}{\mathrm{RuBr}}_3$ $\left(R\overline{3}\right)$ into paramagnetic ${\alpha }^{\prime }\text{−}{\mathrm{RuBr}}_3$ $\left(P\overline{1}\right)$ where short Ru-Ru bonds cause magnetism collapse above 1.3 GPa at 0 K and 2.5 GPa at 295 K. Below this critical pressure, the Néel temperature of $\alpha \text{−}{\mathrm{RuBr}}_3$ increases with the slope of 1.8 K/GPa. Pressure tunes $\alpha \text{−}{\mathrm{RuBr}}_3$ away from the Kitaev limit, whereas increased third-neighbor in-plane coupling and interlayer coupling lead to a further stabilization of the collinear zigzag state. Both $\alpha$- and ${\alpha }^{\prime }\text{−}{\mathrm{RuBr}}_3$ are metastable at ambient pressure, but their transformation into the thermodynamically stable $\beta$ polymorph is kinetically hindered at room temperature.

Recent x-ray spectroscopic studies have revealed spin excitations and charge density waves in thin films of infinite-layer (IL) nickelates. However, clarifying whether the origin of these phenomena is intrinsic to the material class or attributable to impurity phases in the films has presented a major challenge. Here we utilize topotactic methods to synthesize bulk crystals of the IL nickelate ${\mathrm{LaNiO}}_2$ with crystallographically oriented surfaces. We examine these crystals using resonant inelastic x-ray scattering (RIXS) at the Ni $L_3$-edge to elucidate the spin and charge correlations in the bulk of the material. While we detect the presence of prominent spin excitations in the crystals, fingerprints of charge order are absent at the ordering vectors identified in previous thin-film studies. These results contribute to the understanding of the bulk properties of ${\mathrm{LaNiO}}_2$ and establish topotactically synthesized crystals as viable complementary specimens for spectroscopic investigations.

We study numerically Anderson localization on lattices that are tree-like except for the presence of one loop of varying length $L$. The resulting expressions allow us to compute corrections to the Bethe lattice solution on (i) random-regular-graph (RRG) of finite-size $N$ and (ii) Euclidean lattices in finite dimension. In the first case we show that the prefactor of the $1/N$ corrections to the average values of the typical density of states diverges exponentially approaching the critical point. In the second case our results, combined with the $M$-layer expansion, predict that corrections destroy the exotic critical behavior of the Bethe lattice solution in any finite dimension, strengthening the suggestion that the upper critical dimension of Anderson localization is infinity. Our approach explains the puzzling observation that the numerical simulations on finite RRGs deviate spectacularly from the expected asymptotic behavior, and opens the way to the computation of non-mean-field critical exponents by resumming the series of diverging diagrams through the same recipes of the field-theoretical perturbative expansion.

Coherent pumping of magnons by light in an optomagnonic cavity has been recently reported. Here we propose and model the Bose-Einstein condensation of magnons pumped by a coherent pumping source in an optical cavity composed of a ferromagnetic waveguide. To stabilize the condensation, a large number of $\pm k$-magnon pairs should be pumped into the cavity. We calculate and analyze the methods of direct pumping and indirect pumping by multiple confined optical modes. The results show that the condensation can be achieved only by the direct pumping with threshold powers less than 1 watt.

Cavity quantum electrodynamics (QED) studies the interaction between light and matter at the single quantum level and has played a central role in quantum science and technology. Combining the idea of cavity QED with moiré materials, we theoretically show that strong quantum light-matter interaction provides a way to control frustrated magnetism. Specifically, we develop a theory of moiré materials confined in a cavity consisting of thin polar van der Waals crystals. We show that nontrivial quantum geometry of moiré flat bands leads to electromagnetic vacuum dressing of electrons, which produces appreciable changes in single-electron energies and manifests as long-range electron hoppings. We apply our general formulation to a twisted transition metal dichalcogenide heterobilayer encapsulated by ultrathin hexagonal boron nitride layers and predict its phase diagram at different twist angles and light-matter coupling strengths. Our results indicate that the cavity confinement enables one to control magnetic frustration of moiré materials and might allow for realizing various exotic phases such as a quantum spin liquid.

We construct a one-dimensional (1D) topological SSH-like model with chiral symmetry and a superimposed hopping modulation, which we call the chiral Aubry-André model. We show that its topological properties can be described in terms of a pair $(C,W)$ of a two-dimensional (2D) Chern number $C$, stemming from a superspace description of the model, and a 1D winding number $W$, originating in its chiral symmetric nature. Thus, we showcase the explicit coexistence of 1D and 2D topology in a model composed of a single 1D chain. We detail the superspace description by showcasing how our model can be mapped to a Harper-Hofstadter model, familiar from the description of the integer quantum Hall effect, and analyze the vanishing field limit analytically. An extension of the method used for vanishing fields is provided in order to handle any finite fields, corresponding to hopping modulations both commensurate and incommensurate with the lattice. In addition, this formalism allows us to obtain certain features of the 2D superspace model, such as its number of massless Dirac nodes, purely in terms of topological quantities, computed without the need to go into momentum space.

Ultrafast transverse magneto-optical Kerr effect (T-MOKE) spectroscopy in the extreme ultraviolet spectral range provides element-specific information about the magnetization dynamics of complex magnetic structures. However, the relationship between the T-MOKE observable, denoted magnetic asymmetry, and the magnetization of a sample can exhibit significant nonlinearities, even in the case of magnetization changes that are homogeneous along the depth of the sample and without considering any nonequilibrium spin dynamics. Here, we combine static and time-resolved experimental data with simulations based on a wave propagation algorithm for a prototypical magnetic heterostructure that exhibits pronounced deviations from a linear relationship between magnetic asymmetry and magnetization, including increasing values of asymmetry in spite of a reducing magnetization. As an outlook, we describe sample structures and experimental geometries for which a linear response of the T-MOKE observable remains a valid approximation.

Accurately characterizing magnetic resonance of molecules at zero to ultralow magnetic field (nTs-µTs) is challenging, due to vanishingly small sensitivity, which depends on the thermal equilibrium polarization of the nuclear spins and instrumentation. We overcome the former limitation with the parahydrogen-based hyperpolarization method SABRE-SHEATH (signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei). This method allows for the continuous transfer of spin order from parahydrogen to a substrate via chemical exchange, reaching polarization levels of some percent (level equivalent to ${}^{13}\mathrm{C}$ polarization at 20 kT). We address the latter with our application of a superconducting quantum interference device (SQUID)-based detector setup that allows for broadband detection (dc-MHz) with exquisite sensitivity over its entire range. Here, we present the results of our comprehensive characterization of $\left[1{\text{−}}^{13}\mathrm{C}\right]\mathrm{pyruvate}$ and $\left[2{\text{−}}^{13}\mathrm{C}\right]\mathrm{pyruvate}$, hyperpolarized via SABRE-SHEATH, from zero field to 100 µT. To this end, we show low-noise, high-resolution spectra for both molecules, detecting how the NMR spectrum changes from the J-coupling dominated zero-field spectrum to the strongly coupled spectrum, and then finally to the conventional high-field, otherwise known Zeeman-dominated spectrum. We also analytically derive the evolution of product operators in arbitrary magnetic fields, which aid in the understanding of the differences between spin evolution and spin-coupling regimes. We predict and confirm that the absence of spin precession at zero field can result in observable oscillation of magnetization along one axis with a frequency of the J-coupling constant, no observable spin evolution, or observing spin evolution that corresponds to “forbidden” transitions at high field. The zero-field spectra with their near-dc signals reveal different relaxation rates for the different spin states of hyperpolarized ${}^{13}\mathrm{C}$ pyruvates, demonstrating the utility of SQUID detectors and hyperpolarization for the characterization of magnetic resonance phenomena.

By strongly driving a transmon-resonator system, the transmon qubit may eventually escape from its cosine-shaped potential. This process is called transmon ionization (TI) and known to be detrimental to the qubit coherence and operation. In this work we investigate the onset of TI in an irreversible, parametrically driven, frequency conversion process in a system consisting of a superconducting three-dimensional cavity coupled to a fixed-frequency transmon qubit. Above a critical pump power we find a sudden increase in the transmon population. Using Rényi entropy, Floquet modes, and Husimi Q functions, we infer that this abrupt change can be attributed to a quantum-to-classical phase transition. Furthermore, in the context of the single-photon detection, we measure a TI-uncorrected detection efficiency of up to 86% and estimate a TI-corrected value of up to $78\%$ by exploiting the irreversible frequency conversion. Our numerical simulations suggest that increasing the detuning between the pump and qubit frequencies and increasing the qubit anharmonicity can suppress the TI impact. Our findings highlight the general importance of the TI process when operating coupled qubit-cavity systems.

Exciton polaritons are hybrid particles of excitons (bound electron-hole pairs) and cavity photons, which are renowned for displaying Bose-Einstein condensation and other coherent phenomena at elevated temperatures. However, their formation in semiconductor microcavities is often accompanied by the appearance of an incoherent bath of optically dark excitonic states that can interact with polaritons via their matter component. Here we show that the presence of such a dark excitonic medium can dress polaritons with density fluctuations to form coherent polaronlike quasiparticles, thus fundamentally modifying their character. We employ a many-body Green's function approach that naturally incorporates correlations beyond the standard mean-field theories applied to this system. With increasing exciton density, we find a reduction in the light-matter coupling that arises from the polaronic dressing cloud rather than any saturation induced by the fermionic constituents of the exciton. In particular, we observe the strongest effects when the spin of the polaritons is opposite that of the excitonic medium. In this case, the coupling to light generates an additional polaron quasiparticle—the biexciton polariton—which emerges due to the dark-exciton counterpart of a polariton Feshbach resonance. Our results can explain recent experiments on polariton interactions in two-dimensional semiconductors and potentially provide a route to tailoring the properties of exciton polaritons and their correlations.

We present an atomically precise technique to create sulfur vacancies and control their atomic configurations in single-layer ${\mathrm{MoS}}_2$. It involves adsorbed Fe atoms and the tip of a scanning tunneling microscope, which enables single sulfur removal from the top sulfur layer at the initial position of Fe. Using scanning tunneling spectroscopy, we show that the STM tip can also induce two Jahn-Teller distorted states with reduced orbital symmetry in the sulfur vacancies. Density functional theory calculations rationalize our experimental results. Additionally, we provide evidence for molecule-like hybrid orbitals in artificially created sulfur vacancy dimers, which illustrates the potential of our technique for the development of extended defect lattices and tailored electronic band structures.

The use of cavity quantum electrodynamical effects, i.e., of vacuum electromagnetic fields, to modify material properties in cavities has rapidly gained popularity and interest in the last few years. However, there is still a scarcity of general results that provide guidelines for an intuitive understanding and limitations of what kind of effects can be achieved. We provide such a result for the effective interactions between low-energy matter excitations induced either directly by their mutual coupling to the cavity electromagnetic (EM) field or indirectly through coupling to mediator modes that couple to the EM field. We demonstrate that the induced interactions are purely electrostatic in nature and are thus fully described by the EM Green's function evaluated at zero frequency. Our findings imply that reduced models with one or a few cavity modes can easily give misleading results.

We investigated the magnetic helicity of the triple-$\mathit{q}$ magnetic structure of the triangular skyrmion lattice in the A phase of EuPtSi for a magnetic field along the [111] axis by resonant x-ray diffraction using a circularly polarized beam. We show that all three Fourier components of the triple-$\mathit{q}$ structure are perpendicular to the respective $\mathit{q}$ vectors and have the same helicity. They are connected by the rotation operations about the [111] axis. The helicity is the same as that of the single-$\mathit{q}$ helimagnetic phase at low fields, suggesting that the antisymmetric exchange interaction inherent in the chiral structure supports the formation of the triangular skyrmion lattice. We also observe that the helical plane in the helimagnetic phase is tilted to the magnetic field to form a conical structure before the first-order transition to the skyrmion lattice phase.

This paper presents the first sound speed and Grüneisen parameter data for fluid iron compressed to 3 TPa (30 million atmospheres) and 20 $\mathrm{g}/{\mathrm{cm}}^3$ on the Hugoniot. Both the sound speed and Grüneisen parameter are derivatives of the equation of state (EOS) and thus tightly constrain the contours of the EOS surface. The sound speed data are systematically lower than expected from a simple extrapolation of previous data. The Grüneisen parameter shows a 30% drop at pressures and temperatures above the melt transition. Furthermore, while some models compare well with either the sound speed or Grüneisen parameter, none of today's state-of-the-art models can explain both sets of data. Thus these new data will provide pivotal benchmarks for both future theoretical EOSs of warm dense iron and modeling planetary states and processes.

The position operator in a Bloch representation acquires a gauge correction in the momentum space on top of the canonical position, which is called the anomalous position. We show that the anomalous position is generally orbital dependent and thus plays a crucial role in the description of the intrinsic orbital Hall effect in terms of Wannier basis. We demonstrate this from the first-principles calculation of orbital Hall conductivities of transition metals by Wannier interpolation. Our results show that consistent treatment of the velocity operator by including the correction term originating from the anomalous position predicts the orbital Hall conductivities different from those obtained by considering only the group velocity. We find the difference is crucial in several metals. For example, we predict negative signs of the orbital Hall conductivities for elements in groups X and XI such as Cu, Ag, Au, and Pd, for which the previous studies predicted positive signs. In this paper, we suggest the importance of consistently describing the spatial dependence of basis functions by first-principles methods, as it is fundamentally missing in the tight-binding approximation.

The interplay between quantum chaos and integrability has been extensively studied in the past decades. We approach this topic from the point of view of geometry encoded in the quantum geometric tensor, which describes the complexity of adiabatic transformations. In particular, we consider two generic models of spin chains that are parameterized by two independent couplings. In one, the integrability breaking perturbation is global while, in the other, integrability is broken only at the boundary. In both cases, the shortest paths in the coupling space lead towards integrable regions and we argue that this behavior is generic. These regions thus act as attractors of adiabatic flows similar to river basins in nature. Physically, the directions towards integrable regions are characterized by faster relaxation dynamics than those parallel to integrability, and the anisotropy between them diverges in the thermodynamic limit as the system approaches the integrable point. We also provide evidence that the transition from integrable to chaotic behavior is universal for both models, similar to continuous phase transitions, and that the model with local integrability breaking quickly becomes chaotic but avoids ergodicity.

Competing magnetic interactions, frustration-driven quantum fluctuations, and spin correlations offer an ideal route for the experimental realization of emergent quantum phenomena with exotic quasiparticle excitations in three-dimensional frustrated magnets. In this context, trillium lattice, wherein magnetic ions decorate a three-dimensional chiral network of corner-shared equilateral triangular motifs, provides a viable ground. Herein, we present the crystal structure, dc and ac magnetic susceptibilities, specific heat, electron spin-resonance (ESR), muon spin-relaxation ($\mu \mathrm{SR}$) results on the polycrystalline samples of ${\mathrm{K}}_2\mathrm{CrTi}{\left({\mathrm{PO}}_4\right)}_3$ wherein the ${\mathrm{Cr}}^{3+}$ ions form a two-coupled trillium lattice. The Curie-Weiss fit of the magnetic susceptibility data above 100 K yields a Curie-Weiss temperature ${\theta }_{\mathrm{CW}}$ = $-23$ K, which indicates the presence of dominant antiferromagnetic interactions between $S=3/2$ moments of ${\mathrm{Cr}}^{3+}$ ions. For temperatures below 40 K, the Curie-Weiss temperature is reduced to ${\theta }_{\mathrm{CW}}$ = $-3.5$ K, indicative of the appearance of subdominant ferromagnetic interactions. The specific heat measurements reveal the occurrence of two consecutive phase transitions, at temperatures $T_{\mathrm{L}}=4.3$ K and $T_{\mathrm{H}}=8$ K, corresponding to two different magnetic phases. Additionally, it unveils the existence of short-range spin correlations above the ordering temperature $T_{\mathrm{H}}$. The power-law behavior of ESR linewidth suggests the persistence of short-range spin correlations over a relatively wide critical region $(T-T_{\mathrm{H}})/T_{\mathrm{H}}>0.25$ in agreement with the specific heat results. The $\mu \mathrm{SR}$ results provide concrete evidence of two different phases corresponding to two transitions, coupled with the critical slowing down of spin fluctuations above $T_{\mathrm{L}}$ and persistent spin dynamics below $T_{\mathrm{L}}$, consistent with the thermodynamic results. Moreover, the $\mu \mathrm{SR}$ results reveal the coexistence of static and dynamic local magnetic fields below $T_{\mathrm{L}}$, signifying the presence of complex magnetic phases owing to the entwining of spin correlations and competing magnetic interactions in this three-dimensional frustrated magnet.

Nitrogen vacancy (NV) centers in diamond have useful properties for detecting both ac and dc magnetic fields with high sensitivity at nanoscale resolution. The vector detection of ac magnetic fields can be achieved by using NV centers having three different orientations. Here, we propose a method to achieve this by using NV centers of single orientation. In this method, a static magnetic field is applied perpendicular to the NV axis, leading to strong mixing of the $m_s=-1$ and 1 electron spin states. As a result, all three electron spin transitions of the triplet ground state have nonzero dipole moments, with each transition coupling to a single component of the magnetic field. This can be used to measure both the strength and orientation of the applied ac field. To validate the technique, we perform a proof-of-principle experiment using a subset of ensemble NV centers in diamond, all having the same orientation. This method is equally applicable to single NV centers.

We study a Su-Schrieffer-Heeger electron-phonon model on a square lattice by means of auxiliary-field quantum Monte Carlo simulations. The addition of a symmetry-allowed interaction permits analytical integration over the phonons at the expense of discrete Hubbard-Stratonovich fields with imaginary-time correlations. Using single-spin-flip and global updates, we investigate the phase diagram at the O(4)-symmetric point as a function of hopping $t$ and phonon frequency ${\omega }_0$. For $t=0$, where electron hopping is boson assisted, the model maps onto an unconstrained ${\mathbb{Z}}_2$ gauge theory. A key quantity is the emergent effective flux per plaquette, which equals $\pi$ in the assisted-hopping regime and vanishes for large $t$. Phases in the former regime can be understood in terms of instabilities of emergent Dirac fermions. Our results support a direct and continuous transition between a $(\pi ,0)$ valence bond solid (VBS) and an antiferromagnetic (AFM) phase with increasing ${\omega }_0$. For large $t$ and small ${\omega }_0$, we find finite-temperature signatures, a disordered pseudogap phase, of a previously reported $(\pi ,\pi )$ VBS ground state related to a nesting instability. With increasing ${\omega }_0$, AFM order again emerges.

Motivated by the recent discovery of magnetism and heavy quasiparticles in the van der Waals material CeSiI, we develop an effective model that incorporates the conduction electrons residing at the silicene layer interacting with the local moments of the Ce ions. Ce sites are arranged on two layers of triangular lattices, above and below the silicene layer, and they are located at the center of the honeycomb lattice. This arrangement results in an effective extended Kondo interaction along with a predominant ferromagnetic Ruderman-Kittel-Kasuya-Yosida interaction. Via the mean-field theory of Abrikosov fermions, our analysis indicates that the ground state of the monolayer can exhibit a nonmagnetic nematic heavy fermion phase that breaks $C_6$ rotational symmetry for a small Heisenberg exchange and a magnetically ordered phase for a large Heisenberg exchange. For intermediate values, a coexistence of magnetic order and a uniform heavy Fermi liquid is stabilized where they reside on separate Ce layers. We show that this phase can further be enhanced by an external electric field. Our results provide a natural mechanism for the coexistence of magnetic order and heavy fermions in CeSiI and highlight the possibility of unconventional nonmagnetic heavy fermions with broken rotational symmetry.