Exotic States Of Matter Research Articles

Toroidal anapole with point magnetic dipoles

The role of multipolar expansion of the localized current is significant in a broad range of disciplines in the physics of nowadays. It is actively used in such problems as the design of nanoantennas in photonics or the description of exotic states of matter. The toroidal multipoles are the third group of multipoles which complements the electric and magnetic multipolar families. The investigation of toroidal multipoles is important because of their role in the formation of so-called anapole states, non-radiating sources. The further study showed that there are also exist the higher-order toroidal dipoles or mean-square radii, which are equivalent. Here we suggest the structure which consists of point magnetic dipoles and supports the toroidal anapole state, which is obtained with the destructive interference of electric toroidal dipole with the first mean-square radius of the electric toroidal dipole. We also present an analytical condition for toroidal anapole with this system of point magnetic dipoles placed in two circles.

A magnetic continuum in the cobalt-based honeycomb magnet BaCo2(AsO4)2.

Quantum spin liquids (QSLs) are topologically ordered states of matter that host fractionalized excitations. A particular route towards a QSL is via strongly bond-dependent interactions on the hexagonal lattice. A number of Ru- and Ir-based candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions. Using time-domain terahertz spectroscopy, we observed a broad magnetic continuum over a wide range of temperatures and fields in the honeycomb cobalt-based magnet BaCo2(AsO4)2, which has been proposed to be a more ideal version of a Kitaev QSL. Applying an in-plane magnetic field of ~0.5 T suppresses the magnetic order, and at higher fields, applying the field gives rise to a spin-polarized state. Under a 4 T magnetic field that was oriented principally out of plane, a broad magnetic continuum was observed that may be consistent with a field-induced QSL. Our results indicate BaCo2(AsO4)2 is a promising QSL candidate.

Tuning Quantum Phase Transitions at Half Filling in 3L−MoTe2/WSe2 Moiré Superlattices

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 MoTe2 and monolayer WSe2 (3L−MoTe2/WSe2). 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 3L−MoTe2/WSe2 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.Received 27 February 2022Revised 10 August 2022Accepted 14 September 2022DOI:https://doi.org/10.1103/PhysRevX.12.041015Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCritical phenomenaDensity of statesElectrical conductivityFermi surfaceHall effectLandau levelsMagnetotransportPhase diagramsQuantum phase transitionsQuantum transportTransport phenomenaCondensed Matter, Materials & Applied Physics

Photoinduced band renormalization effects in the topological nodal-line semimetal ZrSiS

Out-of-equilibrium effects provide an elegant pathway for probing and understanding the underlying physics of topological materials. In particular, controlling electronic band structure properties using ultrafast optical pulses has shown promise for creating exotic states of matter. Of recent interest is band renormalization in Dirac and Weyl semimetals as it leads to direct physical observables through the enhancement of the effective mass or in the shift of resonant energies. Here we provide experimental and theoretical signatures of photoinduced renormalization of the electronic band structure in the topological nodal-line semimetal ZrSiS. Specifically, we show how the change in the transient reflectivity spectra under femtosecond optical excitations is induced by out-of-equilibrium effects that renormalize the kinetic energy of electrons. We associate the observed spectral features with an enhancement of the effective mass and to a redshift of the resonant frequency as a function of pump field strength. Finally, we show that the transient relaxation dynamics of the reflectivity is primarily an electronic effect with a negligible phononic contribution. Our study presents conclusive modifications of electronic properties in ZrSiS using ultrashort pulses and demonstrates the potential of this approach in creating photoinduced phases in topological quantum matter through an all-optical route.

Quantum spin liquids bootstrapped from Ising criticality in Rydberg arrays

Arrays of Rydberg atoms constitute a highly tunable, strongly interacting venue for the pursuit of exotic states of matter. We develop a new strategy for accessing a family of fractionalized phases known as quantum spin liquids in two-dimensional Rydberg arrays. We specifically use effective field theory methods to study arrays assembled from Rydberg chains tuned to an Ising phase transition that famously hosts emergent fermions propagating within each chain. This highly entangled starting point allows us to naturally access spin liquids familiar from Kitaev's honeycomb model, albeit from an entirely different framework. In particular, we argue that finite-range repulsive Rydberg interactions, which frustrate nearby symmetry-breaking orders, can enable coherent propagation of emergent fermions between the chains in which they were born. Delocalization of emergent fermions across the full two-dimensional Rydberg array yields a gapless Z2 spin liquid with a single massless Dirac cone. Here, the Rydberg occupation numbers exhibit universal power-law correlations that provide a straightforward experimental diagnostic of this phase. We further show that explicitly breaking symmetries perturbs the gapless spin liquid into gapped, topologically ordered descendants: Breaking lattice symmetries generates toric-code topological order, whereas introducing chirality generates non-Abelian Ising topological order. In the toric-code phase, we analytically construct microscopic incarnations of non-Abelian defects, which can be created and transported by dynamically controlling the atom positions in the array. Our work suggests that appropriately tuned Rydberg arrays provide a cold-atoms counterpart of solid-state 'Kitaev materials' and, more generally, spotlights a new angle for pursuing experimental platforms for Abelian and non-Abelian fractionalization.

Photon production rate calculation from quark-gluon plasma using MEQM in heavy-ion collision

It is widely accepted that a huge and intense magnetic field is created along with the exotic state of matter in heavy-ion collision. The photon yield from quark-gluon plasma created in a magnetic environment is studied using a thermal quasi-particle mass. The quarks acquire this mass depending on the temperature and nonzero value of quark chemical potential which gets modified as magnetized effective quark mass (MEQM) in the presence of magnetic field. The total photon rate is shown for one loop process incorporating MEQM. It is found that the emission rate to lowest order increases strongly with increasing of the quark chemical potential and temperature. The enhanced yield in photon is also compared with the recently reported theoretical results.

Dynamical study of the origin of the charge density wave in AV3Sb5 ( A=K , Rb, Cs) compounds

Systems containing the ideal kagome lattice can exhibit several distinct and novel exotic states of matter. One example of such systems is a recently discovered $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, and Cs) family of compounds. Here, the coexistence of the charge density wave (CDW) and superconductivity is observed. In this paper, we study the dynamic properties of the $A$V$_{3}$Sb$_{5}$ systems in context of origin of the CDW phase. We show and discuss the structural phase transition from $P6/mmm$ to $C2/m$ symmetry that are induced by the presence of phonon soft modes. We conclude that the CDW observed in this family of compounds is a consequence of the atom displacement, from the high symmetry position of the kagome net, in low-temperature phase. Additionally, using the numerical {\it ab initio} methods, we discuss the charge distribution on the $A$V$_{3}$Sb$_{5}$ surface. We show that the observed experimental %$4\times 1$ stripe-like modulation of the surface, can be related to surface reconstruction and manifestation of the three dimensional $2 \times 2 \times 2$ bulk CDW. Finally, the consequence of realization of the $C2/m$ structure on the electronic properties are discussed. We show that the electronic band structure reconstruction and the accompanying modification of density of states correspond well to the experimental data.

Simulating strongly interacting Hubbard chains with the variational Hamiltonian ansatz on a quantum computer

Hybrid quantum-classical algorithms have been proposed to circumvent noise limitations in quantum computers. Such algorithms delegate only a calculation of the expectation value to the quantum computer. Among them, the Variational Quantum Eigensolver (VQE) has been implemented to study molecules and condensed matter systems on small size quantum computers. Condensed matter systems described by the Hubbard model exhibit a rich phase diagram alongside exotic states of matter. In this manuscript, we try to answer the question: how much of the underlying physics of a 1D Hubbard chain is described by a problem-inspired Variational Hamiltonian Ansatz (VHA) in a broad range of parameter values ? We start by probing how much does the solution increases fidelity with increasing ansatz complexity. Our findings suggest that even low fidelity solutions capture energy and number of doubly occupied sites well, while spin-spin correlations are not well captured even when the solution is of high fidelity. Our powerful simulation platform allows us to incorporate a realistic noise model and shows a successful implementation of noise-mitigation strategies - post-selection and the Richardson extrapolation. Finally, we compare our results with an experimental realization of the algorithm on IBM Quantum's ibmq_quito device.

Magnetic order and spin liquid behavior in [Mo3]11+ molecular magnets

Molecular magnets with one unpaired electron per molecule offer promise in the ongoing search for exotic states of matter, including quantum spin liquids. Here, the authors investigate a series of molecular magnets based on Mo trimer building blocks with one unpaired electron each and find that the magnetic ground states are very sensitive to small changes in the breathing parameter---the ratio between first and second nearest neighbor bond lengths in the breathing kagome lattice that they form. When this parameter is sufficiently close to 1, these materials show an absence of magnetic order and other hallmarks of quantum spin liquid behavior, which may be correlated with plaquette charge ordering.

Quantum Hydrodynamics of Fractonic Superfluids with Lineon Condensate: From Navier–Stokes-Like Equations to Landau-Like Criterion

Fractonic superfluids are exotic states of matter with spontaneously broken higher-rank U(1) symmetry. The broken symmetry is associated with conserved quantities, including not only particle number (i.e., charge) but also higher moments, such as dipoles, quadrupoles, and angular moments. Owing to the presence of such conserved quantities, the mobility of particles is restricted either completely or partially. Here, we systematically study the hydrodynamical properties of fractonic superfluids, especially focusing on the fractonic superfluids with conserved angular moments. The constituent bosons are called “lineons” with d components in d-dimensional space. From the Euler–Lagrange equation, we derive the continuity equation and Navier–Stokes-like equations, in which the angular moment conservation introduces extra terms. Further, we discuss the current configurations related to the defects. Like the conventional superfluid, we study the critical values of velocity fields and density currents, which gives rise to a Landau-like criterion. Finally, several future directions are discussed.

Electric control of a canted-antiferromagnetic Chern insulator

The interplay between band topology and magnetism can give rise to exotic states of matter. For example, magnetically doped topological insulators can realize a Chern insulator that exhibits quantized Hall resistance at zero magnetic field. While prior works have focused on ferromagnetic systems, little is known about band topology and its manipulation in antiferromagnets. Here, we report that MnBi2Te4 is a rare platform for realizing a canted-antiferromagnetic (cAFM) Chern insulator with electrical control. We show that the Chern insulator state with Chern number C = 1 appears as the AFM to canted-AFM phase transition happens. The Chern insulator state is further confirmed by observing the unusual transition of the C = 1 state in the cAFM phase to the C = 2 orbital quantum Hall states in the magnetic field induced ferromagnetic phase. Near the cAFM-AFM phase boundary, we show that the dissipationless chiral edge transport can be toggled on and off by applying an electric field alone. We attribute this switching effect to the electrical field tuning of the exchange gap alignment between the top and bottom surfaces. Our work paves the way for future studies on topological cAFM spintronics and facilitates the development of proof-of-concept Chern insulator devices.

Spontaneous chiral-spin ordering in spin-orbit coupled honeycomb magnets

Frustrated magnets with highly degenerate ground states are at the heart of hunting exotic states of matter. Recent studies in spin-orbit coupled honeycomb magnets have generated immense interest in bond-dependent interactions, appreciating a symmetric off-diagonal $\Gamma$ interaction which exhibits a macroscopic degeneracy in the classical limit. Here, we study a generic spin model and discover a novel chiral-spin ordering with spontaneously broken time-reversal symmetry near the dominant $\Gamma$ region. The chiral-spin phase is demonstrated to possess a staggered chirality relation in different sublattices, and it exhibits gapless excitations as revealed by the vanishing energy gap and the finite central charge on cylinders. Although there is a vestige of a tiny peak in the corner of the second Brillouin zone, the magnetic order is likely to vanish as the system size increases. Finally, we also attempt to gain insight into the possible topological signature of the chiral-spin phase by calculating the dynamic structure factor and the modular $\mathcal{S}$ matrix.

Hairy black resonators and the AdS4 superradiant instability

The superradiant instability of Kerr-AdS black holes is studied by numerically solving the full 3+1 dimensional Einstein equations. We find that the superradiant instability results in a two stage process with distinct initial and secondary instabilities. At the end of the secondary instability the geometry oscillates at several distinct fundamental frequencies -- a multi-oscillating black hole. The multi-oscillating black hole is remarkably close to a black resonator, albeit with a bit of gravitational hair. During the hairy black resonator epoch, the evolution of the horizon area is consistent with the exponential approach to a constant. By employing different seed perturbations in the initial Kerr-AdS geometry, we also demonstrate that the black resonator's hair is not unique. In the dual quantum field theory description, rotation invariance is spontaneously broken and the energy density is negative in some regions, signaling an exotic state of matter which does not relax to a stationary configuration.

Diamond spin quantum sensing under extreme conditions

Extreme conditions, such as ultra-low temperatures, high pressures, and strong magnetic fields, are critical to producing and studying exotic states of matter. To measure physical properties under extreme conditions, the advanced sensing schemes are required. As a promising quantum sensor, the diamond nitrogen-vacancy (NV) center can detect magnetic field, electronic field, pressure, and temperature with high sensitivity. Considering its nanoscale spatial resolution and ultra-wide working range, the diamond quantum sensing can play an important role in frontier studies involving extreme conditions. This paper reviews the spin and optical properties of diamond NV center under extreme conditions, including low temperature, high temperature, zero field, strong magnetic fields, and high pressures. The opportunities and challenges of diamond quantum sensing under extreme conditions are discussed. The basic knowledge of spin-based quantum sensing and its applications under extreme conditions are also covered.

Topological defects, inherent structures, and hyperuniformity

Disordered hyperuniform systems are exotic states of matter that completely suppress large-scale density fluctuations like crystals, and yet possess no Bragg peaks similar to liquids or glasses. Such systems have been discovered in a variety of equilibrium and non-equilibrium physical and biological systems, and are often endowed with novel physical properties. The mechanisms associated with the emergence of disordered hyperuniformity in nonequilibrium systems, in particular inherent structures are often not well understood, which we will address from a topological perspective in this work. Specifically, we consider a representative class of disordered inherent structures which are constructed by continuously introducing randomly distributed topological defects (dislocations and disclinations) often seen in colloidal systems and atomic-scale two-dimensional materials. We demonstrate that these inherent structures can be viewed as topological variants of ordered hyperuniform states (such as crystals) linked through continuous topological transformation pathways, which remarkably preserve hyperuniformity. Moreover, we develop a continuum theory to demonstrate that the large-scale density fluctuations in these inherent structures are mainly dominated by the elastic displacement fields resulted from the topological defects, which at low defect concentrations can be approximated as superposition of the displacement fields associated with each individual defect (strain source). We rigorously demonstrate that hyperuniformity is preserved as long as certain conditions associated with the displacement fields are met. Our results also highlight the importance of decoupling the positional degrees of freedom from the vibrational degrees of freedom when looking for disordered hyperuniformity, since the hyperuniformity property is often cloaked by thermal fluctuations.

On the link between mean square-radii and high-order toroidal moments

Multipole expansions of the source play an important role in a broad range of disciplines in modern physics, ranging from the description of exotic states of matter to the design of nanoantennas in photonics. Within the context of the latter, toroidal multipoles, a third group of multipoles complementing the well-known electric and magnetic ones, have been widely investigated since they lead to the formation of non-radiating sources. In the last years, however, the photonics community has brought to light the existence of a fourth type of multipoles that is commonly overlooked. Currently, different groups have provided different mathematical expressions to describe such sources, and they have been coined with different names; on the one hand mean-square radii, and on the other hand, as high order toroidal moments. Despite their clear physical similarity, a formal relation between the two has not yet been established. While explicit formulas for both types have been derived, they are not expressed in the same basis, and therefore it is not possible to draw a clear physical connection between them. In this contribution, we will bridge this gap and rigorously derive the connection between the two representations, taking as an example the cases of the nth order mean square radius of the electric dipole and the nth order electric toroidal dipole. Our results conclusively show that both types of representations are exactly equivalent up to a prefactor.

Nesting instability of gapless U(1) spin liquids with spinon Fermi pockets in two dimensions

Quantum spin liquids are exotic states of matter that may be realized in frustrated quantum magnets and feature fractionalized excitations and emergent gauge fields. Here, we consider a gapless U(1) spin liquid with spinon Fermi pockets in two spatial dimensions. Such a state appears to be the most promising candidate to describe the exotic field-induced behavior observed in numerical simulations of the antiferromagnetic Kitaev honeycomb model. A similar such state may also be responsible for the recently-reported quantum oscillations of the thermal conductivity in the field-induced quantum paramagnetic phase of $\alpha$-RuCl$_3$. We consider the regime close to a Lifshitz transition, at which the spinon Fermi pockets shrink to small circles around high-symmetry points in the Brillouin zone. By employing renormalization group and mean-field arguments, we demonstrate that interactions lead to a gap opening in the spinon spectrum at low temperatures, which can be understood as a nesting instability of the spinon Fermi surface. This leads to proliferation of monopole operators of the emergent U(1) gauge field and confinement of spinons. While signatures of fractionalization may be observable at finite temperatures, the gapless U(1) spin liquid state with nested spinon Fermi pockets is ultimately unstable at low temperatures towards a conventional long-range-ordered ground state, such as a valence bond solid. Implications for Kitaev materials in external magnetic fields are discussed.

Magnetic Field Induced Quantum Spin Liquid in the Two Coupled Trillium Lattices of K_{2}Ni_{2}(SO_{4})_{3}.

Quantum spin liquids are exotic states of matter that form when strongly frustrated magnetic interactions induce a highly entangled quantum paramagnet far below the energy scale of the magnetic interactions. Three-dimensional cases are especially challenging due to the significant reduction of the influence of quantum fluctuations. Here, we report the magnetic characterization of K_{2}Ni_{2}(SO_{4})_{3} forming a three-dimensional network of Ni^{2+} spins. Using density functional theory calculations, we show that this network consists of two interconnected spin-1 trillium lattices. In the absence of a magnetic field, magnetization, specific heat, neutron scattering, and muon spin relaxation experiments demonstrate a highly correlated and dynamic state, coexisting with a peculiar, very small static component exhibiting a strongly renormalized moment. A magnetic field B≳4 T diminishes the ordered component and drives the system into a pure quantum spin liquid state. This shows that a system of interconnected S=1 trillium lattices exhibits a significantly elevated level of geometrical frustration.

Strongly-interacting ultralight millicharged particles

We consider the implications of an ultra-light fermionic dark matter candidate that carries baryon number. This naturally arises if dark matter has a small charge under standard model baryon number whilst having an asymmetry equal and opposite to that in the visible universe. A prototypical model is a theory of dark baryons of a non-Abelian gauge group, i.e., a dark Quantum Chromo-Dynamics (QCD). For sub-eV dark baryon masses, the inner region of dark matter halos is naturally at ‘nuclear density’, allowing for the formation of exotic states of matter, akin to neutron stars. The Tremaine-Gunn lower bound on the mass of fermionic dark matter, i.e., the dark baryons, is violated by the strong short-range self-interactions, cooling via emission of light dark pions, and the Cooper pairing of dark quarks that occurs at densities that are high relative to the (ultra-low) dark QCD scale. We develop the astrophysics of these STrongly-interacting Ultra-light Millicharged Particles (STUMPs) utilizing the equation of state of dense quark matter, and find halo cores consistent with observations of dwarf galaxies. These cores are prevented from core-collapse by pressure of the ‘neutron star’, which suggests ultra-light dark QCD as a resolution to core-cusp problem of collisionless cold dark matter. The model is distinguished from ultra-light bosonic dark matter through direct detection and collider signatures, as well as by phenomena associated with superconductivity, such as Andreev reflection and superconducting vortices.

Even-Odd Layer-Dependent Anomalous Hall Effect in Topological Magnet MnBi2Te4 Thin Films.

Recently, MnBi2Te4 has been demonstrated to be an intrinsic magnetic topological insulator and the quantum anomalous Hall (QAH) effect was observed in exfoliated MnBi2Te4 flakes. Here, we used molecular beam epitaxy (MBE) to grow MnBi2Te4 films with thickness down to 1 septuple layer (SL) and performed thickness-dependent transport measurements. We observed a nonsquare hysteresis loop in the antiferromagnetic state for films with thickness greater than 2 SL. The hysteresis loop can be separated into two AH components. We demonstrated that one AH component with the larger coercive field is from the dominant MnBi2Te4 phase, whereas the other AH component with the smaller coercive field is from the minor Mn-doped Bi2Te3 phase. The extracted AH component of the MnBi2Te4 phase shows a clear even-odd layer-dependent behavior. Our studies reveal insights on how to optimize the MBE growth conditions to improve the quality of MnBi2Te4 films.