Correlated Quantum Matter Research Articles

Nonequilibrium quantum critical steady state: Transport through a dissipative resonant level

Nonequilibrium properties of correlated quantum matter are being intensively investigated because of the rich interplay between external driving and the many-body correlations. Of particular interest is the nonequilibrium behavior near a quantum critical point (QCP), where the system is delicately balanced between different ground states. We present both an analytical calculation of the nonequilibrium steady-state current in a critical system and experimental results to which the theory is compared. The system is a quantum dot coupled to resistive leads: a spinless resonant level interacting with an Ohmic dissipative environment. A two-channel Kondo-like QCP occurs when the level is on resonance and symmetrically coupled to the leads, conditions achieved by fine tuning using electrostatic gates. We calculate and measure the nonlinear current as a function of bias (I−V curve) at the critical values of the gate voltages corresponding to the QCP. The quantitative agreement between the experimental data and the theory, with no fitting parameter, is excellent. As our system is fully accessible to both theory and experiment, it provides an ideal setting for addressing nonequilibrium phenomena in correlated quantum matter.Received 16 October 2020Revised 16 January 2021Accepted 21 January 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.013136Published 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 AreasCoulomb blockadeQuantum criticalityTransport phenomenaPhysical SystemsNanotubesQuantum dotsCondensed Matter, Materials & Applied PhysicsGeneral Physics

Dominant Fifth-Order Correlations in Doped Quantum Antiferromagnets.

Traditionally, one- and two-point correlation functions are used to characterize many-body systems. In strongly correlated quantum materials, such as the doped 2D Fermi-Hubbard system, these may no longer be sufficient, because higher-order correlations are crucial to understanding the character of the many-body system and can be numerically dominant. Experimentally, such higher-order correlations have recently become accessible in ultracold atom systems. Here, we reveal strong non-Gaussian correlations in doped quantum antiferromagnets and show that higher-order correlations dominate over lower-order terms. We study a single mobile hole in the t-J model using the density matrix renormalization group and reveal genuine fifth-order correlations which are directly related to the mobility of the dopant. We contrast our results to predictions using models based on doped quantum spin liquids which feature significantly reduced higher-order correlations. Our predictions can be tested at the lowest currently accessible temperatures in quantum simulators of the 2D Fermi-Hubbard model. Finally, we propose to experimentally study the same fifth-order spin-charge correlations as a function of doping. This will help to reveal the microscopic nature of charge carriers in the most debated regime of the Hubbard model, relevant for understanding high-T_{c} superconductivity.

Quantum Smectic Gauge Theory.

We present a gauge theory formulation of a two-dimensional quantum smectic and its relatives, motivated by their realizations in correlated quantum matter. The description gives a unified treatment of phonons and topological defects, respectively, encoded in a pair of coupled gauge fields and corresponding charges. The charges exhibit subdimensional constrained quantum dynamics and anomalously slow highly anisotropic diffusion of disclinations inside a smectic. This approach gives a transparent description of a multistage quantum melting transition of a two-dimensional commensurate crystal (through an incommensurate crystal-a supersolid) into a quantum smectic, which subsequently melts into a quantum nematic and isotropic superfluids, all in terms of a sequence of Higgs transitions.

Quantum Information and Algorithms for Correlated Quantum Matter.

Discoveries in quantum materials, which are characterized by the strongly quantum-mechanical nature of electrons and atoms, have revealed exotic properties that arise from correlations. It is the promise of quantum materials for quantum information science superimposed with the potential of new computational quantum algorithms to discover new quantum materials that inspires this Review. We anticipate that quantum materials to be discovered and developed in the next years will transform the areas of quantum information processing including communication, storage, and computing. Simultaneously, efforts toward developing new quantum algorithmic approaches for quantum simulation and advanced calculation methods for many-body quantum systems enable major advances toward functional quantum materials and their deployment. The advent of quantum computing brings new possibilities for eliminating the exponential complexity that has stymied simulation of correlated quantum systems on high-performance classical computers. Here, we review new algorithms and computational approaches to predict and understand the behavior of correlated quantum matter. The strongly interdisciplinary nature of the topics covered necessitates a common language to integrate ideas from these fields. We aim to provide this common language while weaving together fields across electronic structure theory, quantum electrodynamics, algorithm design, and open quantum systems. Our Review is timely in presenting the state-of-the-art in the field toward algorithms with nonexponential complexity for correlated quantum matter with applications in grand-challenge problems. Looking to the future, at the intersection of quantum information science and algorithms for correlated quantum matter, we envision seminal advances in predicting many-body quantum states and describing excitonic quantum matter and large-scale entangled states, a better understanding of high-temperature superconductivity, and quantifying open quantum system dynamics.

Graphene bilayers with a twist.

Near a magic twist angle, bilayer graphene transforms from a weakly correlated Fermi liquid to a strongly correlated two-dimensional electron system with properties that are extraordinarily sensitive to carrier density and to controllable environmental factors such as the proximity of nearby gates and twist-angle variation. Among other phenomena, magic-angle twisted bilayer graphene hosts superconductivity, interaction-induced insulating states, magnetism, electronic nematicity, linear-in-temperature low-temperature resistivity and quantized anomalous Hall states. We highlight some key research results in this field, point to important questions that remain open and comment on the place of magic-angle twisted bilayer graphene in the strongly correlated quantum matter world.

Facilitating spin squeezing generated by collective dynamics with single-particle decoherence

We study the generation of spin-squeezing in arrays of long-lived dipoles subject to collective emission, coherent drive, elastic interactions, and spontaneous emission. Counter-intuitively, it is found that the introduction of spontaneous emission leads to an enhancement of the achievable spin-squeezing, relative to that which emerges in the steady-state of the purely collective dynamics for the same model parameters. This behavior is connected to the dynamical self-tuning of the system through a dissipative phase transition that is present in the collective system alone. Our findings will be applicable to next-generation quantum sensors harnessing correlated quantum matter, including cavity-QED and trapped ion systems.

Dynamical structure factors of dynamical quantum simulators

The dynamical structure factor is one of the experimental quantities crucial in scrutinizing the validity of the microscopic description of strongly correlated systems. However, despite its long-standing importance, it is exceedingly difficult in generic cases to numerically calculate it, ensuring that the necessary approximations involved yield a correct result. Acknowledging this practical difficulty, we discuss in what way results on the hardness of classically tracking time evolution under local Hamiltonians are precisely inherited by dynamical structure factors and, hence, offer in the same way the potential computational capabilities that dynamical quantum simulators do: We argue that practically accessible variants of the dynamical structure factors are bounded-error quantum polynomial time ([Formula: see text])-hard for general local Hamiltonians. Complementing these conceptual insights, we improve upon a novel, readily available measurement setup allowing for the determination of the dynamical structure factor in different architectures, including arrays of ultra-cold atoms, trapped ions, Rydberg atoms, and superconducting qubits. Our results suggest that quantum simulations employing near-term noisy intermediate-scale quantum devices should allow for the observation of features of dynamical structure factors of correlated quantum matter in the presence of experimental imperfections, for larger system sizes than what is achievable by classical simulation.

Attosecond light source in material science investigation

Since the dawn of the 21st century, attosecond (1 as =10–18 s) science has been explored and is now fast developing. It brought us the unprecedented time-resolved spectroscopy of quasiparticle dynamics in materials. To date, femtosecond (1 fs =10–15 s) time-resolved pump-probe technology is widely used to study the ultrafast quasiparticle dynamics of quantum materials, such as strong correlated quantum materials, topological materials, superconductors, and semiconductors. In a typical pump-probe experiment, the laser source will be divided into two beams, namely the pump and the probe beams. The pump beam excites the quasiparticles in target materials, and the time-delayed probe beam scans the ultrafast dynamics of the excited state carriers and the coherent lattice vibrations. By employing this method, we are able to observe the lifetimes due to various interactions, such as the electron-electron scattering, electron-phonon coupling, phonon-phonon scattering, etc. By manipulating the various laser pulse characteristic parameters, such as the photon energy, polarization, pulse duration and laser fluence, femtosecond ultrafast spectroscopy excels in investigating the physical properties of materials, which are largely determined by the outer electrons. To investigate the excited states of various materials, femtosecond pump-probe spectroscopy is often conducted under extreme conditions, such as low temperature, strong magnetic field, and high pressure. As a comparison, attosecond ultrafast spectroscopy can not only get access to exploring even faster ultrafast processes, but also probe the inner shell electrons of various types of materials. Attosecond pulses are usually prepared by high-order harmonic generation (HHG). The most common way of generating HHG is in an atomic gas. The excited unbound electrons are accelerated to the nuclei, and the electron-core collisions lead to multiple ionizations. The photons are emitted as trains of ultra-shot pulses. Recently, HHG has also been realized in liquids and solids. The separation of isolated attosecond pulses from trains of attosecond harmonics can be achieved by manipulating the polarization gating, the ultrashort few-cycle driving lasers, or both. Both attosecond pulse trains and isolated attosecond pulses can be applied to the investigation of quantum materials. With the aid of attosecond time-resolved ultrafast pump-probe spectroscopy, more and more fundamental physical properties and relevant applications will be revealed, which will greatly lead the development of condensed matter physics, material sciences, and their applications. Conversely, the attosecond science and technology can also benefit from the deeper understanding of the physical properties of solid state materials. In this article, we first introduce two types of attosecond time-resolved pump-probe experimental techniques, attosecond transient absorption spectrum (ATAS) and attosecond time-resolved angle-resolved photoemission spectroscopy (atto-ARPES). We review typical experiments carried out using such facilities in dielectrics, semiconductors, layered materials, transition metals, etc., and make an outlook for the potential applications of attosecond laser spectroscopy in this field of material science. Furthermore, as the attosecond pulses are often generated from the HHG of mid-infrared laser beams, the mid-infrared ultrafast spectroscopy itself can also be applied to material investigations. Moreover, we review the applications of mid-infrared ultrafast spectroscopy in condensed matter physics, with intriguing laser-induced novel phenomena.

Nonequilibrium Electron and Lattice Dynamics of Strongly Correlated Quantum Materials

Nonequilibrium Electron and Lattice Dynamics of Strongly Correlated Quantum Materials

Tuning the structural and antiferromagnetic phase transitions in UCr2Si2 : Hydrostatic pressure and chemical substitution

Structural phase transitions in $f$-electron materials have attracted sustained attention both for practical and basic science reasons, including that they offer an environment to directly investigate relationships between structure and the $f$-state. Here we present results for UCr$_2$Si$_2$, where structural (tetragonal $\rightarrow$ monoclinic) and antiferromagnetic phase transitions are seen at $T_{\rm{S}}$ $=$ 205 K and $T_{\rm{N}}$ $=$ 25 K, respectively. We also provide evidence for an additional second order phase transition at $T_{\rm{X}}$ = 280 K. We show that $T_{\rm{X}}$, $T_{\rm{S}}$, and $T_{\rm{N}}$ respond in distinct ways to the application of hydrostatic pressure and Cr $\rightarrow$ Ru chemical substitution. In particular, hydrostatic compression increases the structural ordering temperature, eventually causes it to merge with $T_{\rm{X}}$ and destroys the antiferromagnetism. In contrast, chemical substitution in the series UCr$_{2-x}$Ru$_x$Si$_2$ suppresses both $T_{\rm{S}}$ and $T_{\rm{N}}$, causing them to approach zero temperature near $x$ $\approx$ 0.16 and 0.08, respectively. The distinct $T-P$ and $T-x$ phase diagrams are related to the evolution of the rigid Cr-Si and Si-Si substructures, where applied pressure semi-uniformly compresses the unit cell and Cr $\rightarrow$ Ru substitution results in uniaxial lattice compression along the tetragonal $c$-axis and an expansion in the $ab$-plane. These results provide insights into an interesting class of strongly correlated quantum materials where degrees of freedom associated with $f$-electron magnetism, strong electronic correlations, and structural instabilities are readily controlled.

Quantum to classical crossover of Floquet engineering in correlated quantum systems

Light-matter coupling involving classical and quantum light offers a wide range of possibilities to tune the electronic properties of correlated quantum materials. Two paradigmatic results are the dynamical localization of electrons and the ultrafast control of spin dynamics, which have been discussed within classical Floquet engineering and in the deep quantum regime where vacuum fluctuations modify the properties of materials. Here we discuss how these two extreme limits are interpolated by a cavity which is driven to the excited states. In particular, this is achieved by formulating a Schrieffer-Wolff transformation for the cavity-coupled system, which is mathematically analogous to its Floquet counterpart. Some of the extraordinary results of Floquet-engineering, such as the sign reversal of the exchange interaction or electronic tunneling, which are not obtained by coupling to a dark cavity, can already be realized with a single-photon state (no coherent states are needed). The analytic results are verified and extended with numerical simulations on a two-site Hubbard model coupled to a driven cavity mode. Our results generalize the well-established Floquet-engineering of correlated electrons to the regime of quantum light. It opens up a new pathway of controlling properties of quantum materials with high tunability and low energy dissipation.

Breakdown of emergent Lifshitz symmetry in holographic matter with Harris-marginal disorder

We revisit the theory of strongly correlated quantum matter perturbed by Harris-marginal random-field disorder, using the simplest holographic model. We argue that for weak disorder, the ground state of the theory is not Lifshitz invariant with a non-trivial disorder-dependent dynamical exponent, as previously found. Instead, below a non-perturbatively small energy scale, we predict infrared physics becomes independent of the disorder strength.

Strongly Correlated Molecular Magnet with Curie Temperature above 60 K

Summary Two-dimensional crystals with ferromagnetic ordering would lead to unprecedented possibilities for magnetic, magneto-optic, and magnetoelectric applications. However, long-range ferromagnetic ordering in two-dimensional van der Waals materials is strongly diminished by thermal fluctuation, while three-dimensional systems show the stiffness of magnetic ordering temperature to magnetic fields. Here, we report the discovery of intrinsic ferromagnetic ordering temperature of 60 K in an iron/tetracyanoquinodimethane layer via superconducting precursor. In such a molecular ferromagnet, an unprecedented control of ferromagnetic fluctuation temperature by ∼200% improvement is realized through magnetic fields. We demonstrate magnetodielectric coupling with a concomitant transition in magnetic ordering. A small magnetic field of 0.01 T enables an effective route to realize a sizable transition shift in magnetodielectric states. Stimulated by photoirradiation, it manifests a hidden metallic conducting state, declaring a metallic ferromagnet. The molecular ferromagnet and photoirradiation-induced hidden metallic phase present a “holy grail” in the field of molecular conducting magnets.

Ising model in a light-induced quantized transverse field

This paper investigates the effect of light-matter interactions on correlated quantum matter. The non-local bosonic light field of an optical resonator drastically changes the phase transition in an Ising chain, with a tuning parameter that resembles a non-classical transverse magnetic field with quantized values

Colloquium : Heavy-electron quantum criticality and single-particle spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) have become indispensable tools in the study of correlated quantum materials. Both probe complementary aspects of the single-particle excitation spectrum. Taken together, ARPES and STM have the potential to explore properties of the electronic Green's function, a central object of many-body theory. This review explicates this potential with a focus on heavy-electron quantum criticality, especially the role of Kondo destruction. A discussion on how to probe the Kondo destruction effect across the quantum-critical point using ARPES and STM measurements is presented. Particular emphasis is placed on the question of how to distinguish between the signatures of the initial onset of hybridization-gap formation, which is the "high-energy" physics to be expected in all heavy-electron systems, and those of Kondo destruction, which characterizes the low-energy physics and, hence, the nature of quantum criticality. Recent progress and possible challenges in the experimental investigations are surveyed, the STM and ARPES spectra for several quantum-critical heavy-electron compounds are compared, and the prospects for further advances are outlined.

From Spinon Band Topology to the Symmetry Quantum Numbers of Monopoles in Dirac Spin Liquids

The interplay of symmetry and topology has been at the forefront of recent progress in quantum matter. Here we uncover an unexpected connection between band topology and the description of competing orders in a quantum magnet. Specifically we show that aspects of band topology protected by crystalline symmetries determine key properties of the Dirac spin liquid (DSL) which can be defined on the honeycomb, square, triangular and kagom\'e lattices. At low energies, the DSL on all these lattices is described by an emergent Quantum Electrodynamics (QED$_3$) with $N_f=4$ flavors of Dirac fermions coupled to a $U(1)$ gauge field. However the symmetry properties of the magnetic monopoles, an important class of critical degrees of freedom, behave very differently on different lattices. In particular, we show that the lattice momentum and angular momentum of monopoles can be determined from the charge (or Wannier) centers of the corresponding spinon insulator. We also show that for DSLs on bipartite lattices, there always exists a monopole that transforms trivially under all microscopic symmetries owing to the existence of a parent SU(2) gauge theory. We connect our results to generalized Lieb-Schultz-Mattis theorems and also derive the time-reversal and reflection properties of monopoles. Our results indicate that recent insights into free fermion band topology can also guide the description of strongly correlated quantum matter.

Novel magnetism and spin dynamics of strongly correlated electron systems: Microscopic insights

Abstract Spin correlation, competing interactions and subtle interplay between various degrees of freedom can lead to novel quantum states in correlated electron systems. Insights into the intrinsic magnetic susceptibility and the origin of complex magnetic ordering on a microscopic scale set an attractive setting in correlated quantum matter. In this context, Nuclear Magnetic Resonance (NMR) probe offers an unprecedented approach to explore the underlying physical mechanism of correlated quantum phenomena in condensed matter physics. NMR is a site selective microscopic technique extremely sensitive to low energy spin dynamics defining the ground state properties and is useful in mapping the tomography of correlated electron materials. NMR is a powerful microscopic probe to extract the intrinsic magnetic susceptibility and to probe topological order and elucidate the nature of enigmatic elementary excitations in correlated quantum matter. NMR shares an interface with muon spin relaxation (µSR) and neutron scattering and the combination of these techniques on a complementary scale provides an outstanding track to shed insights into the dynamics of dressed quasi-particles in the ground state of correlated electron materials. In this short review, we focus on some of the emergent physical phenomena in a few correlated electron materials sharing common experimental signatures in magnetization, specific heat, and NMR spin susceptibility and spin lattice relaxation rates.

Modern Scattering-Type Scanning Near-Field Optical Microscopy for Advanced Material Research.

Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering-type scanning near-field optical microscopy (s-SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5-3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm-1 ), and temporal (<10 fs) resolution. The history of s-SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research.

Phase-Change Hyperbolic Heterostructures for Nanopolaritonics: A Case Study of hBN/VO2.

Unlike conventional plasmonic media, polaritonic van der Waals (vdW) materials hold promise for active control of light-matter interactions. The dispersion relations of elementary excitations such as phonons and plasmons can be tuned in layered vdW systems via stacking using functional substrates. In this work, infrared nanoimaging and nanospectroscopy of hyperbolic phonon polaritons are demonstrated in a novel vdW heterostructure combining hexagonal boron nitride (hBN) and vanadium dioxide (VO2 ). It is observed that the insulator-to-metal transition in VO2 has a profound impact on the polaritons in the proximal hBN layer. In effect, the real-space propagation of hyperbolic polaritons and their spectroscopic resonances can be actively controlled by temperature. This tunability originates from the effective change in local dielectric properties of the VO2 sublayer in the course of the temperature-tuned insulator-to-metal phase transition. The high susceptibility of polaritons to electronic phase transitions opens new possibilities for applications of vdW materials in combination with strongly correlated quantum materials.

A dissipatively stabilized Mott insulator of photons.

Superconducting circuits are a competitive platform for quantum computation because they offer controllability, long coherence times and strong interactions-properties that are essential for the study of quantum materials comprising microwave photons. However, intrinsic photon losses in these circuits hinder the realization of quantum many-body phases. Here we use superconducting circuits to explore strongly correlated quantum matter by building a Bose-Hubbard lattice for photons in the strongly interacting regime. We develop a versatile method for dissipative preparation of incompressible many-body phases through reservoir engineering and apply it to our system to stabilize a Mott insulator of photons against losses. Site- and time-resolved readout of the lattice allows us to investigate the microscopic details of the thermalization process through the dynamics of defect propagation and removal in the Mott phase. Our experiments demonstrate the power of superconducting circuits for studying strongly correlated matter in both coherent and engineered dissipative settings. In conjunction with recently demonstrated superconducting microwave Chern insulators, we expect that our approach will enable the exploration of topologically ordered phases of matter.