Showing papers on "Mott transition published in 2021"

Quantum anomalous Hall effect from intertwined moiré bands

Abstract: Electron correlation and topology are two central threads of modern condensed matter physics. Semiconductor moire materials provide a highly tunable platform for studies of electron correlation. Correlation-driven phenomena, including the Mott insulator, generalized Wigner crystals, stripe phases and continuous Mott transition, have been demonstrated. However, nontrivial band topology has remained elusive. Here we report the observation of a quantum anomalous Hall (QAH) effect in AB-stacked MoTe2/WSe2 moire heterobilayers. Unlike in the AA-stacked structures, an out-of-plane electric field controls not only the bandwidth but also the band topology by intertwining moire bands centered at different high-symmetry stacking sites. At half band filling, corresponding to one particle per moire unit cell, we observe quantized Hall resistance, h/e2 (with h and e denoting the Planck's constant and electron charge, respectively), and vanishing longitudinal resistance at zero magnetic field. The electric-field-induced topological phase transition from a Mott insulator to a QAH insulator precedes an insulator-to-metal transition; contrary to most known topological phase transitions, it is not accompanied by a bulk charge gap closure. Our study paves the path for discovery of a wealth of emergent phenomena arising from the combined influence of strong correlation and topology in semiconductor moire materials.

Continuous Mott transition in semiconductor moiré superlattices

Abstract: The evolution of a Landau Fermi liquid into a non-magnetic Mott insulator with increasing electronic interactions is one of the most puzzling quantum phase transitions in physics1–6. The vicinity of the transition is believed to host exotic states of matter such as quantum spin liquids4–7, exciton condensates8 and unconventional superconductivity1. Semiconductor moire materials realize a highly controllable Hubbard model simulator on a triangular lattice9–22, providing a unique opportunity to drive a metal–insulator transition (MIT) via continuous tuning of the electronic interactions. Here, by electrically tuning the effective interaction strength in MoTe2/WSe2 moire superlattices, we observe a continuous MIT at a fixed filling of one electron per unit cell. The existence of quantum criticality is supported by the scaling collapse of the resistance, a continuously vanishing charge gap as the critical point is approached from the insulating side, and a diverging quasiparticle effective mass from the metallic side. We also observe a smooth evolution of the magnetic susceptibility across the MIT and no evidence of long-range magnetic order down to ~5% of the Curie–Weiss temperature. This signals an abundance of low-energy spinful excitations on the insulating side that is further corroborated by the Pomeranchuk effect observed on the metallic side. Our results are consistent with the universal critical theory of a continuous Mott transition in two dimensions4,23. The interaction strength in moire superlattices is tuned to drive a continuous metal-to-insulator transition at a fixed electron density.

Strongly correlated excitonic insulator in atomic double layers

Abstract: Excitonic insulators (EI) arise from the formation of bound electron-hole pairs (excitons) in semiconductors and provide a solid-state platform for quantum many-boson physics. Strong exciton-exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations. Although spectroscopic signatures of EIs have been reported, conclusive evidence for strongly correlated EI states has remained elusive. Here, we demonstrate a strongly correlated spatially indirect two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. An equilibrium interlayer exciton fluid is formed when the bias voltage applied between the two electrically isolated TMD layers, is tuned to a range that populates bound electron-hole pairs, but not free electrons or holes. Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible - direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We further construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing the exotic quantum phases of excitons, as well as multi-terminal exciton circuitry for applications.

Strongly correlated excitonic insulator in atomic double layers

Abstract: Excitonic insulators (EIs) arise from the formation of bound electron–hole pairs (excitons)1,2 in semiconductors and provide a solid-state platform for quantum many-boson physics3–8. Strong exciton–exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations8–11. Although spectroscopic signatures of EIs have been reported6,12–14, conclusive evidence for strongly correlated EI states has remained elusive. Here we demonstrate a strongly correlated two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. A quasi-equilibrium spatially indirect exciton fluid is created when the bias voltage applied between the two electrically isolated TMD layers is tuned to a range that populates bound electron–hole pairs, but not free electrons or holes15–17. Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible—direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing exotic quantum phases of excitons8, as well as multi-terminal exciton circuitry for applications18–20. So far only signatures of excitonic insulators have been reported, but here direct thermodynamic evidence is provided for a strongly correlated excitonic insulating state in transition metal dichalcogenide semiconductor double layers.

Room temperature Mott metal–insulator transition in V2O3 compounds induced via strain-engineering

Abstract: Vanadium sesquioxide (V2O3) is an archetypal Mott insulator in which the atomic positions and electron correlations change as temperature, pressure, and doping are varied, giving rise to different structural, magnetic, or electronic phase transitions. Remarkably, the isostructural Mott transition in Cr-doped V2O3 between paramagnetic metallic and insulating phase observed in bulk has been elusive in thin film compounds so far. Here, via continuous lattice deformations induced by heteroepitaxy, we demonstrate a room temperature Mott metal–insulator transition in 1.5% Cr-doped and pure V2O3 thin films. By means of a controlled epitaxial strain, not only the structure but also the intrinsic electronic and optical properties of the thin films are stabilized at different intermediate states between the metallic and insulating phases, inaccessible in bulk materials. This leads to films with unique features such as a colossal change in room temperature resistivity (ΔR/R up to 100 000%) and a broad range of optical constant values as consequence of a strain-modulated bandgap. We propose a new phase diagram for pure and Cr-doped V2O3 thin films with the engineered in-plane lattice constant as a tunable parameter. Our results demonstrate that controlling phase transitions in correlated systems by epitaxial strain offers a radical new approach to create the next generation of Mott devices.

Pressure-induced Anderson-Mott transition in elemental tellurium

Abstract: Elemental tellurium is a small band-gap semiconductor, which is always p-doped due to the natural occurrence of vacancies. Its chiral non-centrosymmetric structure, characterized by helical chains arranged in a triangular lattice, and the presence of a spin-polarized Fermi surface, render tellurium a promising candidate for future applications. Here, we use a theoretical framework, appropriate for describing the corrections to conductivity from quantum interference effects, to show that a high-quality tellurium single crystal undergoes a quantum phase transition at low temperatures from an Anderson insulator to a correlated disordered metal at around 17 kbar. Such insulator-to-metal transition manifests itself in all measured physical quantities and their critical exponents are consistent with a scenario in which a pressure-induced Lifshitz transition shifts the Fermi level below the mobility edge, paving the way for a genuine Anderson-Mott transition. We conclude that previously puzzling quantum oscillation and transport measurements might be explained by a possible Anderson-Mott ground state and the observed phase transition. Elemental tellurium is a natural p-type semiconductor with a chiral structure and spin-polarized Fermi surface. Here, the authors show that the pressure-induced topological change of the Fermi surface at 17 kbar triggers an Anderson-Mott insulator-to-metal transition.

Rise and fall of Landau's quasiparticles while approaching the Mott transition

Abstract: Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m⋆. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe–Regel–Mott limit is accompanied by a ‘displaced Drude peak’ in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition. Charge transport in strongly correlated electron systems is not fully understood. Here, the authors show that resilient quasiparticles at finite frequency persist into the bad-metal regime near a Mott insulator, where dynamical localization results in a ‘displaced Drude peak’ and strongly enhanced dc resistivity.

Pole and screening masses of neutral pions in a hot and magnetized medium: A comprehensive study in the Nambu-Jona-Lasinio model

Abstract: In this work, we investigate not only the pole masses but also the screening masses of neutral pions at finite temperature and magnetic field by utilizing the random phase approximation (RPA) approach in the framework of the two-flavor Nambu--Jona-Lasinio model. And two equivalent formalisms in the presence of a magnetic field, i.e., the Landau level representation and the proper-time representation (PTR), are applied to obtain the corresponding analytical expressions of the polarization functions (except the expressions for the pole masses in the PTR). In order to evaluate the applicable region of the low-momentum expansion (LME), we compare the numerical results within the full RPA (FRPA) with those within the reduced RPA, i.e., the RPA in the LME. It is confirmed that the pole masses of ${\ensuremath{\pi}}^{0}$ in the FRPA suffer a sudden mass jump at the Mott transition temperature when in the presence of external magnetic field, the Mott transition temperature is catalyzed by the magnetic field. And by analyzing the behaviors of the directional sound velocities of ${\ensuremath{\pi}}^{0}$, which are associated with the breaking of the Lorentz invariance by the heat bath and the magnetic field, we clarify the two problems existing in previous literatures: one is that the transverse sound velocities in the medium are always larger than unity and thus violate the law of causality on account of the noncovariant regularization scheme, and the other is that the longitudinal sound velocities are identically equal unity at finite temperature on account of the limitation of the derivative expansion method used.

Tunable magnetism in layered CoPS 3 by pressure and carrier doping

Abstract: Despite extensive research on recently discovered layered ferromagnetic (FM) materials, their further development is hampered by the limited number of candidate materials with desired properties. As a much bigger family, layered antiferromagnetic (AFM) materials represent excellent platforms to not only deepen our understanding of fundamental physics but also push forward high-performance spintronics applications. Here, by systematic first-principles calculations, we demonstrate pressure and carrier doping control of magnetic properties in layered AFM CoPS3, a representative of transition metal phosphorus trichalcogenides. In particular, pressure can drive isostructural Mott transition, in sharp contrast to other transition metal thiophosphates. Intriguingly, both pressure and carrier doping can realize the long-sought FM half-metallic states with 100% spin polarization percentage, which is good for improving the injection and detection efficiency of spin currents among others. Moreover, the Mott transition is accompanied by instantaneous spin-crossover (SCO) in CoPS3, and such cooperative SCO facilitates the implementation of fast-response reversible devices, such as data storage devices, optical displays and sensors. We further provide an in-depth analysis for the mechanisms of FM half-metallicity and SCO. Tunable magnetism in layered AFM materials opens vast opportunities for purposeful device design with various functionalities.

Ultrafast Mott transition driven by nonlinear electron-phonon interaction

Abstract: Nonlinear phononics holds the promise for controlling properties of quantum materials on the ultrashort timescale. Using nonequilibrium dynamical mean-field theory, we solve a model for the description of organic solids, where correlated electrons couple nonlinearly to a quantum phonon mode. Unlike previous works, we exactly diagonalize the local phonon mode within the noncrossing approximation to include the full phononic fluctuations. By exciting the local phonon in a broad range of frequencies near resonance with an ultrashort pulse, we show it is possible to induce a Mott insulator-to-metal phase transition. Conventional semiclassical and mean-field calculations, where the electron-phonon interaction decouples, underestimate the onset of the quasiparticle peak. This fact, together with the nonthermal character of the photoinduced metal, suggests a leading role of the phononic fluctuations and of the dynamic nature of the state in the vibrationally induced quasiparticle coherence.

Mott Switching and Structural Transition in the Metal Phase of VO2 Nanodomain

Abstract: VO2 undergoes the insulator–metal transition (IMT) and monoclinic–rutile structural phase transition (SPT) near 67 °C. The IMT switching has many applications. However, there is an unresolved issue...

Competing Nodal d-Wave Superconductivity and Antiferromagnetism.

Abstract: Competing unconventional superconductivity and antiferromagnetism widely exist in several strongly correlated quantum materials whose direct simulation generally suffers from fermion sign problem. Here, we report unbiased quantum Monte Carlo (QMC) simulations on a sign-problem-free repulsive toy model with same on site symmetries as the standard Hubbard model on a 2D square lattice. Using QMC simulations, supplemented with mean-field and continuum field-theory arguments, we find that it hosts three distinct phases: a nodal $d$-wave phase, an antiferromagnet, and an intervening phase which hosts coexisting antiferromagnetism and nodeless $d$-wave superconductivity. The transition from the coexisting phase to the antiferromagnet is described by the $2+1\text{\ensuremath{-}}D$ $XY$ universality class, while the one from the coexisting phase to the nodal $d$-wave phase is described by the Heisenberg-Gross-Neveu theory. The topology of our phase diagram resembles that of layered organic materials which host pressure tuned Mott transition from antiferromagnet to unconventional superconductor at half-filling.

Nano-imaging of strain-tuned stripe textures in a Mott crystal

Abstract: The 4d transition metal perovskites Can+1RunO3n+1 have attracted interest for their strongly interacting electronic phases showing pronounced sensitivity to controllable stimuli like strain, temperature, and even electrical current. Through multi-messenger low-temperature nano-imaging, we reveal a spontaneous striped texture of coexisting insulating and metallic domains in single crystals of the bilayer ruthenate Ca3(TixRu1-x)2O7 across its first-order Mott transition at $$T \approx 95$$ K. We image on-demand anisotropic nucleation and growth of these domains under in situ applied uniaxial strain rationalized through control of a spontaneous Jahn-Teller distortion. Our scanning nano-susceptibility imaging resolves the detailed susceptibility of coexisting phases to strain and temperature at the transition threshold. Comparing these nano-imaging results to bulk-sensitive elastoresistance measurements, we uncover an emergent “domain susceptibility” sensitive to both the volumetric phase fractions and elasticity of the self-organized domain lattice. Our combined susceptibility probes afford nano-scale insights into strain-mediated control over the insulator-metal transition in 4d transition metal oxides.

Chemical tuning of molecular quantum materials κ-[(BEDT-TTF)1−x(BEDT-STF)x]2Cu2(CN)3: from the Mott-insulating quantum spin liquid to metallic Fermi liquid

Abstract: The electronic properties of molecular conductors can be readily varied via physical or chemical pressure as it enlarges the bandwidth W. This enables them to cross the Mott insulator-to-metal phase transition by reducing electronic correlations U/W. Here we introduce an alternative path by spatially expanding the molecular orbitals when partially replacing sulfur by selenium in the constituting bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) molecules of the title compound. We characterize how the insulating quantum-spin-liquid state is tuned via a Mott transition to the metallic Fermi-liquid state crossing a narrow region of superconductivity. The transport, dielectric, and optical measurements reveal that at this first-order phase transition, metallic regions coexist in the insulating matrix leading to pronounced percolative effects, which are most obvious in the strong enhancement of the dielectric constant at low temperatures.

Certifying the Adiabatic Preparation of Ultracold Lattice Bosons in the Vicinity of the Mott Transition.

Abstract: Thermometry of ultracold bosons in an optical lattice shows adiabatic preparation of finite-entropy states across the superfluid---Mott-insulator quantum critical point.

Interacting stochastic topology and Mott transition from light response

Abstract: We develop a stochastic description of the topological properties in an interacting Chern insulator. We confirm the Mott transition's first-order nature in the interacting Haldane model on the honeycomb geometry from a mean-field variational approach supported by density matrix renormalization group results and Ginzburg-Landau arguments. From the Bloch sphere, we make predictions for circular dichroism of light related to the quantum Hall conductivity on the lattice and in the presence of interactions. This analysis shows that the topological number can be measured from the light response at the Dirac points. Electron-electron interactions can also produce a substantial number of particle-hole pairs above the band gap, which leads us to propose a stochastic Chern number as an interacting measure of the topology. The stochastic Chern number can describe disordered situations with a fluctuating staggered potential, and we build an analogy between interaction-induced particle-hole pairs and temperature effects. Our stochastic approach is physically intuitive, easy to implement, and leads the way to further studies of interaction effects.

Low-temperature dielectric anomaly arising from electronic phase separation at the Mott insulator-metal transition

Abstract: Coulomb repulsion among conduction electrons in solids hinders their motion and leads to a rise in resistivity. A regime of electronic phase separation is expected at the first-order phase transition between a correlated metal and a paramagnetic Mott insulator, but remains unexplored experimentally as well as theoretically nearby T = 0. We approach this issue by assessing the complex permittivity via dielectric spectroscopy, which provides vivid mapping of the Mott transition and deep insight into its microscopic nature. Our experiments utilizing both physical pressure and chemical substitution consistently reveal a strong enhancement of the quasi-static dielectric constant e1 when correlations are tuned through the critical value. All experimental trends are captured by dynamical mean-field theory of the single-band Hubbard model supplemented by percolation theory. Our findings suggest a similar ’dielectric catastrophe’ in many other correlated materials and explain previous observations that were assigned to multiferroicity or ferroelectricity.

Resistivity Exponents in 3D Dirac Semimetals From Electron-Electron Interaction.

Abstract: We study the resistivity of three-dimensional semimetals with linear dispersion in the presence of on-site electron-electron interaction. The well-known quadratic temperature dependence of the resistivity of conventional metals is turned into an unusual ${T}^{6}$ behavior. An analogous change affects the thermal transport, preserving the linearity in $T$ of the ratio between thermal and electrical conductivities. These results hold from weak coupling up to the nonperturbative region of the Mott transition. Our findings yield a natural explanation for the hitherto not understood large exponents characterizing the temperature dependence of transport experiments on various topological semimetals.

Probing charge density wave phases and the Mott transition in 1 T − TaS 2 by inelastic light scattering

Abstract: We present a polarization-resolved, high-resolution Raman scattering study of the three consecutive charge density wave (CDW) regimes in $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ single crystals, supported by ab initio calculations. Our analysis of the spectra within the low-temperature commensurate (C-CDW) regime shows $P\overline{3}$ symmetry of the system, thus excluding the previously proposed triclinic stacking of the ``star-of-David'' structure, and promoting trigonal or hexagonal stacking instead. The spectra of the high-temperature incommensurate (IC-CDW) phase directly project the phonon density of states due to the breaking of the translational invariance, supplemented by sizable electron-phonon coupling. Between 200 and 352 K, our Raman spectra show contributions from both the IC-CDW and the C-CDW phases, indicating their coexistence in the so-called nearly commensurate (NC-CDW) phase. The temperature dependence of the symmetry-resolved Raman conductivity indicates the stepwise reduction of the density of states in the CDW phases, followed by a Mott transition within the C-CDW phase. We determine the size of the Mott gap to be ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{gap}}\ensuremath{\approx}170\text{--}190$ meV, and track its temperature dependence.

Complexity of the dipolar exciton Mott transition in GaN/(AlGa)N nanostructures

Abstract: The Mott transition from a dipolar excitonic liquid to an electron-hole plasma is demonstrated in a wide GaN/(Al,Ga)N quantum well at $T=7$K by means of spatially-resolved magneto-photoluminescence spectroscopy. Increasing optical excitation density we drive the system from the excitonic state, characterized by a diamagnetic behavior and thus a quadratic energy dependence on the magnetic field, to the unbound electron-hole state, characterized by a linear shift of the emission energy with the magnetic field. The complexity of the system requires to take into account both the density-dependence of the exciton binding energy and the exciton-exciton interaction and correlation energy that are of the same order of magnitude. We estimate the carrier density at Mott transition as $n_\mathrm{Mott}\approx 2\times 10^{11}$cm$^{-2}$ and address the role played by excitonic correlations in this process. Our results strongly rely on the spatial resolution of the photoluminescence and the assessment of the carrier transport. We show, that in contrast to GaAs/(Al,Ga)As systems, where transport of dipolar magnetoexcitons is strongly quenched by the magnetic field due to exciton mass enhancement, in GaN/(Al,Ga)N the band parameters are such that the transport is preserved up to $9$T.

Chiral crossover characterized by Mott transition at finite temperature

Abstract: We discuss the proper definition for the chiral crossover at finite temperature, based on Goldstone's theorem. Different from the commonly used maximum change in chiral condensate, we propose defining the crossover temperature using the Mott transition of pseudo-Goldstone bosons, which, by definition, guarantees Goldstone's theorem. We analytically and numerically demonstrate this property in the frame of a Pauli-Villars regularized NJL model. In an external magnetic field, we find that the Mott transition temperature shows an inverse magnetic catalysis effect.

Continuous Mott transition in semiconductor moir\'e superlattices

Abstract: The evolution of a Landau Fermi liquid into a nonmagnetic Mott insulator with increasing electronic interactions is one of the most puzzling quantum phase transitions in physics. The vicinity of the transition is believed to host exotic states of matter such as quantum spin liquids, exciton condensates and unconventional superconductivity. Semiconductor moire materials realize a highly controllable Hubbard model simulator on a triangular lattice, providing a unique opportunity to drive a metal-insulator transition (MIT) via continuous tuning of the electronic interactions. Here, by electrically tuning the effective interaction strength in MoTe2/WSe2 moire superlattices, we observe a continuous MIT at a fixed filling of one electron per unit cell. The existence of quantum criticality is supported by the scaling behavior of the resistance, a continuously vanishing charge-gap as the critical point is approached from the insulating side, and a diverging quasiparticle effective mass from the metallic side. We also observe a smooth evolution of the low-temperature magnetic susceptibility across the MIT and find no evidence of long-range magnetic order down to ~ 5% of the Curie-Weiss temperature. The results signal an abundance of low-energy spinful excitations on the insulating side that is further corroborated by the presence of the Pomeranchuk effect on the metallic side. Our results are consistent with the universal critical theory of a continuous MIT from a Landau Fermi liquid to a nonmagnetic Mott insulator in two dimensions.

Mott transition in a cavity-boson system: A quantitative comparison between theory and experiment

Abstract: The competition between short-range and cavity-mediated infinite-range interactions in a cavity-boson system leads to the existence of a superfluid phase and a Mott-insulator phase within the self-organized regime. In this work, we quantitatively compare the steady-state phase boundaries of this transition measured in experiments and simulated using the Multiconfigurational Time-Dependent Hartree Method for Indistinguishable Particles. To make the problem computationally feasible, we represent the full system by the exact many-body wave function of a two-dimensional four-well potential. We argue that the validity of this representation comes from the nature of both the cavity-atomic system and the Bose-Hubbard physics. Additionally we show that the chosen representation only induces small systematic errors, and that the experimentally measured and theoretically predicted phase boundaries agree reasonably. We thus demonstrate a new approach for the quantitative numerical determination of the superfluid--Mott-insulator phase boundary.

Sachdev-Ye-Kitaev Models and Beyond: A Window into Non-Fermi Liquids

Abstract: We present a review of the Sachdev-Ye-Kitaev (SYK) model of compressible quantum many-body systems without quasiparticle excitations, and its connections to various theoretical studies of non-Fermi liquids in condensed matter physics. The review is placed in the context of numerous experimental observations on correlated electron materials. Strong correlations in metals are often associated with their proximity to a Mott transition to an insulator created by the local Coulomb repulsion between the electrons. We explore the phase diagrams of a number of models of such local electronic correlation, employing a dynamical mean field theory in the presence of random spin exchange interactions. Numerical analyses and analytical solutions, using renormalization group methods and expansions in large spin degeneracy, lead to critical regions which display SYK physics. The models studied include the single-band Hubbard model, the $t$-$J$ model and the two-band Kondo-Heisenberg model in the presence of random spin exchange interactions. We also examine non-Fermi liquids obtained by considering each SYK model with random four fermion interactions to be a multi-orbital atom, with the SYK-atoms arranged in an infinite lattice. We connect to theories of sharp Fermi surfaces without any low-energy quasiparticles in the absence of spatial disorder, obtained by coupling a Fermi liquid to a gapless boson; a systematic large $N$ theory of such a critical Fermi surface, with SYK characteristics, is obtained by averaging over an ensemble of theories with random boson-fermion couplings. Finally, we present an overview of the links between the SYK model and quantum gravity and end with an outlook on open questions.

Theory of a Continuous Bandwidth-tuned Wigner-Mott Transition

Abstract: We develop a theory for a continuous bandwidth-tuned transition at fixed $\textit{fractional}$ electron filling from a metal with a generic Fermi surface to a `Wigner-Mott' insulator that spontaneously breaks crystalline space-group symmetries. Across the quantum critical point, (i) the entire electronic Fermi surface disappears abruptly upon approaching from the metallic side, and (ii) the insulating charge gap and various order-parameters associated with the spontaneously broken space-group symmetries vanish continuously upon approaching from the insulating side. Additionally, the insulating side hosts a Fermi surface of neutral spinons. We present a framework for describing such continuous metal-insulator transitions (MIT) and analyze the example of a bandwidth-tuned transition at a filling, $ u=1/6$, for spinful electrons on the triangular lattice. By extending the theory to a certain large-$N$ limit, we provide a concrete example of such a continuous MIT and discuss numerous experimental signatures near the critical point. We place our results in the context of recent experiments in moir\'e transition metal dichalcogenide materials.

Robust gapless superconductivity in 4 H b − TaS 2

Abstract: The superconducting transition metal dichalcogenide (TMD) $4Hb\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ consists of alternating layers of $H$ and $T$ structures, which in their bulk form are metallic and Mott insulating, respectively. Recently, this compound has been proposed as a candidate chiral superconductor, due to an observed enhancement of the muon-spin relaxation at ${T}_{c}$. $4Hb\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ also exhibits a puzzling $T$-linear specific heat at low temperatures, which is unlikely to be caused by disorder. Elucidating the origin of this behavior is an essential step in discerning the true nature of the superconducting ground state. Here, we propose a simple model that attributes the $T$-linear specific heat to the emergence of a robust multiband gapless superconducting state. We show that an extended regime of gapless superconductivity naturally appears when the pair-breaking scattering rate on distinct Fermi-surface pockets differs significantly, and the pairing interaction is predominantly intrapocket. Using a tight-binding model derived from first-principle calculations, we show that the pair-breaking scattering rate promoted by slow magnetic fluctuations on the $T$ layers, which arise from proximity to a Mott transition, can be significantly different in the various $H$-layer dominated Fermi pockets depending on their hybridization with $T$-layer states. Thus, our results suggest that the ground state of $4Hb\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ consists of Fermi pockets displaying gapless superconductivity, which are shunted by superconducting Fermi pockets that are nearly decoupled from the $T$ layers.

Accelerated Hot-Carrier Cooling in MAPbI3 Perovskite by Pressure-Induced Lattice Compression.

Abstract: Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hot-phonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC. On the contrary, above the Mott transition, HCC in methylammonium lead iodide is around 2-3 times faster at 0.3 GPa than at ambient pressure. Our electron-phonon coupling calculations reveal ∼2-fold stronger electron-phonon coupling for the inorganic cage mode at 0.3 GPa. However, our experiments reveal that pressure promotes faster HCC only above the Mott transition. Altogether, these findings suggest a change in the nature of excited carriers above the Mott transition threshold, providing insights into the electronic behavior of devices operating at such high charge-carrier densities.

Mott transition and magnetism in a fragile topological insulator

Abstract: We study the effects of electronic correlations on fragile topology using dynamical mean-field theory. Fragile topological insulators (FTIs) offer obstruction to the formation of exponentially localized Wannier functions, but they can be trivialized by adding certain trivial degrees of freedom. For the same reason, FTIs do not host symmetry-protected flow of edge states between bulk bands in cylindrical boundary conditions but are expected to have a spectral flow between the fragile bands and other bands under certain twisted boundary conditions. We here analyze commonly observed effects of strong correlations, such as the Mott insulator transition and magnetism, on a known model hosting fragile topology. We show that in the nonmagnetic case, fragile topology, along with the twisted boundary states, is stable with interactions below a critical interaction strength. Above this interaction strength, a transition to the Mott insulating phase occurs, and the twisted boundary states disappear. Furthermore, by applying a homogeneous magnetic field, the fragile topology is destroyed. However, we show that a magnetic field can induce a topological phase transition which converts a fragile topological insulator to a Chern insulator. Finally, we study ferromagnetic solutions of the fragile topological model.