Ultrafast dynamics in photo-excited Mott insulator Sr_(3)Ir_(2)O_7 at high pressure

High-pressure ultrafast dynamics,as a new crossed research direction,are sensitive to subtle non-equilibrium state changes that might be unresolved by equilibrium states measurements,providing crucial information for studying delicate phase transitions caused by complex interactions in Mott insulators.With time-resolved transient reflectivity measurements,we identified the new phases in the spin–orbit Mott insulator Sr_(3)Ir_(2)O_7 at 300 K that was previously unidentified using conventional approaches such as x-ray diffraction.Significant pressure-dependent variation of the amplitude and lifetime obtained by fitting the reflectivity?R/R reveal the changes of electronic structure caused by lattice distortions,and reflect the critical phenomena of phase transitions.Our findings demonstrate the importance of ultrafast nonequilibrium dynamics under extreme conditions for understanding the phase transition of Mott insulators.

Evolution of Bond-Order-Wave Phase in One-Dimensional Mott Insulators

In this paper, by using the level spectroscopy method and bosonization theory, we discuss the evolution of the bond-order-wave （BOW） phase in a one-dimensional half-filled extended Hubbard model wlth the on-site Coulomb repulsion U as well as the inter-site Coulomb repulsion V and antiferromagnetic exchange J. After clarifying the generic phase diagrams in three limiting cases with one of the parameters being fixed at zero individually, we find that the BOW phase in the U-V phase diagram is initially enlarged as J increases from zero but is eventually suppressed as J increases further in the strong-coupling regime. A three-dimensional phase diagram is suggested where the BOW phase exists in an extended region separated from the spin-density-wave and charge-density-wave phases.

Experimental observation of pseudogap in a modulation-doped Mott insulator:Sn/Si(111)-(√3×√3)R30°

Unusual quantum phenomena usually emerge upon doping Mott insulators.Using a molecular beam epitaxy system integrated with cryogenic sc√annin√g tunneling microscope,we investigate the electronic structure of a modulation-doped Mott insulator Sn/Si(111)-(√3×√3)R30°.In underdoped regions,we observe a universal pseudogap opening around the Fermi level,which changes little with the applied magnetic field and the occurrence of Sn vacancies.The pseudogap gets smeared out at elevated temperatures and alters in size with the spatial confinement of the Mott insulating phase.Our findings,along with the previously observed superconductivity at a higher doping level,are highly reminiscent of the electronic phase diagram in the doped copper oxide compounds.

Mott insulator-density ordered superfluid transition and‘shamrock transition’in a frustrated triangle lattice

Density order is usually a consequence of the competition between long-range and short-range interactions.Here we report a density ordered superfluid emergent from a homogeneous Mott insulator due to the competition between frustrations and local interactions.This transition is found in a Bose–Hubbard model on a frustrated triangle lattice with an extra pairing term.Furthermore,we find a quantum phase transition between two different density ordered superfluids,which is beyond the Landau–Ginzburg(LG)paradigm.A U(1)symmetry is emergent at the critical point,while the symmetry in each density ordered superfluid is Z_(2)×Z_(3).We call the transition a‘shamrock transition’,due to its degenerate ground state in the parameter space being a shamrock-like curve rather than a circle in an LG-type transition.Effective low energy theories are established for the two transitions mentioned above and we find their resemblance and differences with clock models.

A review of Mott insulator in memristors:The materials,characteristics,applications for future computing systems and neuromorphic computing

Mott insulator material,as a kind of strongly correlated electronic system with the characteristic of a drastic change in electrical conductivity,shows excellent application prospects in neuromorphological calculations and has attracted significant attention in the scientific community.Especially,computing systems based on Mott insulators can overcome the bottleneck of separated data storage and calculation in traditional artificial intelligence systems based on the von Neumann architecture,with the potential to save energy,increase operation speed,improve integration,scalability,and three-dimensionally stacked,and more suitable to neuromorphic computing than a complementary metal-oxide-semiconductor.In this review,we have reviewed Mott insulator materials,methods for driving Mott insulator transformation(pressure-,voltage-,and temperature-driven approaches),and recent relevant applications in neuromorphic calculations.The results in this review provide a path for further study of the applications in neuromorphic calculations based on Mott insulator materials and the related devices.

Superfluid-Mott-Insulator Phase .Transition of Bosons in an Optical Lattice

The Bose-Hubbard model describing interacting bosons in an optical lattice is reduced to a simple spin-1 XY model with single-ion anisotropy in the vicinity of the Mort phase. We propose a mean-field theory based on a constraint SU（3） pseudo-boson representation on the effective model to study the properties of the superfluid-Mott-insulator phase transition. By calculating the elementary excitation spectra and the average particle number tluctuation in the Brillouin zone center, we lind that the energy gaps vanish continuously around （JXY/Jz）c≈ 0.175 and （JxY/Jz）c ≈ 0.094 for 2D and 3D cubic lattices respectively, where the superfluid order parameters come up from zero and the Mort insulator state changes into a superfluid state.

Imaging the atomic-scale electronic states induced by a pair of hole dopants in Ca_(2)CuO_(2)Cl_(2) Mott insulator

We use scanning tunneling microscopy to visualize the atomic-scale electronic states induced by a pair of hole dopants in Ca_(2)CuO_(2)Cl_(2)parent Mott insulator of cuprates.We find that when the two dopants approach each other,the transfer of spectral weight from high energy Hubbard band to low energy ingap state creates a broad peak and nearly V-shaped gap around the Fermi level.The peak position shows a sudden drop at distance around 4 a_(0)and then remains almost constant.The in-gap states exhibit peculiar spatial distributions depending on the configuration of the two dopants relative to the underlying Cu lattice.These results shed important new lights on the evolution of low energy electronic states when a few holes are doped into parent cuprates.

Interface dipole induced by asymmetric exchange effect in Mott-insulator/Mott-insulator heterojunction

We study theoretically the interracial electronic property of a heterojunction made from two Mott insulators （MI） with different magnetic structures. By means of unrestricted Hartree-Fock calculations in real space, we find that a charge dipole can form spontaneously near the interface of the MI/MI heterojunction. The magnitude of this charge dipole depends strongly on the magnetic states of both sides of the heterojunction. Combining with the result from an exactly solvable two-site toy model, we argue that the interface dipole arises from exchange effects as well as its asymmetry intrinsic to the heterojunction near the interface. Our study may shed light on the fabrication of ultrathin ferroelectric and magnetoelectric devices.

Mottness,phase string,and high-T_(c)superconductivity

It is a great discovery in physics of the twentieth century that the elementary particles in nature are dictated by gauge forces,characterized by a nonintegrable phase factor that an elementary particle of charge q acquires from A to B points:P exp(iq/hc∫A^(B)A_(μ)dxμ),where Aμis the gauge potential and P stands for path ordering.In a many-body system of strongly correlated electrons,if the so-called Mott gap is opened up by interaction,the corresponding Hilbert space will be fundamentally changed.A novel nonintegrable phase factor known as phase-string will appear and replace the conventional Fermi statistics to dictate the low-lying physics.Protected by the Mott gap,which is clearly identified in the high-Tc cuprate with a magnitude>1.5 e V,such a singular phase factor can enforce a fractionalization of the electrons,leading to a dual world of exotic elementary particles with a topological gauge structure.A non-Fermi-liquid“parent”state will emerge,in which the gapless Landau quasiparticle is only partially robust around the so-called Fermi arc regions,while the main dynamics are dominated by two types of gapped spinons.Antiferromagnetism,superconductivity,and a Fermi liquid with full Fermi surface can be regarded as the low-temperature instabilities of this new parent state.Both numerics and experiments provide direct evidence for such an emergent physics of the Mottness,which lies in the core of a high-Tc superconducting mechanism.

Fast impurity solver for dynamical mean field theory based on second order perturbation around the atomic limit

This paper proposes an impurity solver for the dynamical mean field theory （DMFT） study of the Mott insulators, which is based on the second order perturbation of the hybridization function. After careful benchmarking with quantum Monte Carlo results on the anti-ferromagnetic phase of the Hubbard model, it concludes that this impurity solver can capture the main physical features in the strong coupling regime and can be a very useful tool for the LDA （local density approximation） ＋ DMFT studies of the Mort insulators with long range order.

Quantum Monte Carlo study of hard-core bosons in Creutz ladder with zero flux

The quantum phase of hard-core bosons in Creutz ladder with zero flux is studied. For a specific regime of the parameters （tx = tp,ty 〈 0）, the exact ground-state is found analytically, which is a dimerized insulator with one electron bound in each rung of the ladder. For the case tx, ty, tp 〉 0, the system is exactly studied using quantum Monte Carlo （QMC） method without a sign problem. It is found that the system is a Mott insulator for small tp and a quantum phase transition to a superfluid phase is driven by increasing tp. The critical t~ is determined precisely by a scaling analysis. Since it is possible that the Creutz ladder is realized experimentally, the theoretical results are interesting to the cold-atom experiments.

Evidence for distortion-induced local electric polarization in α-RuCl_(3)

The spin-orbit assisted Mott insulator α-RuCl_(3) is a prime candidate for material realization of the Kitaev quantum spin liquid.While little attention has been paid to charge degrees of freedom,charge effects,such as electric polarization,may arise in this system.Here,we report distortion-induced local electric polarization in α-RuCl_(3) as evidenced by single-crystal X-ray diffraction,second harmonic generation(SHG)and dielectric measurements.The SHG signal appears at room temperature and develops substantially in the Kitaev paramagnetic state when short-range spin correlations come into play.Despite sizable pyroelectric currents in the Kitaev paramagnetic state,the absence of hysteresis in the polarization-electric field(P-E)points to the shortrange nature of electric polarization.This localized electric polarization is likely the result of distortion-induced charge dimerization,achieved through virtual hopping-induced charge redistribution.In addition,the electric polarization is boosted by short-range spin correlations via spin-phonon coupling in the Kitaev paramagnetic state.Our results emphasize the importance of charge degrees of freedom in α-RuCl_(3),which establish a novel platform to investigate charge effects in Kitaev materials.

Designing topological and correlated 2D magnetic states via superatomic lattice constructions of zirconium dichloride

Magnetic materials could realize the intriguing quantum anomalous Hall effect and metal-to-insulator transition when combined with band topology or electronic correlation,which have broad prospects in quantum information,spintronics,and valleytronics.Here,we propose the approach of designing novel two-dimensional(2D)magnetic states via d-orbital-based superatomic lattices.Specifically,we chose triangular zirconium dichloride disks as superatoms to construct the honeycomb superatomic lattices.Using first-principles calculations,we identified a series of 2D magnetic states with varying sizes of superatoms.We found the non-uniform stoichiometries and geometric effect of superatomic lattice give rise to spin-polarized charges arranged in different magnetic configurations,containing ferromagnetic coloring triangles,antiferromagnetic honeycomb,and ferromagnetic kagome lattices.Attractively,these magnetic states are endowed with nontrivial band topology or strong correlation,forming an ideal Chern insulator or antiferromagnetic Dirac Mott insulator.Our work not only reveals the potential of d-orbital-based superatoms for generating unusual magnetic configurations,but also supplies a new avenue for material engineering at the nanoscale.

Measurement of interacting quantum phases:A band mapping scheme

Band mapping is widely used in various scenarios of cold atom physics to measure the quasi-momentum distribution and band population.However,conventional methods fail in strongly interacting systems.Here we propose and experimentally realize a novel scheme of band mapping that can accurately measure the quasi-momentum of interacting manybody systems.Through an anisotropic control in turning down the threedimensional optical lattice,we can eliminate the effect of interactions on the band mapping process.Then,based on a precise measurement of the quasi-momentum distribution,we introduce the incoherent fraction as a physical quantity that can quantify the degree of incoherence of quantum many-body states.This method enables precise measurement of processes such as the superfluid to Mott insulator phase transition.Additionally,by analyzing the spatial correlation derived from the quasi-momentum of superfluid-Mott insulator phase transitions,we obtain results consistent with the incoherent fraction.Our scheme broadens the scope of band mapping and provides a method for studying quantum many-body problems.

Modulation of electronic state in copper-intercalated 1T-TaS_(2)

Intercalation is an effective method to modify physical properties and induce novel electronic states of transition metal dichalcogenide(TMD)materials.However,it is difficult to reveal the microscopic electronic state evolution in the intercalated TMDs.Here we successfully synthesize the copper-intercalated 1T-TaS_(2) and characterize the structural and electronic modification combining resistivity measurements,atomic-resolution scanning transmission electron microscopy(ADF-STEM),and scanning tunneling microscopy(STM).The intercalated Cu atom is determined to be directly below the Ta atom and suppresses the commensurate charge density wave(CCDW)phase.Two specific electronic modulations are discovered in the near-commensurate(NC)CDW phase:the electron doping state near the defective star of Davids(SDs)in metallic domains and the spatial evolution of the Mott gap in insulating domains.Both modulations reveal that intercalated Cu atoms act as a medium to enhance the interaction between intralayer SDs,in addition to the general charge transfer effect.It also solidifies the Mott foundation of the insulating gap in pristine samples.The intriguing electronic evolution in Cu-intercalated 1T-TaS_(2) will motivate further exploration of novel electronic states in the intercalated TMD materials.

Higgs type excitations in cold atom systems

Higgs type excitations are the excitations which give mass to particles. The Higgs type excitations has a critical role both in particle physics and condensed matter physics. In particle physics, the suspected Higgs boson has been found by the Large Hadron Collider （LHC） in 2012. In condensed matter physics, the Higgs type excitations relate to order phase of the system. In this review, we present an overview of recent studies on the Higgs type excitations both in non-interacting and interacting cold atom systems. First, in non-interacting cold atom system, by synthesizing artificial non-Abelian gauge potential, we demonstrate that when a non- Abelian gauge potential is reduced to Abeliau potential, the Abelian part constructs spin-orbit coupling, and the non-Abelian part emerges Higgs excitations. Secondly, the Higgs excitations which are the reputed Higgs amplitude mode in interacting cold atom system are discussed. We review the theoretical model and the experimental detection of Higgs amplitude mode in two dimensional superfluid. The observation of both Higgs type excitations in real experiments are also discussed.