Spectral-weight transfer: Breakdown of low-energy-scale sum rules in correlated systems

Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi2 Sr2 CaCu2 O8 + δ crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O 1s component arising from the Cu-O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu-O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O 1s core-level emission originating from the oxygen ions in the Cu-O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu-O plane in the normal and superconducting phases.

Interplay between Local Moment and Itinerant Magnetism in the Layered Metallic Antiferromagnet TaFe1.14Te3

Two-dimensional antiferromagnets have garnered considerable interest for the next generation of functional spintronics. However, many bulk materials from which two-dimensional antiferromagnets are isolated are limited by their air sensitivity, low ordering temperatures, and insulating transport properties. TaFe1+yTe3 aims to address these challenges with increased air stability, metallic transport, and robust antiferromagnetism. Here, we synthesize TaFe1+yTe3 (y = 0.14), identify its structural, magnetic, and electronic properties, and elucidate the relationships between them. Axial-dependent high-field magnetization measurements on TaFe1.14Te3 reveal saturation magnetic fields ranging between 27 and 30 T with saturation magnetic moments of 2.05-2.12 μB. Magnetotransport measurements confirm that TaFe1.14Te3 is metallic with strong coupling between magnetic order and electronic transport. Angle-resolved photoemission spectroscopy measurements across the magnetic transition uncover a complex interplay between itinerant electrons and local magnetic moments that drives the magnetic transition. We demonstrate the ability to isolate few-layer sheets of TaFe1.14Te3, establishing TaFe1.14Te3 as a potential platform for two-dimensional spintronics.

Observation of Electronic Strong Correlation in VTe_{2}-2sqrt[3]×2sqrt[3] Monolayer

Strong electron correlation under two-dimensional limit is intensely studied in the transition metal dichalcogenides monolayers, mostly within their charge density wave (CDW) states that host a star of David period. Here, by using scanning tunneling microscopy and spectroscopy and density functional theory calculations with on-site Hubbard corrections, we study the VTe_{2} monolayer with a different 2sqrt[3]×2sqrt[3] CDW period. We find that the dimerization of neighboring Te-Te and V-V atoms occurs during the CDW transition, and that the strong correlation effect opens a Mott-like full gap at Fermi energy (E_{F}). We further demonstrate that such a Mott phenomenon is ascribed to the combination of the CDW transition and on-site Coulomb interactions. Our work provides a new platform for exploring Mott physics in 2D materials.

Unrevealing tunable resonant excitons and correlated plasmons and their coupling in new amorphous carbon-like for highly efficient photovoltaic devices

An understanding on roles of excitons and plasmons is important in excitonic solar cells and photovoltaic (PV) technologies. Here, we produce new amorphous carbon (a-C) like films on Indium Tin Oxide (ITO) generating PV cells with efficiency three order of magnitude higher than the existing biomass-derived a-C. The amorphous carbon films are prepared from the bioproduct of palmyra sap with a simple, environmentally friendly, and highly reproducible method. Using spectroscopic ellipsometry, we measure simultaneously complex dielectric function, loss function as well as reflectivity and reveal coexistence of many-body resonant excitons and correlated-plasmons occurring due to strong electronic correlations. X-ray absorption and photoemission spectroscopies show the nature of electron and hole in defining the energy of the excitons and plasmons as a function of N or B doping. Our result shows new a-C like films and the importance of the coupling of resonant excitons and correlated plasmons in determining efficiency of photovoltaic devices.

Stranger than metals

In traditional metals, the temperature ( T ) dependence of electrical resistivity vanishes at low or high T , albeit for different reasons. Here, we review a class of materials, known as “strange” metals, that can violate both of these principles. In strange metals, the change in slope of the resistivity as the mean free path drops below the lattice constant, or as T → 0, can be imperceptible, suggesting continuity between the charge carriers at low and high T . We focus on transport and spectroscopic data on candidate strange metals in an effort to isolate and identify a unifying physical principle. Special attention is paid to quantum criticality, Planckian dissipation, Mottness, and whether a new gauge principle is needed to account for the nonlocal transport seen in these materials.

Role of Surface Termination in the Metal-Insulator Transition of V2O3(0001) Ultrathin Films

Surface termination is known to play an important role in determining the physical properties of materials. It is crucial to know how surface termination affects the metal-insulator transition (MIT) of V₂O₃ films for both fundamental understanding and its applications. By changing growth parameters, we achieved a variety of surface terminations in V₂O₃ films that are characterized by low-energy electron diffraction (LEED) and photoemission spectroscopy techniques. Depending upon the terminations, our results show that MIT can be partially or fully suppressed near the surface region due to the different fillings of the electrons at the surface and subsurface layers and the change of screening length compared to the bulk. Across MIT, a strong redistribution of spectral weight and its transfer from a high-to-low-binding energy regime is observed in a wide energy scale. Our results show that the total spectral weight in the low-energy regime is not conserved across MIT, indicating a breakdown of the "sum rules of spectral weight", signature of a strongly correlated system. Such a change in spectral weight is possibly linked to the change in hybridization, lattice volume (i.e., effective carrier density), and the spin degree of freedom in the system that occurs across MIT. We find that MIT in this system is strongly correlation-driven, where the electron-electron interactions play a pivotal role. Moreover, our results provide better insight into the understanding of the electronic structure of strongly correlated systems and highlight the importance of accounting for surface effects during interpretation of the physical property data mainly using surface-sensitive probes, such as surface resistivity.

Ultrafast melting and recovery of collective order in the excitonic insulator Ta2NiSe5

The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system's low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature.

Momentum-resolved visualization of electronic evolution in doping a Mott insulator

High temperature superconductivity in cuprates arises from doping a parent Mott insulator by electrons or holes. A central issue is how the Mott gap evolves and the low-energy states emerge with doping. Here we report angle-resolved photoemission spectroscopy measurements on a cuprate parent compound by sequential in situ electron doping. The chemical potential jumps to the bottom of the upper Hubbard band upon a slight electron doping, making it possible to directly visualize the charge transfer band and the full Mott gap region. With increasing doping, the Mott gap rapidly collapses due to the spectral weight transfer from the charge transfer band to the gapped region and the induced low-energy states emerge in a wide energy range inside the Mott gap. These results provide key information on the electronic evolution in doping a Mott insulator and establish a basis for developing microscopic theories for cuprate superconductivity.

How a Mott insulating state evolves into a conducting or superconducting state is a central issue in doping a Mott insulator and important to understand the physics in high temperature cuprate superconductors. Here, the authors visualize the electronic structure evolution of a Mott insulator within the full Mott gap region and address the fundamental issues.

Direct Visualization of Ambipolar Mott Transition in Cuprate CuO_{2} Planes

Identifying the essence of doped Mott insulators is one of the major outstanding problems in condensed matter physics and the key to understanding the high-temperature superconductivity in cuprates. We report real space visualization of Mott insulator-metal transition in Sr_{1-x}La_{x}CuO_{2+y} cuprate films that cover both the electron- and hole-doped regimes. Tunneling conductance measurements directly on the copper-oxide (CuO_{2}) planes reveal a systematic shift in the Fermi level, while the fundamental Mott-Hubbard band structure remains unchanged. This is further demonstrated by exploring the atomic-scale electronic response of CuO_{2} to substitutional dopants and intrinsic defects in a sister compound Sr_{0.92}Nd_{0.08}CuO_{2}. The results may be better explained in the framework of self-modulation doping, similar to that in semiconductor heterostructures, and form a basis for developing any microscopic theories for cuprate superconductivity.

Modulation of Manganite Nanofilm Properties Mediated by Strong Influence of Strontium Titanate Excitons

Hole-doped perovskite manganites have attracted much attention because of their unique optical, electronic, and magnetic properties induced by the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, a comprehensive investigation of the optical, electronic, and magnetic properties of La_0.7Sr_0.3MnO₃ thin films on SrTiO₃ (LSMO/STO) and other substrates is conducted using a combination of temperature-dependent transport, spectroscopic ellipsometry, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. A significant difference in the optical property of LSMO/STO that occurs even in thick (87.2 nm) LSMO/STO from that of LSMO on other substrates is discovered. Several excitonic features are observed in thin film nanostructure LSMO/STO at ∼4 eV, which could be attributed to the formation of anomalous charged excitonic complexes. On the basis of the spectral weight transfer analysis, anomalous excitonic effects from STO strengthen the electronic correlation in LSMO films. This results in the occurrence of optical spectral changes related to the intrinsic Mott-Hubbard properties in manganites. We find that while lattice strain from the substrate influences the optical properties of the LSMO thin films, the coexistence of strong electron-electron (e-e) and electron-hole (e-h) interactions which leads to the resonant excitonic effects from the substrate plays a much more significant role. Our result shows that the onset of anomalous excitonic dynamics in manganite oxides may potentially generate new approaches in manipulating exciton-based optoelectronic applications.

Realization of a Hole-Doped Mott Insulator on a Triangular Silicon Lattice

The physics of doped Mott insulators is at the heart of some of the most exotic physical phenomena in materials research including insulator-metal transitions, colossal magnetoresistance, and high-temperature superconductivity in layered perovskite compounds. Advances in this field would greatly benefit from the availability of new material systems with a similar richness of physical phenomena but with fewer chemical and structural complications in comparison to oxides. Using scanning tunneling microscopy and spectroscopy, we show that such a system can be realized on a silicon platform. The adsorption of one-third monolayer of Sn atoms on a Si(111) surface produces a triangular surface lattice with half filled dangling bond orbitals. Modulation hole doping of these dangling bonds unveils clear hallmarks of Mott physics, such as spectral weight transfer and the formation of quasiparticle states at the Fermi level, well-defined Fermi contour segments, and a sharp singularity in the density of states. These observations are remarkably similar to those made in complex oxide materials, including high-temperature superconductors, but highly extraordinary within the realm of conventional sp-bonded semiconductor materials. It suggests that exotic quantum matter phases can be realized and engineered on silicon-based materials platforms.

Tunable inverted gap in monolayer quasi-metallic MoS2 induced by strong charge-lattice coupling

Polymorphism of two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS₂) exhibit fascinating optical and transport properties. Here, we observe a tunable inverted gap (~0.50 eV) and a fundamental gap (~0.10 eV) in quasimetallic monolayer MoS₂. Using spectral-weight transfer analysis, we find that the inverted gap is attributed to the strong charge–lattice coupling in two-dimensional transition metal dichalcogenides (2D-TMDs). A comprehensive experimental study, supported by theoretical calculations, is conducted to understand the transition of monolayer MoS₂ on gold film from trigonal semiconducting 1H phase to the distorted octahedral quasimetallic 1T’ phase. We clarify that electron doping from gold, facilitated by interfacial tensile strain, is the key mechanism leading to its 1H–1T’ phase transition, thus resulting in the formation of the inverted gap. Our result shows the importance of charge–lattice coupling to the intrinsic properties of the inverted gap and polymorphism of MoS₂, thereby unlocking new possibilities for 2D-TMD-based device fabrication.

MoS₂ exhibits multiple electronic properties associated with different crystal structures. Here, the authors observe inverted and fundamental gaps through a designed annealing-based strategy, to induce a semiconductor-to-metal phase transition in monolayer-MoS₂ on Au, facilitated by interfacial strain and electron transfer from Au to MoS₂.

Tunable and low-loss correlated plasmons in Mott-like insulating oxides

Plasmonics has attracted tremendous interests for its ability to confine light into subwavelength dimensions, creating novel devices with unprecedented functionalities. New plasmonic materials are actively being searched, especially those with tunable plasmons and low loss in the visible–ultraviolet range. Such plasmons commonly occur in metals, but many metals have high plasmonic loss in the optical range, a main issue in current plasmonic research. Here, we discover an anomalous form of tunable correlated plasmons in a Mott-like insulating oxide from the Sr_1−xNb_1−yO_3+δ family. These correlated plasmons have multiple plasmon frequencies and low loss in the visible–ultraviolet range. Supported by theoretical calculations, these plasmons arise from the nanometre-spaced confinement of extra oxygen planes that enhances the unscreened Coulomb interactions among charges. The correlated plasmons are tunable: they diminish as extra oxygen plane density or film thickness decreases. Our results open a path for plasmonics research in previously untapped insulating and strongly-correlated materials.

Conventional plasmons in metals often suffer from high plasmonic loss in the optical range. Here, the authors report a distinct form of tunable correlated plasmons in Mott-like insulating Sr_1−xNbO_3+δ films, with multiple plasmon frequencies and low loss in the visible-ultraviolet range.

Coexistence of Midgap Antiferromagnetic and Mott States in Undoped, Hole- and Electron-Doped Ambipolar Cuprates

We report the first observation of the coexistence of a distinct midgap state and a Mott state in undoped and their evolution in electron and hole-doped ambipolar Y_{0.38}La_{0.62}(Ba_{0.82}La_{0.18})_{2}Cu_{3}O_{y} films using spectroscopic ellipsometry and x-ray absorption spectroscopies at the O K and Cu L_{3,2} edges. Supported by theoretical calculations, the midgap state is shown to originate from antiferromagnetic correlation. Surprisingly, while the magnetic state collapses and its correlation strength weakens with dopings, the Mott state in contrast moves toward a higher energy and its correlation strength increases. Our result provides important clues to the mechanism of electronic correlation strengths and superconductivity in cuprates.

Defects, Disorder, and Strong Electron Correlations in Orbital Degenerate, Doped Mott Insulators

We elucidate the effects of defect disorder and e-e interaction on the spectral density of the defect states emerging in the Mott-Hubbard gap of doped transition-metal oxides, such as Y(1-x)Ca(x)VO(3). A soft gap of kinetic origin develops in the defect band and survives defect disorder for e-e interaction strengths comparable to the defect potential and hopping integral values above a doping dependent threshold; otherwise only a pseudogap persists. These two regimes naturally emerge in the statistical distribution of gaps among different defect realizations, which turns out to be of Weibull type. Its shape parameter k determines the exponent of the power-law dependence of the density of states at the chemical potential (k-1) and hence distinguishes between the soft gap (k≥2) and the pseudogap (k<2) regimes. Both k and the effective gap scale with the hopping integral and the e-e interaction in a wide doping range. The motion of doped holes is confined by the closest defect potential and the overall spin-orbital structure. Such a generic behavior leads to complex nonhydrogenlike defect states that tend to preserve the underlying C-type spin and G-type orbital order and can be detected and analyzed via scanning tunneling microscopy.

Electronic screening-enhanced hole pairing in two-leg spin ladders studied by high-resolution resonant inelastic x-ray scattering at Cu M edges

We study the electronic screening mechanisms of the effective Coulomb on-site repulsion in hole-doped Sr(14)Cu(24)O(41) compared to undoped La(6)Ca(8)Cu(24)O(41) using polarization dependent high-resolution resonant inelastic x-ray scattering at Cu M edges. By measuring the energy of the effective Coulomb on-site repulsion and the spin excitations, we estimate superexchange and hopping matrix element energies along rungs and legs, respectively. Interestingly, hole doping locally screens the Coulomb on-site repulsion reducing it by as much as 25%. We suggest that the increased ratio of the electronic kinetic to the electronic correlation energy contributes to the local superexchange mediated pairing between holes.

Oxygen-driven anisotropic transport in ultra-thin manganite films

Transition metal oxides have a range of unique properties due to coupling of charge, spin, orbital and lattice degrees of freedom and nearly degenerate multiple ground states. These properties make them interesting for applications and for fundamental investigations. Here we report a new phase with abnormal transport anisotropy in La(0.7)Sr(0.3)MnO3 ultra-thin films under large tensile strain. This anisotropy is absent in films under smaller tensile strain or compressive strain. Furthermore, thickness and magnetic-field-dependent experiments suggest that the tensile-strain-induced two-dimensional character is crucial for the observed phenomena. X-ray absorption spectroscopy results indicate that this anisotropy is likely driven by O 2p orbital, which hybridizes with Mn 3d. Ab initio calculations confirm this result. Our results may help to understand the anisotropic transport behaviour observed in other systems.

Numerical solution of the t-J model with random exchange couplings in d=∞ dimensions

To explore the nature of the metallic state near the transition to a Mott insulator, we investigate the t-J model with random exchange interaction in d=∞ dimensions. A numerically exact solution is obtained by an extension of the continuous-time quantum Monte Carlo method to the case of a vector bosonic field coupled to a local spin. We show that the paramagnetic solution near the Mott insulator describes an incoherent metal with a residual moment, and that single-particle excitations produce an additional band, which is separated from the Mott-Hubbard band.

Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates

In strongly correlated systems the electronic properties at the Fermi energy (E(F)) are intertwined with those at high-energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high-energy scale physics associated with Mott-like excitations (|E-E(F)|>1 eV) is involved in the condensate formation. Here, we report the interplay between the many-body high-energy CuO(2) excitations at 1.5 and 2 eV, and the onset of HTSC. This is revealed by a novel optical pump-supercontinuum-probe technique that provides access to the dynamics of the dielectric function in Bi(2)Sr(2)Ca(0.92)Y(0.08)Cu(2)O(8+δ) over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (T(c)).

Mottness collapse and T-linear resistivity in cuprate superconductors

Central to the normal state of cuprate high-temperature superconductors is the collapse of the pseudo-gap, briefly reviewed here, at a critical point and the subsequent onset of the strange metal characterized by a resistivity that scales linearly with temperature. A possible clue to the resolution of this problem is the inter-relation between two facts: (i) a robust theory of T-linear resistivity resulting from quantum criticality requires an additional length scale outside the standard one-parameter scaling scenario and (ii) breaking the Landau correspondence between the Fermi gas and an interacting system with short-range repulsions requires non-fermionic degrees. We show that a low-energy theory of the Hubbard model that correctly incorporates dynamical spectral weight transfer has the extra degrees of freedom needed to describe this physics. The degrees of freedom that mix into the lower band as a result of dynamical spectral weight transfer are shown to either decouple beyond a critical doping, thereby signalling Mottness collapse, or unbind above a critical temperature, yielding strange metal behaviour characterized by T-linear resistivity.

Dynamically generated Mott gap from holography

In the fermionic sector of top-down approaches to holographic systems, one generically finds that the fermions are coupled to gravity and gauge fields in a variety of ways, beyond minimal coupling. In this Letter, we take one such interaction-a Pauli, or dipole, interaction-and study its effects on fermion correlators. We find that this interaction modifies the fermion spectral density in a remarkable way. As we change the strength of the interaction, we find that spectral weight is transferred between bands, and beyond a critical value, a gap emerges in the fermion density of states. A possible interpretation of this bulk interaction then is that it drives the dynamical formation of a (Mott) gap, in the absence of continuous symmetry breaking.

Composite-fermion theory for pseudogap, Fermi arc, hole pocket, and non-Fermi liquid of underdoped cuprate superconductors

We propose that an extension of the exciton concept to doped Mott insulators offers a fruitful insight into challenging issues of the copper oxide superconductors. In our extension, new fermionic excitations called cofermions emerge in conjunction to generalized excitons. The cofermions hybridize with conventional quasiparticles. Then a hybridization gap opens, and is identified as the pseudogap observed in the underdoped cuprates. The resultant Fermi-surface reconstruction naturally explains a number of unusual properties of the underdoped cuprates, such as the Fermi arc and/or pocket formation.

X-ray absorption spectra reveal the inapplicability of the single-band Hubbard model to overdoped cuprate superconductors

X-ray absorption spectra on the overdoped high-temperature superconductors Tl2Ba2CuO(6+delta) and La(2-x)SrxCuO(4+/-delta) reveal a striking departure in the electronic structure from that of the underdoped regime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygen K edge, while the lowest-energy prepeak gains less intensity than expected above p approximately 0.21. This suggests a breakdown of the Zhang-Rice singlet approximation and a loss of correlation effects or a significant shift in the most fundamental parameters of the system, rendering single-band Hubbard models inapplicable. Such fundamental changes suggest that the overdoped regime may offer a distinct route to understanding in the cuprates.

Quantum critical point at finite doping in the 2D Hubbard model: a dynamical cluster quantum Monte Carlo study

We explore the Matsubara quasiparticle fraction and the pseudogap of the two-dimensional Hubbard model with the dynamical cluster quantum Monte Carlo method. The character of the quasiparticle fraction changes from non-Fermi-liquid, to marginal Fermi liquid, to Fermi liquid as a function of doping, indicating the presence of a quantum critical point separating non-Fermi-liquid from Fermi-liquid character. Marginal Fermi-liquid character is found at low temperatures at a very narrow range of doping where the single-particle density of states is also symmetric. At higher doping the character of the quasiparticle fraction is seen to cross over from Fermi liquid to marginal Fermi liquid as the temperature increases.

Visualizing pair formation on the atomic scale and the search for the mechanism of superconductivity in high-T(c) cuprates

We have developed several new experimental techniques, based on the scanning tunneling microscope, to visualize the process of pair formation on the atomic scale and to probe with high precision what controls the strength of pairing in high-T(c) cuprate superconductor compounds. These new experiments provide evidence that pairing in these exotic superconductors occurs above the bulk transition temperature and in nanoscale regions with sizes of 1-3 nm. The high temperature nucleation and proliferation of these nanoscale puddles have a strong connection to the temperature-doping phase diagram of these superconductors. On average we have found that the pairing gap Δ and the temperature at which they first nucleate T(p) follow the simple relation: 2Δ/k(B)T(p)∼8. Moreover, the variations of the pairing strength on the nanoscale can be examined to find microscopic clues to the mechanism of pairing. Specifically, we have found evidence that suggests that strong electronic correlation, as opposed to coupling of electrons to bosons, is responsible for the pairing mechanism in the cuprates. Surprisingly, we have found that nanoscale measurements of electronic correlations in the normal state (at temperatures as high as twice T(c)) can be used to predict the strength of the local pairing interaction at low temperatures.

Hidden charge 2e Boson in doped Mott insulators

We construct the low-energy theory of a doped Mott insulator, such as the high-temperature superconductors, by explicitly integrating over the degrees of freedom far away from the chemical potential. For either hole or electron doping, a charge 2e bosonic field emerges at low energy. The charge 2e boson mediates dynamical spectral weight transfer across the Mott gap and creates a new charge e excitation by binding a hole. The result is a bifurcation of the electron dispersion below the chemical potential as observed recently in angle-resolved photoemission on Pb-doped Bi2Sr2CaCu2O8+delta (Pb2212).

Doped Mott insulators are insulators: hole localization in the cuprates

We demonstrate that a Mott insulator lightly doped with holes is still an insulator at low temperature even without disorder. Hole localization obtains because the chemical potential lies in a pseudogap which has a vanishing density of states at zero temperature. The energy scale for the pseudogap is set by the nearest-neighbor singlet-triplet splitting. As this energy scale vanishes if transitions, virtual or otherwise, to the upper Hubbard band are not permitted, the fundamental length scale in the pseudogap regime is the average distance between doubly occupied sites. Consequently, the pseudogap is tied to the noncommutativity of the two limits U-->infinity (U the on-site Coulomb repulsion) and L -->infinity (the system size).

Particle-hole asymmetry in doped mott insulators: implications for tunneling and photoemission spectroscopies

We use exact sum rules for the one-particle spectral function to quantify the idea that it is more difficult to add an electron than to extract one in a system with strong local repulsion. Our results explain the striking asymmetry in the tunneling spectra of underdoped cuprates which increases with underdoping. We also propose a novel method, based on ratios of sum rules, to estimate local density variations in inhomogeneous materials. Using a variational approach, we show that the origin of the particle-hole asymmetry lies in the incoherent spectral weight.

Absence of asymptotic freedom in doped mott insulators: breakdown of strong coupling expansions

We show that doped Mott insulators such as the copper-oxide superconductors are asymptotically slaved in that the quasiparticle weight Z near half-filling depends critically on the existence of the high-energy scale set by the upper Hubbard band. In particular, near half-filling, the following dichotomy arises: Z not equal to 0 when the high-energy scale is integrated out but Z=0 in the thermodynamic limit when it is retained. Slavery to the high-energy scale arises from quantum interference between electronic excitations across the Mott gap. Broad spectral features seen in photoemission in the normal state of the cuprates are argued to arise from high-energy slavery.

Pseudogap in doped Mott insulators is the near-neighbor analogue of the Mott gap

We show that the strong-coupling physics inherent to the insulating Mott state in 2D leads to a jump in the chemical potential upon doping and the emergence of a pseudogap in the single-particle spectrum below a characteristic temperature. The pseudogap arises because any singly occupied site not immediately neighboring a hole experiences a maximum energy barrier for transport equal to t(2)/U, t the nearest-neighbor hopping integral and U the on-site repulsion. The resultant pseudogap cannot vanish before each lattice site, on average, has at least one hole as a near neighbor. The ubiquity of this effect in all doped Mott insulators suggests that the pseudogap in the cuprates has a simple origin.

Doping dependence of an n-type cuprate superconductor investigated by angle-resolved photoemission spectroscopy

We present an angle-resolved photoemission doping dependence study of the n-type cuprate superconductor Nd(2-x)Ce(x)CuO(4+/-delta), from the half-filled Mott insulator to the T(c) = 24 K superconductor. In Nd2CuO4, we reveal the charge-transfer band for the first time. As electrons are doped into the system, this feature's intensity decreases with the concomitant formation of near- E(F) spectral weight. At low doping, the Fermi surface is an electron-pocket (with volume approximately x) centered at (pi,0). Further doping leads to the creation of a new holelike Fermi surface (volume approximately 1+x) centered at (pi,pi). These findings shed light on the Mott gap, its doping evolution, as well as the anomalous transport properties of the n-type cuprates.

Spectral weight of the hubbard model through cluster perturbation theory

We calculate the spectral weight of the one- and two-dimensional Hubbard models by performing exact diagonalizations of finite clusters and treating intercluster hopping with perturbation theory. Even with relatively modest clusters (e.g., 12 sites), the spectra thus obtained give an accurate description of the exact results. Spin-charge separation (i.e., an extended spectral weight bounded by singularities dispersing with wave vector) is clearly recognized in the one-dimensional Hubbard model, and so is extended spectral weight in the two-dimensional Hubbard model.