Suppression Of Spectral Weight Research Articles

Electron Glass Phase with Resilient Zhang-Rice Singlets in LiCu_{3}O_{3}.

LiCu_{3}O_{3} is an antiferromagnetic mixed valence cuprate where trilayers of edge-sharing Cu(II)O (3d^{9}) are sandwiched in between planes of Cu(I) (3d^{10}) ions, with Li stochastically substituting Cu(II). Angle-resolved photoemission spectroscopy (ARPES) and density functional theory reveal two insulating electronic subsystems that are segregated in spite of sharing common oxygen atoms: a Cu d_{z^{2}}/O p_{z} derived valence band (VB) dispersing on the Cu(I) plane, and a Cu 3d_{x^{2}-y^{2}}/O 2p_{x,y} derived Zhang-Rice singlet (ZRS) band dispersing on the Cu(II)O planes. First-principle analysis shows the Li substitution to stabilize the insulating ground state, but only if antiferromagnetic correlations are present. Li further induces substitutional disorder and a 2D electron glass behavior in charge transport, reflected in a large 530meV Coulomb gap and a linear suppression of VB spectral weight at E_{F} that is observed by ARPES. Surprisingly, the disorder leaves the Cu(II)-derived ZRS largely unaffected. This indicates a local segregation of Li and Cu atoms onto the two separate corner-sharing Cu(II)O_{2} sub-lattices of the edge-sharing Cu(II)O planes, and highlights the ubiquitous resilience of the entangled two hole ZRS entity against impurity scattering.

Signature of Kondo hybridisation with an orbital-selective Mott phase in 4d Ca2−xSrxRuO4

AbstractThe heavy fermion state with Kondo-hybridisation (KH), usually manifested in f-electron systems with lanthanide or actinide elements, was recently discovered in several 3d transition metal compounds without f-electrons. However, KH has not yet been observed in 4d/5d transition metal compounds, since more extended 4d/5d orbitals do not usually form flat bands that supply localised electrons appropriate for Kondo pairing. Here, we report a substitution- and temperature-dependent angle-resolved photoemission study on 4d Ca2−xSrxRuO4, which shows the signature of KH. We observed a spectral weight transfer in the γ-band, reminiscent of an orbital-selective Mott phase (OSMP). The Mott localised γ-band induces the KH with an itinerant β-band, resulting in spectral weight suppression around the Fermi level. Our work demonstrates the evolution of the OSMP with possible KH among 4d electrons, and thereby expands the material boundary of Kondo physics to 4d multi-orbital systems.

Proximity-Induced Novel Ferromagnetism Accompanied with Resolute Metallicity in NdNiO3 Heterostructure.

Employing X‐ray magnetic circular dichroism (XMCD), angle‐resolved photoemission spectroscopy (ARPES), and momentum‐resolved density fluctuation (MRDF) theory, the magnetic and electronic properties of ultrathin NdNiO3 (NNO) film in proximity to ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) layer are investigated. The experimental data shows the direct magnetic coupling between the nickelate film and the manganite layer which causes an unusual ferromagnetic (FM) phase in NNO. Moreover, it is shown the metal–insulator transition in the NNO layer, identified by an abrupt suppression of ARPES spectral weight near the Fermi level (E F), is absent. This observation suggests that the insulating AFM ground state is quenched in proximity to the FM layer. Combining the experimental data (XMCD and AREPS) with the momentum‐resolved density fluctuation calculation (MRDF) reveals a direct link between the MIT and the magnetic orders in NNO systems. This work demonstrates that the proximity layer order can be broadly used to modify physical properties and enrich the phase diagram of RENiO3 (RE = rare‐earth element).

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the temperature dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and double-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-Liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

Charge order from structured coupling in VSe$_2$

Charge order–ubiquitous among correlated materials–is customarily described purely as an instability of the electronic structure. However, the resulting theoretical predictions often do not match high-resolution experimental data. A pertinent case is 1T1T-VSe_22, whose single-band Fermi surface and weak-coupling nature make it qualitatively similar to the Peierls model underlying the traditional approach. Despite this, its Fermi surface is poorly nested, the thermal evolution of its charge density wave (CDW) ordering vectors displays an unexpected jump, and the CDW gap itself evades detection in direct probes of the electronic structure. We demonstrate that the thermal variation of the CDW vectors is naturally reproduced by the electronic susceptibility when incorporating a structured, momentum-dependent electron-phonon coupling, while the evasive CDW gap presents itself as a localized suppression of spectral weight centered above the Fermi level. Our results showcase the general utility of incorporating a structured coupling in the description of charge ordered materials, including those that appear unconventional.

Gap-like feature observed in the non-magnetic topological insulators

Non-magnetic gap at the Dirac point of topological insulators remains an open question in the field. Here, we present angle-resolved photoemission spectroscopy experiments performed on Cr-doped Bi2Se3 and showed that the Dirac point is progressively buried by the bulk bands and a low spectral weight region in the vicinity of the Dirac point appears. These two mechanisms lead to spectral weight suppression region being mistakenly identified as an energy gap in earlier studies. We further calculated the band structure and found that the original Dirac point splits into two nodes due to the impurity resonant states and the energy separation between the nodes is the low density of state region which appears to be like an energy gap in potoemission experiments. We supported our arguments by presenting photoemission experiments carried out with on- and off- resonant photon energies. Our observation resolves the widely debated questions of apparent energy gap opening at the Dirac point without long range ferromagnetic order in topological insulators.

Spectral weight suppression and Fermi arc-like features with strong holographic lattices

We investigate holographic fermions in uni-directional striped phases, where the breaking of translational invariance can be generated either spontaneously or explicitly. We solve the Dirac equation for a probe fermion in the associated background geometry. When the spatial modulation effect becomes sufficiently strong, we see a spectral weight suppression whenever the Fermi surface is larger than the first Brillouin zone. This leads to the gradual disappearance of the Fermi surface along the symmetry breaking direction, in all of the cases we have examined. This effect appears to be a generic consequence of strong inhomogeneities, independently of whether translational invariance is broken spontaneously or explicitly. The resulting Fermi surface is segmented and has features reminiscent of Fermi arcs.

Spectra of correlated many-electron systems: From a one- to a two-particle description

State-of-the-art spectroscopic techniques allow for a comprehensive understanding of one-electron excitations in various physically interesting and/or technologically relevant materials. While for weakly-correlated systems the corresponding one-particle spectral function A(ω, k) contains essentially all information about their physical properties the situation is much more complicated in the presence of strong electronic correlations. In fact, in the latter case different theoretical treatments often lead to very different explanations of the origin of specific features in the spectrum. A typical example is the pseudogap in the cuprates, i.e., the momentum-selective suppression of spectral weight at the Fermi level, which has been related to spin, charge or (d-wave) pairing fluctuations by different authors. This ambiguity about the underlying physical mechanism at work can be overcome by considering two-particle correlation functions as they are able to describe the collective modes of the system and can be also related to certain ground state properties. In this work, we will present different theoretical approaches for analyzing the spectrum of correlated systems by exploiting the information contained in these two-particle correlation functions. For the specific case of the pseudogap these procedures have allowed us (1) to identify antiferromagnetic spin fluctuations as microscopic origin of this spectral feature which (2) can be related to the formation of a resonating valence bond ground state in the system.

High- Tc superconductivity in CaKFe4As4 in absence of nematic fluctuations

We employ polarization-resolved Raman spectroscopy to study multi-band stoichiometric superconductor CaKFe$_4$As$_4$. The B$_{2g}$ symmetry Raman response shows no signatures of Pomeranchuk-like electronic nematic fluctuations which is observed for many other Fe-based superconductors. In the superconducting state, we identify three pair-breaking peaks at 13.8, 16.9 and 21 meV and full spectral weight suppression at low energies. The pair-breaking peak energies in Raman response are about 20% lower than twice the gap energies as measured by single-particle spectroscopy, implying a sub-dominant $d$-wave symmetry interaction. We analyze the superconductivity induced phonon self-energy effects and give an estimation of weak electron-phonon coupling constant $\lambda^\Gamma$=0.0015.

A Novel One-Dimensional Electronic State at IrTe2 Surface

Highly one-dimensional (1D) Fermi sheets are realized at the surface of a layered Ir telluride IrTe2 which exhibits a stripe-type charge and orbital order below ∼280 K. The 1D Fermi sheets appear in the low temperature range where the stripe order is well established. The 1D Fermi sheets are truncated by the bulk Fermi surfaces, and the spectral weight suppression at the Fermi level deviates from the typical Tomonaga–Luttinger behavior. The 1D band runs along the stripe and is accompanied by several branches which can be derived from the quantization in the perpendicular direction.

Revealing correlation effect of Co 3d electrons in La3Co4Sn13 and Ce3Co4Sn13 by infrared spectroscopy study

We report resistivity, specific heat and optical spectroscopy measurements on single crystal samples of and . We observed clear temperature-induced spectral weight suppression below 4000 for both compounds in the conductivity spectra , indicating the progressive formation of gap-like features with decreasing temperature. The suppressed spectral weight transfers mostly to the higher energy region. This observation reflects the presence of the correlation effect in the compounds. We attribute the correlation effect to the Co 3d electrons.

Spectral weight suppression near a metal-insulator transition in a double-layer electron-doped iridate

The perovskite iridates Sr2IrO4 and Sr3Ir2O7 represent novel systems for exploring the electronic structure that is characteristic of Mott insulators upon carrier doping. Using angle-resolved photoemission spectroscopy (ARPES), we reveal a previously unobserved suppression of spectral weight near the Fermi level in the conduction band of very lightly electron-doped (Sr1-xLax)(3)Ir2O7 followed by a loss of coherence at high temperature. The doping and temperature dependence of this suppression suggests a correspondence with the antiferromagnetic Mott state. These results connect (Sr1-xLax)(3)Ir2O7 to other doped Mott insulators and add to the growing evidence of universal physics in these systems.

Absence of a Large Superconductivity-Induced Gap in Magnetic Fluctuations of Sr_{2}RuO_{4}.

Inelastic neutron scattering experiments on Sr_{2}RuO_{4} determine the spectral weight of the nesting induced magnetic fluctuations across the superconducting transition. There is no observable change at the superconducting transition down to an energy of ∼0.35 meV, which is well below the 2Δ values reported in several tunneling experiments. At this and higher energies magnetic fluctuations clearly persist in the superconducting state. Only at energies below ∼0.3 meV can evidence for partial suppression of spectral weight in the superconducting state be observed. This strongly suggests that the one-dimensional bands with the associated nesting fluctuations do not form the active, highly gapped bands in the superconducting pairing in Sr_{2}RuO_{4}.

Spectral weight suppression in response functions of ultracold fermion-boson mixtures

We study the dynamical response of ultracold fermion-boson mixture in the Bogoliubov regime, where the interactions between fermionic impurities and bosonic excitations (phonons) are described by an effective Frohlich model under the Bogoliubov approximation. A characteristic suppression of the single-particle spectral weight is found in the small momentum region where the impurity band and phonon mode intersect. Using diagrammatic technique we compute the Bragg spectra as well as the momentum dependent force-force correlation function. We fnd that both of them are heavily affected by the spectral weight suppression effect at low impurity densities in both 1D and 2D systems. We show that the the spectral weight suppression feature in Bragg spectra, which was previously found in the quantum Monte Carlo simulations and which cannot be recovered by the random phase approximation, can be accurately reproduced with the help of vertex corrections.

Large power factor and anomalous Hall effect and their correlation with observed linear magneto resistance in Co-doped Bi2Se3 3D topological insulator

Magnetoresistance (MR), thermo power, magnetization and Hall effect measurements have been performed on Co-doped Bi2Se3 topological insulators. The undoped sample shows that the maximum MR as a destructive interference due to a π-Berry phase leads to a decrease of MR. As the Co is doped, the linearity in MR is increased. The observed MR of Bi2Se3 can be explained with the classical model. The low temperature MR behavior of Co doped samples cannot be explained with the same model, but can be explained with the quantum linear MR model. Magnetization behavior indicates the establishment of ferromagnetic ordering with Co doping. Hall effect data also supports the establishment of ferromagnetic ordering in Co-doped Bi2Se3 samples by showing the anomalous Hall effect. Furthermore, when spectral weight suppression is insignificant, Bi2Se3 behaves as a dilute magnetic semiconductor. Moreover, the maximum power factor is observed when time reversal symmetry (TRS) is maintained. As the TRS is broken the power factor value is decreased, which indicates that with the rise of Dirac cone above the Fermi level the anomalous Hall effect and linearity in MR increase and the power factor decreases.

Holographic Lifshitz fermions and exponentially suppressed spectral weight

The absence of fixed momentum excitations in a theory with Lifshitz scale invariance gives rise to exponential suppression of spectral weight in the low-frequency limit. In the holographic dual, this suppression arises as a consequence of a tunneling barrier that decouples the horizon from the boundary. We compute the spin-1/2 holographic Green's function and show that the form of the barrier is identical to that of the scalar case. We furthermore demonstrate that the suppression factor is universal in the $\hat\omega\to0$ limit where $\hat\omega=\omega/|\vec k|^z$. In particular, it depends only on $\hat\omega$ and the critical exponent $z$, and is independent of scaling dimension and spin.

Temperature-dependent Fermi surface of the misfit cobaltite[Bi2Sr2O4]0.51CoO2: A comparison with[Bi2Ba2O4]0.50CoO2

Angle-resolved photoemission spectroscopy has been performed to investigate the Fermi surface (FS) and the electronic structure of misfit cobaltites, by comparing ${[{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{O}}_{4}]}_{p}{\mathrm{CoO}}_{2}$ with $[{\mathrm{Bi}}_{2}{\mathrm{Ba}}_{2}{\mathrm{O}}_{4}]{}_{0.50}{\mathrm{CoO}}_{2}$. In $[{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{O}}_{4}]{}_{0.51}{\mathrm{CoO}}_{2}$, we observed a suppression of spectral weight near the Fermi energy. Nevertheless, the FS can be mapped out by measuring the photoelectron intensity distribution. The FS size changes with temperature. Additionally, results show that the electronic band structure at a higher binding energy was changed by changing the sample temperature. Our results provide insight into the characteristics of the pseudogap state in the misfit-layered cobaltites. The unusual properties are probably related to the local deformations of the incommensurate structure.

Neutron scattering studies of spin-phonon hybridization and superconducting spin gaps in the high-temperature superconductorLa2−x(Sr,Ba)xCuO4

We present time-of-fight neutron-scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2-x}Sr_{x}CuO_{4}$ (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional (3D) commensurate long range antiferromagnetic order, for x $\leq$ 0.02, to two dimensional (2D) incommensurate antiferromagnetism co-existing with superconductivity for x $\geq$ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clear resonant enhancement of the inelastic scattering coincident with the low energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore we show that the low temperature, low energy magnetic spectral weight is substantially larger for samples with non-superconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO.

Electronic structure of the titanium-based oxypnictide superconductorBa0.95Na0.05Ti2Sb2Oand direct observation of its charge density wave order

${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$ is a titanium-based oxypnictide superconductor with possible density wave order. We have performed high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy (STM) studies on ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$. Our STM studies find a charge density wave (CDW) order with the wave vector of ($\ensuremath{\pi}, \ensuremath{\pi}$). The electronic structure shows both multiorbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surfaces are well nested along the ($\ensuremath{\pi}, \ensuremath{\pi}$) direction, which might drive the CDW transition. Likely due to the weak strength of the CDW order, we do not observe a leading-edge gap. Instead, we observe an overall spectral weight enhancement with decreasing temperature, but the rate of this enhancement is abruptly reduced or diminished in the CDW state, suggesting the suppression of spectral weight or opening of a partial CDW gap. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$.

Bifundamental superfluids from holography

We study the holographic dual of a (2+1)-dimensional s-wave superfluid that breaks an abelian U(1) x U(1) global symmetry group to the diagonal U(1)_V. The model is inspired by Sen's tachyonic action, and the operator that condenses transforms in the bifundamental representation of the symmetry group. We focus on two configurations: the first one describes a marginal operator, and the phase diagram at finite temperature contains a first or a second order phase transition, depending on the parameters that determine the theory. In the second model the operator is relevant and the finite temperature transitions are always second order. In the latter case the conductivity for the current associated to the broken symmetry shows quasiparticle excitations at low temperatures, with mass given by the width of the superconducting gap. The suppression of spectral weight at low frequencies is also observed in the conductivity associated to the conserved symmetry, for which the DC value decreases as the temperature is reduced.