Coherent Quasiparticle Research Articles

Bose–Einstein condensation and superfluidity of trapped polaritons in graphene and quantum wells embedded in a microcavity

The theory for spontaneous coherence of short-lived quasiparticles in two-dimensional excitonic systems is reviewed, in particular, quantum wells (QWs) and graphene layers (GLs) embedded in microcavities. Experiments with polaritons in an optical microcavity have already shown evidence of Bose-Einstein condensation (BEC) in the lowest quantum state in a harmonic trap. The theory of BEC and superfluidity of the microcavity excitonic polaritons in a harmonic potential trap is presented. Along the way, we determine a general method for defining the superfluid fraction in a two-dimensional trap, within the angular momentum representation. We discuss BEC of magnetoexcitonic polaritons (magnetopolaritons) in a QW and GL embedded in an optical microcavity in high magnetic field. It is shown that Rabi splitting in graphene is tunable by the external magnetic field B, while in a QW the Rabi splitting does not depend on the magnetic field in the strong B limit.

Loss of antinodal coherence with a singled-wave superconducting gap leads to two energy scales for underdoped cuprate superconductors

We address the question of the two energy scales in the superconducting state of hole-doped copper-oxide superconductors below the optimal doping level. We show by performing temperature-dependent Raman scattering measurements on ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ that the two energy scales often assigned to two distinct gaps, are both associated with coherent Bogoliubov quasiparticles and can be simply explained by a one single $d$-wave gap and the loss of antinodal coherence. We then establish that the critical temperature ${T}_{c}$ is controlled by the fraction of coherent Fermi surface ${f}_{c}$ around the nodes such as: ${k}_{B}{T}_{c}\ensuremath{\propto}{f}_{c}\ensuremath{\cdot}{\ensuremath{\Delta}}_{max}$, where ${\ensuremath{\Delta}}_{max}$ is the maximum amplitude of the superconducting gap.

Strong correlations and spin-density-wave phase induced by a massive spectral weight redistribution inα-Fe1.06Te

The electronic structure of $\ensuremath{\alpha}{\text{-Fe}}_{1.06}\text{Te}$ is studied with angle-resolved photoemission spectroscopy. We show that there is substantial spectral weight around $\ensuremath{\Gamma}$ and X, and line shapes are intrinsically broad in the paramagnetic state. The magnetic transition is characterized by a massive spectral weight transfer over an energy range as large as the bandwidth, which even exhibits a hysteresis loop that marks the strong first-order transition. Coherent quasiparticles emerge in the magnetically ordered state due to decreased spin fluctuations, which accounts for the change in transport properties from insulating behavior to metallic behavior. Our observation demonstrates that ${\text{Fe}}_{1.06}\text{Te}$ distinguishes itself from other iron-based systems with more local characters and much stronger interactions, and how a spin-density wave is formed in the presence of strong correlation.

Interplane charge dynamics in a valence-bond dynamical mean-field theory of cuprate superconductors

We present calculations of the interplane charge dynamics in the normal state of cuprate superconductors within the valence-bond dynamical mean-field theory. We show that by varying the hole doping, the c-axis optical conductivity and resistivity dramatically change character, going from metallic-like at large doping to insulating-like at low-doping. We establish a clear connection between the behavior of the c-axis optical and transport properties and the destruction of coherent quasiparticles as the pseudogap opens in the antinodal region of the Brillouin zone at low doping. We show that our results are in good agreement with spectroscopic and optical experiments.

Interference of nematic quantum critical quasiparticles: A route to the octet model

Repeated observations of inhomogeneity in cuperate superconductors[1-5] make one immediately question the existance of coherent quasiparticles(qp's) and the applicability of a momentum space picture. Yet, obversations of interference effects[6-9] suggest that the qp's maintain a remarkable coherence under special circumstances. In particular, quasi-particle interference (QPI) imaging using scanning tunneling spectroscopy revealed a highly unusual form of coherence: accumulation of coherence only at special points in momentum space with a particular energy dispersion[5-7]. Here we show that nematic quantum critical fluctuations[10], combined with the known extreme velocity anisotropy[11] provide a natural mechanism for the accumulation of coherence at those special points. Our results raise the intriguing question of whether the nematic fluctuations provide the unique mechanism for such a phenomenon.

Possibility of magnetic-field-induced reconstruction of the Fermi surface in underdoped cuprates: Constraints from infrared magneto-optics

We present an analysis of infrared optical and magneto-optical conductivity data for a range of underdoped cuprate superconductors including ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{y}$ and ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$. In light of recent magneto-oscillation experiments which have been interpreted in terms of Fermi surface reconstruction in magnetic field, we search for far-infrared signatures of field-induced pockets of coherent quasiparticles. Analysis of the conductivity spectra in magnetic field reveals no sign of field-induced Drude-like response, as well as no evidence for modification of the pseudogap. By considering changes of the low-frequency spectral weight, we are able to place limits on the oscillator strength of coherent modes deriving from proposed field-induced pockets in the Fermi surface. In underdoped ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$ we observe a complete suppression of the superfluid density but no evidence for a coherent contribution to the conductivity. Other limits are established for ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{y}$. We further discuss these results in the context of cuprate systems in which stripe order can account for the observed nodal effects but does not lead to the development of antinodal pockets with long-lived quasiparticles.

Coherent particle response of two-dimensional organic conductors upon crossing the Mott transition

The two-dimensional organic charge-transfer salt κ-(BEDT-TTF)2Cu[N(CN)2]-BrxCl1–x is tuned across the metal-insulator transition by varying the Br content. Temperature dependent measurements of the optical conductivity yield information on the electrodynamics of the half-filled correlated electron system. At room temperature a broad incoherent response is observed with no clear Drude peak. As the temperature drops below 50 K, an energy gap develops in the Cl-rich samples which grows to approximately 1000 cm−1 for T → 0. With increasing Br concentration spectral weight shifts into the gap region and eventually fills it up completely. A Drude-like component develops due to the coherent quasiparticles.

Observation of a d-wave nodal liquid in highly underdoped Bi2Sr2CaCu2O8+δ

We use angle resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the antiferromagnetic Mott insulator at zero doping and the superconducting state at larger dopings in Bi_2Sr_2CaCu_2O_{8+\delta}. We find that this state is a nodal liquid whose excitation gap becomes zero only at points in momentum space. Despite exhibiting a resistivity characteristic of an insulator and the absence of coherent quasiparticle peaks, this material has the same gap structure as the d-wave superconductor. We observe a smooth evolution of the spectrum across the insulator-to-superconductor transition, which suggests that high temperature superconductivity emerges when quantum phase coherence is established in a non-superconducting nodal liquid.

Bulk-sensitive laser-ARPES study on the cuprate superconductor [formula omitted

We investigate the bulk superconducting electronic structure in optimally doped YBa2Cu3O7-δ (YBCO) using laser-excited angle-resolved photoemission spectroscopy. The band dispersions with the dx2-y2-wave like gap opening are observed without being hindered by surface state and CuO chain band due to bulk-sensitivity and/or matrix element effect at hν=6.994eV. The momentum-dependence of the superconducting gap exhibits a nonzero minimum of ∼12meV along the (0,0)–(π,π) line. In addition, the unusual suppression of the quasiparticle coherence is observed at T/Tc>0.5–0.7.

Quasiparticle interference in temperature dependent-STM as a probe of superconducting coherence

In this proceeding we explore the behavior of the quasiparticle interference pattern (QPI) of scanning tunneling microscopy in moderately underdoped cuprates as a function of temperature, T. After ensuring a minimal consistency with angle-resolved photoemission spectroscopy (ARPES), we find that the QPI pattern is profoundly sensitive to quasiparticle coherence and that it manifests two energy gap scales. In particular, we show that the superconducting QPI pattern vanishes at the same temperature as that at which the Fermi arcs appear. Experimental support for this conclusion comes from the observation of a nearly dispersionless QPI pattern which appears above Tc in moderately underdoped cuprates. To illustrate the important two energy scale physics we present predictions of the QPI-inferred energy gaps as a function of T in moderately underdoped cuprates. This defines the so-called “Bogoliubov arcs”, which possess an extinction point which is controlled by the size of the superconducting order parameter. A second larger energy scale is obtained by extrapolating the low-energy gap to the antinodes, presuming a simple d-wave shape. The validity of this extrapolation procedure is supported by our analysis of the ARPES gap and reveals the pseudogap energy scale.

Model for the temperature dependence of the quasiparticle interference pattern in the measured scanning tunneling spectra of underdoped cuprate superconductors

In this paper we explore the behavior of the quasiparticle interference (QPI) pattern of scanning tunneling microscopy as a function of temperature $T$, for moderately underdoped cuprates. After ensuring a minimal consistency with angle-resolved photoemission spectroscopy, we find that the QPI pattern is profoundly sensitive to quasiparticle coherence and that it manifests two energy-gap scales. In particular, we find that the superconducting QPI pattern vanishes at the same temperature as that at which the Fermi arcs appear. Experimental support for this conclusion comes from the observation of a nearly dispersionless QPI pattern which has been observed to appear above ${T}_{c}$ in moderately underdoped cuprates. To illustrate the important two energy scale physics we present predictions of the QPI-inferred energy gaps as a function of $T$ for future experiments on moderately underdoped cuprates. This defines the so-called ``Bogoliubov arcs,'' which we find have an extinction point controlled by the size of the superconducting order parameter. This extinction, thus, seems unrelated to the magnetic zone boundary. Moreover, the calculated Bogoliubov arcs persist across the full nodal region, as might be expected, since that is where the superconductivity is thought to be dominant. A second larger energy scale is implied from extrapolation to the antinodes, presuming a simple $d$-wave shape and that the temperatures are not too close to ${T}_{c}$. This corresponds to the pseudogap energy scale in the literature.

Momentum-resolved spectroscopy of correlated metals: A view from dynamical mean field theory

Abstract In this review we discuss how theoretical momentum-resolved many-body spectral functions can help understanding the physics underlying angular resolved photoemission spectra (ARPES). Special focus is set on phenomena induced by electronic Coulomb correlations. Among these effects are transfers of spectral weight, the loss of quasi-particle coherence, and the sensitivity of these phenomena on external parameters, such as temperature or pressure. For the examples of the metallic phases of VO 2 and V 2 O 3 we review results obtained within dynamical mean-field theory, and assess the limits of band-structure approaches. Our discussion emphasizes the need for true many-body techniques even for certain metallic materials. To cite this article: J.M. Tomczak et al., C. R. Physique 10 (2009).

Bandwidth-controlled Mott transition inκ−(BEDT−TTF)2Cu[N(CN)2]BrxCl1−x: Optical studies of correlated carriers

In the two-dimensional organic charge-transfer salts $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]{\mathrm{Br}}_{x}{\mathrm{Cl}}_{1\ensuremath{-}x}$ a systematic variation in the Br content from $x=0$ to 0.9 allows us to tune the Mott transition by increasing the bandwidth. At temperatures below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, an energy gap develops in the Cl-rich samples and grows to approximately $1000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for $T\ensuremath{\rightarrow}0$. With increasing Br concentration spectral weight shifts into the gap region and eventually fills it up completely. As the samples with $x=0.73$, 0.85, and 0.9 become metallic at low temperatures, a Drude-type response develops due to the coherent quasiparticles. Here, the quasiparticle scattering rate shows a ${\ensuremath{\omega}}^{2}$ dependence and the effective mass of the carriers is enhanced in agreement with the predictions for a Fermi liquid. These typical signatures of strong electron-electron interactions are more pronounced for compositions close to the critical value ${x}_{c}\ensuremath{\approx}0.7$, where the metal-to-insulator transition occurs.

Crossover from coherent quasiparticles to incoherent hole carriers in underdoped cuprates

In underdoped cuprates, only a portion of the Fermi surface survives as Fermi arcs due to pseudogap opening. In hole-doped La$_{2}$CuO$_4$, we have deduced the "coherence temperature" $T_{coh}$ of quasi-particles on the Fermi arc above which the broadened leading edge position in angle-integrated photoemission spectra is shifted away from the Fermi level and the quasi-particle concept starts to lose its meaning. $T_{coh}$ is found to rapidly increase with hole doping, an opposite behavior to the pseudogap temperature $T^*$. The superconducting dome is thus located below both $T^*$ and $T_{coh}$, indicating that the superconductivity emerges out of the coherent Fermionic quasi-particles on the Fermi arc. $T_{coh}$ remains small in the underdoped region, indicating that incoherent charge carriers originating from the Fermi arc are responsible for the apparently metallic transport at high temperatures.

Electron beam coherence measurements using diffracted beam interferometry/holography

The intensity and coherence of elastically and inelastically scattered electrons have been studied by the interference of electron-diffracted beams using a method of diffracted beam interferometry/holography (DBI/H). In the interferograms produced, fringes were found to exist from low to high scattering angles. The intensity and coherence of the fringes are useful for understanding the contrast mismatch between experimental and simulated images found in atomic resolution images of crystals produced by transmission electron microscopy (TEM) and annular dark-field (ADF) scanning transmission electron microscopy (STEM). The fringes disappear when the interfering beams are separated from an exact overlay position, which produces a measurement of the beam's lateral coherence and holds promise for measuring the coherence of the respective quasi-particles associated with the energy loss electrons.

Quantum oscillations in an overdoped high-Tc superconductor

As well as their elevated superconducting transition temperatures (Tcs), the copper-oxide superconductors display a variety of novel physical properties that have inspired many theories to explain their electronic ground state. One topic of note is the nature of the metallic phase in these compounds, in particular, how coherent electronic states or quasiparticles emerge from the antiferromagnetic insulator with doping — the addition of trace amounts of impurities. Vignolle et al. report the observation of quantum oscillations, that is, oscillations in the electrical and magnetic response as a function of magnetic field, in the overdoped superconductor Tl2Ba2CuO6+δ. These oscillations point to the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. The measurements confirm the applicability of a generalized Fermi-liquid picture to the overdoped side of the superconducting dome and mean that the important charge carriers involved in superconductivity are essentially similar to those found in normal metals. This paper reports the observation of quantum oscillations in the overdoped superconductor Tl2Ba2CuO6+δ that show the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. These measurements firmly establish the applicability of a generalized Fermi-liquid picture on the overdoped side of the superconducting dome. The nature of the metallic phase in the high-transition-temperature (high-Tc) copper oxide superconductors, and its evolution with carrier concentration, has been a long-standing mystery1. A central question is how coherent electronic states, or quasiparticles, emerge from the antiferromagnetic insulator with doping. Recent quantum oscillation experiments on lightly doped copper oxides have shown evidence for small pockets of Fermi surface2,3,4,5, the formation of which has been associated with the opening of the pseudogap—an anisotropic gap in the normal state excitation spectrum of unknown origin1. As the doping is increased, experiments suggest that the full Fermi surface is restored6,7, although the doping level at which the pseudogap closes and the nature of the electronic ground state beyond this point have yet to be determined. Here we report the observation of quantum oscillations in the overdoped superconductor Tl2Ba2CuO6+δ that show the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. These measurements confirm that, in overdoped superconducting copper oxides, coherence is established at all Fermi wavevectors, even near the zone boundary where the pseudogap is maximal and electronic interactions are strongest; they also firmly establish the applicability of a generalized Fermi-liquid picture on the overdoped side of the superconducting phase diagram.

ELECTRONIC STRUCTURE OF KINETIC ENERGY DRIVEN CUPRATE SUPERCONDUCTORS

In this paper, we review the low energy electronic structure of the kinetic energy driven d-wave cuprate superconductors. We give a general description of the charge-spin separation fermion-spin theory, where the constrained electron is decoupled as the gauge invariant dressed holon and spin. In particular, we show that under the decoupling scheme, the charge-spin separation fermion-spin representation is a natural representation of the constrained electron defined in a restricted Hilbert space without double electron occupancy. Based on the charge-spin separation fermion-spin theory, we have developed the kinetic energy driven superconducting mechanism, where the superconducting state is controlled by both superconducting gap parameter and quasiparticle coherence. Within this kinetic energy driven superconductivity, we have discussed the low energy electronic structure of the single layer and bilayer cuprate superconductors in both superconducting and normal states, and qualitatively reproduced all main features of the angle-resolved photoemission spectroscopy measurements on the single layer and bilayer cuprate superconductors. We show that the superconducting state in cuprate superconductors is the conventional Bardeen-Cooper-Schrieffer like with the d-wave symmetry, so that the basic Bardeen-Cooper-Schrieffer formalism with the d-wave gap function is still valid in discussions of the low energy electronic structure of cuprate superconductors, although the pairing mechanism is driven by the kinetic energy by exchanging spin excitations. We also show that the well-pronounced peak-dip-hump structure of the bilayer cuprate superconductors in the superconducting state and double-peak structure in the normal state are mainly caused by the bilayer splitting.

Quasiparticles, coherence, and nonlinearity: Exact simulations of rf spectroscopy of strongly interacting one-dimensional Fermi gases

We consider rf spectroscopy of ultracold Fermi gases by exact simulations of the many-body state and the coherent dynamics in one dimension. Deviations from the linear response sum rule result are found to suppress the pairing contribution to the rf line shifts. We compare the coherent rotation and quasiparticle descriptions of rf spectroscopy which are analogous to NMR experiments in superfluid $^{3}\mathrm{He}$ and tunneling in solids, respectively. We suggest that rf spectroscopy in ultracold gases provides an interesting crossover between these descriptions that could be used for studying decoherence in quantum measurement, in the context of many-body quantum states.

Coherence and metamagnetism in the two-dimensional Kondo lattice model

We report the results of dynamical mean-field calculations for the metallic Kondo lattice model subject to an applied magnetic field. High-quality spectral functions reveal that the picture of rigid, hybridized bands, which are Zeeman shifted in proportion to the field strength, is qualitatively correct. We find evidence of a zero-temperature magnetization plateau whose onset coincides with the chemical potential entering the spin up hybridization gap. The plateau appears at the field scale predicted by a (static) large-$N$ mean-field theory and has a magnetization value consistent with that of $x=1\ensuremath{-}{n}_{c}$ spin-polarized heavy holes, where ${n}_{c}l1$ is the conduction band filling of the noninteracting system. We argue that the emergence of the plateau at a low temperature marks the onset of quasiparticle coherence.

Fermi-arc superconductivity, pseudogap and charge order in Bi 2Sr 2CaCu 2O 8+δ

We report STM/STS observations on the 4 a×4 a charge order in the pseudogap (PG) and superconducting (SC) states of Bi 2Sr 2CaCu 2O 8+ δ , and suggest that the charge order is associated with incoherent quasiparticle or pair states around the antinodal fermi surface (FS), which are also responsible for the PG, and it can coexist with the superconductivity caused by the pairing of coherent quasiparticles on the nodal fermi arc. We also suggest that the nanometer-scale gap inhomogeneity in the SC state, reported previously [Pan, et al., Nature 413 (2001) 282; Mommo, et al., J. Phys. Soc. Jpn. 74 (2005) 2400; Hashimoto, et al., Phys. Rev. B74 (2006) 064508] arises from that in the PG state, which occurs around the antinodal FS.