Coherent Quasiparticle Research Articles

Coherent quasiparticles with a small fermi surface in lightly doped Sr(3)Ir(2)O(7).

We characterize the electron doping evolution of (Sr_{1-x}La_{x})_{3}Ir_{2}O_{7} by means of angle-resolved photoemission. Concomitant with the metal insulator transition around x≈0.05 we find the emergence of coherent quasiparticle states forming a closed small Fermi surface of volume 3x/2, where x is the independently measured La concentration. The quasiparticle weight Z remains large along the entire Fermi surface, consistent with the moderate renormalization of the low-energy dispersion, and no pseudogap is observed. This indicates a conventional, weakly correlated Fermi liquid state with a momentum independent residue Z≈0.5 in lightly doped Sr_{3}Ir_{2}O_{7}.

Absence of coherent peaks in a Z2 fractionalized BCS superconducting state

We explore a Z2 fractionalized Bardeen–Cooper–Schrieffer (BCS) superconducting state, which is a minimal extension of usual BCS framework. It is found that this state has similar thermal and transport properties, but its single-particle feature strongly deviates from the coherent quasiparticle behavior of the classic/conventional BCS superconducting state. The fingerprint of such Z2 BCS state is the absence of the BCS coherent peaks and instead a kink in the local density of state occurs, which in principle could be probed by scanning tunneling microscopy or point-contact spectroscopy experiments. The corresponding exactly soluble models that realize the desirable Z2 fractionalized BCS state are presented. In addition, we also study the extended t–U–J model by using Z2 slave-spin representation and find that the Z2 BCS state may exist when the paring structure is fully gapped or has nodes. The prototypical wave-function of such a Z2 BCS state is also proposed, which could be taken as trial wave-function in current numerical techniques. Furthermore, the pairing mechanism of Z2 BCS state is argued from both weak and strong coupling perspective. The present work may be helpful to further study the unconventional superconductivity and its relation to non-Fermi liquids.

Nature of strong hole pairing in doped Mott antiferromagnets.

Cooper pairing instability in a Fermi liquid is well understood by the BCS theory, but pairing mechanism for doped Mott insulators still remains elusive. Previously it has been shown by density matrix renormalization group (DMRG) method that a single doped hole is always self-localized due to the quantum destructive interference of the phase string signs hidden in the t-J ladders. Here we report a DMRG investigation of hole binding in the same model, where a novel pairing-glue scheme beyond the BCS realm is discovered. Specifically, we show that, in addition to spin pairing due to superexchange interaction, the strong frustration of the phase string signs on the kinetic energy gets effectively removed by pairing the charges, which results in strong binding of two holes. By contrast, if the phase string signs are “switched off” artificially, the pairing strength diminishes significantly even if the superexchange coupling remains the same. In the latter, unpaired holes behave like coherent quasiparticles with pairing drastically weakened, whose sole origin may be attributed to the resonating-valence-bond (RVB) pairing of spins. Such non-BCS pairing mechanism is therefore beyond the RVB picture and may shed important light on the high-Tc cuprate superconductors.

Doublon-holon binding, Mott transition, and fractionalized antiferromagnet in the Hubbard model

We argue that the binding between doubly occupied (doublon) and empty (holon) sites governs the incoherent excitations and plays a key role in the Mott transition in strongly correlated Mott-Hubbard systems. We construct a new saddle point solution with doublon-holon binding in the Kotliar-Ruckenstein slave-boson functional integral formulation of the Hubbard model. On the half-filled honeycomb lattice and square lattice, the ground state is found to exhibit a continuous transition from the paramagnetic semimetal/metal to an antiferromagnetic ordered Slater insulator with coherent quasiparticles at $U_{c1}$, followed by a Mott transition into an electron-fractionalized AF$^*$ phase without coherent excitations at $U_{c2}$. Such a phase structure appears generic of bipartite lattices without frustration. We show that doublon-holon binding unites the three important ideas of strong correlation: the coherent quasiparticles, the incoherent Hubbard bands, and the deconfined Mott insulator.

Optical conductivity of a two-dimensional metal at the onset of spin-density-wave order

We consider the optical conductivity of a clean two-dimensional metal near a quantum spin-density-wave transition. Critical magnetic fluctuations are known to destroy fermionic coherence at "hot spots" of the Fermi surface but coherent quasiparticles survive in the rest of the Fermi surface. A large part of the Fermi surface is not really "cold" but rather "lukewarm" in a sense that coherent quasiparticles in that part survive but are strongly renormalized compared to the non-interacting case. We discuss the self-energy of lukewarm fermions and their contribution to the optical conductivity, $\sigma(\Omega)$, focusing specifically on scattering off composite bosons made of two critical magnetic fluctuations. Recent study [S.A. Hartnoll et al., Phys. Rev. B {\bf 84}, 125115 (2011)] found that composite scattering gives the strongest contribution to the self-energy of lukewarm fermions and suggested that this may give rise to non-Fermi liquid behavior of the optical conductivity at the lowest frequencies. We show that the most singular term in the conductivity coming from self-energy insertions into the conductivity bubble, $\sigma'(\Omega)\propto \ln^3\Omega/\Omega^{1/3}$, is canceled out by the vertex-correction and Aslamazov-Larkin diagrams. However, the cancelation does not hold beyond logarithmic accuracy, and the remaining conductivity still diverges as $1/\Omega^{1/3}$. We further argue that the $1/\Omega^{1/3}$ behavior holds only at asymptotically low frequencies, well inside the frequency range affected by superconductivity. At larger $\Omega$, up to frequencies above the Fermi energy, $\sigma'(\Omega)$ scales as $1/\Omega$, which is reminiscent of the behavior observed in the superconducting cuprates.

In-plane terahertz response of thin filmSr2RuO4

The low-energy electron dynamics of the quasi-two-dimensional electron-hole multiorbital system ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is investigated by time-domain terahertz spectroscopy for a thin film sample. The direct observation of the real and imaginary part of the optical conductivity below 8 meV clearly demonstrates a narrowing of the coherent quasiparticle response with decreasing temperature. The optical conductivity spectra, dc conductivity, and Hall coefficient can be quantitatively well described by the multiband Drude-Lorentz model, from which band dependent scattering rates can be extracted. The resulting modest band dependent scattering rates, which are lower than 2 meV at 4 K, support a rather isotropic relaxation regime.

Persistent spin excitations in doped antiferromagnets revealed by resonant inelastic light scattering

How coherent quasiparticles emerge by doping quantum antiferromagnets is a key question in correlated electron systems, whose resolution is needed to elucidate the phase diagram of copper oxides. Recent resonant inelastic X-ray scattering (RIXS) experiments in hole-doped cuprates have purported to measure high-energy collective spin excitations that persist well into the overdoped regime and bear a striking resemblance to those found in the parent compound, challenging the perception that spin excitations should weaken with doping and have a diminishing effect on superconductivity. Here we show that RIXS at the Cu L3-edge indeed provides access to the spin dynamical structure factor once one considers the full influence of light polarization. Further we demonstrate that high-energy spin excitations do not correlate with the doping dependence of Tc, while low-energy excitations depend sensitively on doping and show ferromagnetic correlations. This suggests that high-energy spin excitations are marginal to pairing in cuprate superconductors.

Direct Observation of the Bandwidth Control Mott Transition in theNiS2−xSexMultiband System

The bulk electronic structure of NiS$_{2-x}$Se$_x$ is studied across the bandwidth-control Mott transition (BCMT) by soft X-ray angle-resolved photoemission spectroscopy. The microscopic picture of the BCMT in this multiband non-half-filled system is revealed for the first time. We show that Se doping does not alter the Fermi surface volume. When approaching the insulating phase with decreasing Se concentration, we observed that the Fermi velocity continuously decreases. Meanwhile, the coherent quasiparticle weight continuously decreases and is transferred to higher binding energies, until it suddenly disappears across the Mott transition. In the insulating phase, there is still finite spectral weight at the Fermi energy, but it is incoherent and dispersionless due to strong correlations. Our results provide a direct observation of BCMT, and unveil its distinct characters in a multiband non-half-filled system.

Novel High-Pressure Monoclinic Metallic Phase ofV2O3

Vanadium sesquioxide, V2O3, is a prototypical metal-to-insulator system where, in temperature-dependent studies, the transition always coincides with a corundum-to-monoclinic structural transition. As a function of pressure, V2O3 follows the expected behavior of increased metallicity due to a larger bandwidth for pressures up to 12.5GPa. Surprisingly, for higher pressures when the structure becomes unstable, the resistance starts to increase. Around 32.5GPa at 300K, we observe a novel pressure-induced corundum-to-monoclinic transition between two metallic phases, showing that the structural phase transition can be decoupled from the metal-insulator transition. Using x-ray Raman scattering, we find that screening effects, which are strong in the corundum phase, become weakened at high pressures. Theoretical calculations indicate that this can be related to a decrease in coherent quasiparticle strength, suggesting that the high-pressure phase is likely a critical correlated metal, on the verge of Mott-insulating behavior.

Why there is a difference between optimal doping for maximal [formula omitted] and critical doping for highest [formula omitted] in cuprate superconductors?

A long-standing puzzle is why there is a difference between the optimal dopingδoptimal≈0.15 for the maximal superconducting (SC) transition temperature Tc and the critical dopingδcritical≈0.19 for the highest superfluid density ρs in cuprate superconductors? This puzzle is calling for an explanation. Within the kinetic energy driven SC mechanism, it is shown that except the quasiparticle coherence, ρs is dominated by the bare pair gap, while Tc is set by the effective pair gap. By calculation of the ratio of the effective and the bare pair gaps, it is shown that the coupling strength decreases with increasing doping. This doping dependence of the coupling strength induces a shift from the critical doping for the maximal value of the bare pair gap parameter to the optimal doping for the maximal value of the effective pair gap parameter, which leads to a difference between the optimal doping for the maximal Tc and the critical doping for the highest ρs.

Lattice distortion-induced phase transformations in La1 − yPr y MnO3 + δ manganites

The structural and magnetic phase transformations that occur in the system of self-doped La1 − yPr y MnO3 + δ (δ ≈ 0.1, 0 ≤ y ≤ 1) manganites in the temperature range 4.2–300 K are studied by X-ray diffraction and measuring the temperature and field dependences of dc magnetization. The low-temperature magnetic phase transformations induced by the substitution of Pr for La correlate well with the structural phase transformations at T = 300 K, which indicates a strong coupling of the electronic and magnetic subsystems of La1 − yPr y MnO3 + δ manganites with the crystal lattice. The anomalies of the magnetic and structural properties detected in this work in the form of peaks and inflection points in the concentration dependences of the magnetization and lattice parameters of the pseudocubic phase of La1 − yPr y MnO3 + δ (0.1 ≤ y ≤ 0.7) in the temperature range 4.2–300 K are explained in terms of the existing concepts of the effect of Fermi surface nesting on the renormalization of the density of states and the hole dispersion near EF in the presence of a strong coupling of holes with low-frequency optical phonons, which results in their transformation into quasiparticles. The narrow peak in the magnetization curve M(y) of La1 − yPr y MnO3 + δ that is detected near y = 0.3 at T = 4.2 K is assumed to correspond to the peak of coherence of quasiparticles with a low energy of coupling with the crystal lattice near EF, which was found earlier in the photoelectron emission spectra of manganites. The disappearance of the narrow magnetization peak with increasing Pr concentration is explained by the transition of charge carriers from the mode of “light” holes weakly coupled to one of the soft phonons to the mode of “heavy” holes strongly coupled to several phonons. The transition between phases with strongly different effective quasiparticle masses proceeds jumpwise; that is, it has features of the metal-insulator Mott transition and is accompanied by the transition from 3D to 2D quasiparticle motion in planes ab.

Quasiparticle Mass Enhancement and Temperature Dependence of the Electronic Structure of FerromagneticSrRuO3Thin Films

We report high-resolution angle-resolved photoemission studies of epitaxial thin films of the correlated 4d transition metal oxide ferromagnet SrRuO(3). The Fermi surface in the ferromagnetic state consists of well-defined Landau quasiparticles exhibiting strong coupling to low-energy bosonic modes which contributes to the large effective masses observed by transport and thermodynamic measurements. Upon warming the material through its Curie temperature, we observe a substantial decrease in quasiparticle coherence but negligible changes in the ferromagnetic exchange splitting, suggesting that local moments play an important role in the ferromagnetism in SrRuO(3).

Orbital selectivity in Hund's metals: The iron chalcogenides

We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion $U$ and Hund's rule coupling $J$. The evolution of the quasiparticle weight $Z$ as a function of the interaction terms reveals a clear crossover at $U\ensuremath{\simeq}2.5$ eV. In the weak coupling limit $Z$ decreases for all correlated $d$ orbitals as a function of $U$ and beyond the crossover coupling they become weakly dependent on $U$ while strongly dependent on $J$. A marked orbital dependence of the $Z$'s emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of $U$. This two-stage reduction of the quasiparticle coherence due to the combined effect of Hubbard $U$ and the Hund's $J$ suggests that the iron-based superconductors can be referred to as Hund's correlated metals.

Observation of a coherence peak in the microwave conductivity of optimally-doped Ba2Fe1.8Co0.2As2 single crystals with a critical temperature of 24 K

We study the microwave surface impedance of 170-µm-thick BaFe1.8Co0.2As2 single crystals with a T C of 24 K by using an open-ended sapphire resonator. The surface resistance values measured at two different frequencies of 23.3 and 19.3 GHz revealed exponential behaviors at low temperatures when the residual surface resistance values were subtracted, which was attributed to quasiparticle excitations in the smaller energy gap, Δ1, having an s-wave nature. The measured Δ1 values were (2.1 ± 0.1) meV at 23.3 GHz and (2.3 ± 0.1) meV at 19.3 GHz. A coherence peak was observed at T/T C ∼ 0.85 at 23.3 GHz and at ∼0.71 at 19.3 GHz in the real part of the complex conductivity as a function of temperature, which was consistent with the s-wave nature of the energy gaps in the BaFe1.8Co0.2As2 single crystals. Our results support that the coherence peak, reported earlier for BaFe2−x Co x As2 films, is indeed due to the intrinsic properties of the BaFe2−x Co x As2 single crystals, and that, unlike the case in nuclear magnetic resonance experiments, the quasiparticle coherence in BaFe1.8Co0.2As2 single crystals can be detected in microwave experiments.

Interplay between magnetism and charge transport in antiperovskite manganese nitrides: Extremely low temperature coefficient of resistance due to strong magnetic scattering

The recently discovered low temperature coefficient of resistance (low TCR) in Mn3Ag1-xCuxN exemplifies the peculiar magnetic and electronic states of this class of manganites. Despite its overall metallic character, a broad maximum appears in the temperature–resistivity curve in the paramagnetic region, and extremely low TCR is achieved over a temperature window that includes room temperature. The peak temperature can be tuned via Cu content x. It is apparently related to the magnetic transition temperature. These peculiar behaviors are possibly a result of the collapse of coherent quasiparticle states by strong magnetic scattering. We discuss the interplay between magnetism and charge transport in terms of magnetoresistance.

Janus-Faced Influence of Hund’s Rule Coupling in Strongly Correlated Materials

We show that in multiband metals the correlations are strongly affected by Hund's rule coupling, which depending on the filling promotes metallic, insulating or bad-metallic behavior. The quasiparticle coherence and the proximity to a Mott insulator are influenced distinctly and, away from single- and half-filling, in opposite ways. A strongly correlated bad metal far from a Mott phase is found there. We propose a concise classification of 3d and 4d transition-metal oxides within which the ubiquitous occurrence of strong correlations in Ru- and Cr-based oxides, as well as the recently measured high Néel temperatures in Tc-based perovskites are naturally explained.

Finite-temperature spectra and quasiparticle interference in Kondo lattices: From light electrons to coherent heavy quasiparticles

Recent advances in scanning tunneling spectroscopy performed on heavy-fermion metals provide a window onto local electronic properties of composite heavy-electron quasiparticles. Here we theoretically investigate the energy and temperature evolution of single-particle spectra and their quasiparticle interference caused by point-like impurities in the framework of a periodic Anderson model. By numerically solving dynamical-mean-field-theory equations, we are able to access all temperatures and to capture the crossover from weakly interacting c and f electrons to fully coherent heavy quasiparticles. Remarkably, this crossover occurs in a dynamical fashion at an energy-dependent crossover temperature. We study in detail the associated Fermi-surface reconstruction and characterize the incoherent regime near the Kondo temperature. Finally, we link our results to current heavy-fermion experiments.

Electronic Raman scattering in copper oxide superconductors: Understanding the phase diagram

Electronic Raman scattering measurements have been performed on hole doped copper oxide superconductors as a function of temperature and doping level. In the superconducting state coherent Bogoliubov quasiparticles develop preferentially over the nodal region in the underdoped regime. We can then define the fraction of coherent Fermi surface, $f_c$ around the nodes for which quasiparticles are well defined and superconductivity sets in. We find that $f_c$ is doping dependent and leads to the emergence of two energy scales. We then establish in a one single gap shem, that the critical temperature $T_{c} \propto f_{c}\Delta_{max}$ where $\Delta_{max}$ is the maximum amplitude of the d-wave superconducting gap. In the normal state, the loss of antinodal quasiparticles spectral weight detected in the superconducting state persists and the spectral weight is only restored above the pseudogap temperature $T*$. Such a dichotomy in the quasiparticles dynamics is then responsible for the emergence of the two energy scales in the superconducting state and the appearance of the pseudogap in the normal state. We propose a 3D phase diagram where both the temperature and the energy phase diagrams have been plotted together. We anticipate that the development of coherent excitations on a restricted part of the Fermi surface only is a general feature in high $T_c$ cuprate superconductors as the Mott insulating is approaching.

High-energy anomaly in Nd2−xCexCuO4investigated by angle-resolved photoemission spectroscopy and quantum Monte Carlo simulations

Recent high-binding-energy angle-resolved photoemission spectroscopy (ARPES) experiments reveal a change in band dispersion in the high-temperature superconducting cuprates (HTSCs) known as the high-energy anomaly (HEA). Despite considerable experimental and theoretical attention, the origin of the HEA remains a topic of some controversy. In this paper we present systematic and comprehensive experimental evidence on the origin of the HEA from ARPES measurements on the electron-doped HTSC material Nd${}_{2\ensuremath{-}x}$Ce${}_{x}$CuO${}_{4}$ at a number of dopings across the phase diagram and over the entire Brillouin zone (BZ). Comparing these new experimental findings to quantum Monte Carlo simulations of the single-band Hubbard model across the BZ and for various dopings demonstrates that this simple model qualitatively reproduces the key experimental features of the HEA and points to significant self-energy and band renormalization effects accompanying strong electron correlations as its origin rather than coupling to any one emergent bosonic mode, e.g., antiferromagnetic spin fluctuations. We conclude from comparison to this simple model that the HEA in these systems should be regarded as a crossover from a coherent quasiparticle band at low binding energies, emergent from the upper Hubbard band in electron-doped HTSCs due to doping and modified by subsequent strong band renormalization effects, to oxygen valence bands at higher binding energy that would be revealed in simulations explicitly incorporating these important orbital degrees of freedom.

Dichotomy in the T -linear resistivity in hole-doped cuprates

From analysis of the in-plane resistivity ρ(ab)(T) of La(2-x)Sr(x)CuO(4), we show that normal state transport in overdoped cuprates can be delineated into two regimes in which the electrical resistivity varies approximately linearly with temperature. In the low-temperature limit, the T-linear resistivity extends over a very wide doping range, in marked contrast to expectations from conventional quantum critical scenarios. The coefficient of this T-linear resistivity scales with the superconducting transition temperature T(c), implying that the interaction causing this anomalous scattering is also associated with the superconducting pairing mechanism. At high temperatures, the coefficient of the T-linear resistivity is essentially doping independent beyond a critical doping p(crit)=0.19 at which the ratio of the two coefficients is maximal. Taking our cue from earlier thermodynamic and photoemission measurements, we conclude that the opening of the normal-state pseudogap at p(crit) is driven by the loss of coherence of anti-nodal quasi-particles at low temperatures.