Coupling Of Quasiparticles Research Articles

Spin Transport in High‐Field Superconductors

AbstractHigh‐field superconductors are characterized by a spin splitting of the density of states, giving rise to spin transport phenomena. This includes spin‐polarized tunneling, spin‐dependent thermoelectric effects, long‐range quasiparticle spin transport, and spin‐dependent coupling of supercurrents and quasiparticles. This review gives a brief overview of the theory background, recent experimental progress in the field, and an outlook on open problems and possible applications.

Spin-dependent coupling of supercurrent and nonequilibrium quasiparticles in high-field superconductors

We report on an experimental investigation of the combined effect of nonequilibrium quasiparticle injection and supercurrent in superconducting aluminum wires. At low temperature, we observe the supercurrent-induced coupling of energy and charge imbalance with spectral resolution. At high magnetic fields, in the presence of a Zeeman splitting of the density of states, we find evidence for an additional spin-dependent coupling which has been recently predicted theoretically.

Superlattice of spherical quantum dots with a trapezoidal potential profile with weak coupling of quasiparticles between layers of quantum dots

A theoretical study of the spectrum of quasiparticles in a superlattice of spherical quantum dots (QDs) with a trapezoidal potential profile with a weak coupling of quasiparticles between QD layers is carried out. The calculations were performed using the example of spherical QDs InGaAs embedded in GaAs in the form of a superlattice. The model of the trapezoidal QD profile with three variable parameters is especially suitable for considering such zero-dimensional structures when, due to the different lattice constants of the QD material and the matrix medium, the boundary between them is fuzzy and is modeled by a certain transition region. The dependences of the energies of the underlying bands on the geometric parameters of the superlattice are investigated, which can be used to determine the metric of zero-dimensional structures by comparing it with the experimental intraband absorption spectra. The sensitivity of the position of the bands to changes in the parameters of the potential profile is revealed, which makes it possible to efficiently analyze the tunneling passage of carriers between QDs, to study the processes of controlled relaxation of carriers in QDs, which is important for the operation of lasers based on zero-dimensional structures with mismatched lattice constants of the QD material and the matrix medium.

Fragile versus stable two-dimensional fermionic quasiparticles

We provide a comprehensive theoretical investigation of the Fermi liquid quasiparticle description in two-dimensional electron gas interacting via the long-range Coulomb interaction by calculating the electron self-energy within the leading-order approximation, which is exact in the high-density limit. We find that the quasiparticle energy is larger than the imaginary part of the self-energy up to very high energies, implying that the basic Landau quasiparticle picture is robust up to far above the Fermi energy. We find, however, that the quasiparticle picture becomes fragile in a small discrete region around a critical wave vector where the quasiparticle spectral function strongly deviates from the expected quasiparticle Lorentzian line shape with a vanishing renormalization factor. We show that such a non-Fermi liquid behavior arises due to the coupling of quasiparticles with the collective plasmon mode. This situation is somewhat intermediate between the one-dimensional interacting electron gas (i.e., Luttinger liquid), where the Landau Fermi liquid theory completely breaks down since only bosonic collective excitations exist, and three-dimensional electron gas, where quasiparticles are well-defined and more stable against interactions than in one and two dimensions. We use a number of complementary definitions for a quasiparticle to examine the interacting spectral function, contrasting two-dimensional and three-dimensional situations critically.

Tunable Kondo Resonance at a Pristine Two-Dimensional Dirac Semimetal on a Kondo Insulator.

The proximity of two different materials leads to an intricate coupling of quasiparticles so that an unprecedented electronic state is often realized at the interface. Here, we demonstrate a resonance-type many-body ground state in graphene, a nonmagnetic two-dimensional Dirac semimetal, when grown on SmB6, a Kondo insulator, via thermal decomposition of fullerene molecules. This ground state is typically observed in three-dimensional magnetic materials with correlated electrons. Above the characteristic Kondo temperature of the substrate, the electron band structure of pristine graphene remains almost intact. As temperature decreases, however, the Dirac Fermions of graphene become hybridized with the Sm 4f states. Remarkable enhancement of the hybridization and Kondo resonance is observed with further cooling and increasing charge-carrier density of graphene, evidencing the Kondo screening of the Sm 4f local magnetic moment by the conduction electrons of graphene at the interface. These findings manifest the realization of the Kondo effect in graphene by the proximity of SmB6 that is tuned by the temperature and charge-carrier density of graphene.

Unconventional superconductivity in the strong-coupling limit for the heavy fermion system CeCoIn5

We present scanning tunneling spectroscopy measurements of the local quasiparticles' excitation spectra of the heavy fermion CeCoIn5 between 440mK and 3K in samples with a bulk Tc=2.25K. The spectral shape of our low-temperature tunneling data, quite textbook nodal-Δ conductance, allow us to confidently fit the spectra with a d-wave density of states considering also a shortening of quasiparticles' lifetime term Γ. The Δ(0) value obtained from the fits yields a BCS ratio 2Δ/kTc=7.73 suggesting that CeCoIn5 is an unconventional superconductor in the strong coupling limit. The fits also reveal that the height of coherence peaks in CeCoIn5 is reduced with respect to a pure BCS spectra and therefore the coupling of quasiparticles with spin excitations should play a relevant role. The tunneling conductance shows a depletion at energies smaller than Δ for temperatures larger than the bulk Tc, giving further support to the existence of a pseudogap phase that in our samples span up to T*∼1.2Tc. The phenomenological scaling of the pseudogap temperature observed in various families of cuprates, 2Δ/kT*∼4.3, is not fulfilled in our measurements. This suggests that in CeCoIn5 the strong magnetic fluctuations might conspire to close the local superconducting gap at a smaller pesudogap temperature-scale than in cuprates.

Induced nonlinear cross sections of conductive electrons scattering on the charged impurities in doped graphene

Relativistic quantum theory of induced scattering of 2D Dirac particles by electrostatic field of impurity ion (in the Born approximation) in the doped graphene at the presence of an external electromagnetic radiation field (actually terahertz radiation, to exclude the valence electrons excitations at high Fermi energies) has been developed. It is shown that the strong coupling of massless quasiparticles in the quantum nanostructures to a strong electromagnetic radiation field leads to the strongly nonlinear response of graphene, which opens diverse ways for manipulating the electronic transport properties of conductive electrons by coherent radiation fields.

Strong lattice correlation of non-equilibrium quasiparticles in a pseudospin-1/2 Mott insulator Sr2IrO4.

In correlated oxides the coupling of quasiparticles to other degrees of freedom such as spin and lattice plays critical roles in the emergence of symmetry-breaking quantum ordered states such as high temperature superconductivity. We report a strong lattice coupling of photon-induced quasiparticles in spin-orbital coupling Mott insulator Sr2IrO4 probed via optical excitation. Combining time-resolved x-ray diffraction and optical spectroscopy techniques, we reconstruct a spatiotemporal map of the diffusion of these quasiparticles. Due to the unique electronic configuration of the quasiparticles, the strong lattice correlation is unexpected but extends the similarity between Sr2IrO4 and cuprates to a new dimension of electron-phonon coupling which persists under highly non-equilibrium conditions.

Pair–pair interactions as a mechanism for high-Tc superconductivity

The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above Tc, pre-existing but fluctuating Cooper pairs give rise to the unconventional pseudogap (PG) state, well-characterized by experiment. At the critical temperature, the pair–pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below Tc, both the condensation energy and the pair–pair interaction β are proportional to the condensate density Noc(T), while the usual Fermi-level spectral gap is independent of temperature. The new order parameter , can be followed as a function of temperature, carrier concentration and disorder—i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the coupling of quasiparticles to Cooper-pair excitations. The latter interpretation is strongly supported by the observed quasiparticle spectral function.

First-principles theory of electron-spin fluctuation coupling and superconducting instabilities in iron selenide

We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with $s_\pm$-symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an $s_\pm$-symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements.

In search for the pairing glue in cuprates by non-equilibrium optical spectroscopy

In strongly correlated materials the electronic and optical properties are significantly affected by the coupling of fermionic quasiparticles to different degrees of freedom, such as lattice vibrations and bosonic excitations of electronic origin. Broadband ultrafast spectroscopy [1, 2] is emerging as the premier technique to unravel the subtle interplay between quasiparticles and electronic or phononic collective excitations, by their different characteristic timescales and spectral responses. By investigating the femtosecond dynamics of the optical properties of Bi2Sr2Ca0.92Y0.08Cu2O8+δ (Y-Bi2212) crystals over the 0.5–2 eV energy range, we disentangle the electronic and phononic contributions to the generalized electron-boson Eliashberg function [3, 4], showing that the spectral distribution of the electronic excitations, such as spin fluctuations and current loops, and the strength of their interaction with quasiparticles can account for the high critical temperature of the superconducting phase transition [5]. Finally, we discuss how the use of this technique can be extended to the underdoped region of the phase diagram of cuprates, in which a pseudogap in the quasiparticle density of states opens.The microscopic modeling of the interaction of ultrashort light pulses with unconventional superconductors will be one of the key challenges of the next-years materials science, eventually leading to the full understanding of the role of the electronic correlations in controlling the dynamics on the femtosecond timescale.

Evidence for phonon-like charge and spin fluctuations from an analysis of angle-resolved photoemission spectra of La2−xSrxCuO4superconductors

In high temperature superconductors we provide evidence of spin and mixed phonon-charge collective modes as mediators of the effective electron-electron interaction and suggestive of a charge and spin density wave instability competing with superconductivity. Indeed, we show that the so-called kinks and waterfalls observed in angle-resolved photoemission spectra of La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$, a prototypical high-${T}_{c}$ superconducting cuprate, are due to the coupling of quasiparticles with two distinct nearly critical collective modes with finite characteristic wave vectors, typical of charge and spin fluctuations. The simultaneous presence of these two modes reconciles the long standing dichotomy whether kinks are due to phonons or spin waves.

Dynamically induced Fermi arcs and pockets: A model for the pseudogap in underdoped cuprates

We investigate the effects of the dynamic bosonic fluctuations on the Fermi surface reconstruction in two dimensions as a model for the pseudogap in underdoped cuprates. At energies larger than the boson energy ωb, the dynamic nature of the fluctuations is not important and the quasi-particle dispersion exhibits the shadow feature like that induced by a static long range order. At lower energies, however, the shadow feature is pushed away by the finite ωb. The detailed low energy features are determined by the bare dispersion and the coupling of quasi-particles to the dynamic fluctuations. We present how these factors truncate the Fermi surface to produce the Fermi arcs or the Fermi pockets, or their coexistence. Our principal result is that the dynamic nature of the fluctuations, without invoking a yet-to-be-established translational symmetry breaking hidden order, can produce the Fermi arc or Fermi pocket centered away from the (π/2, π/2) towards the zone center. This is discussed in comparison with the experimental observations.

Nuclear surface dynamics and pairing correlations

We solve the Nambu-Gor'kov equations in 120Sn, demonstrating that the coupling of quasiparticles with collective surface vibrations renormalizes the low-lying part of the nuclear spectrum and influences pairing correlations in a substantial way.

Local Quasiparticle Density of States of SuperconductingSmFeAsO1−xFxSingle Crystals: Evidence for Spin-Mediated Pairing

We probe the local quasiparticles density of states in micron-sized SmFeAsO(1-x)F(x) single crystals by means of scanning tunnelling spectroscopy. Spectral features resemble those of cuprates, particularly a dip-hump-like structure developed at energies larger than the gap that can be ascribed to the coupling of quasiparticles to a collective mode, quite likely a resonant spin mode. The energy of the collective mode revealed in our study decreases when the pairing strength increases. Our findings support spin-fluctuation-mediated pairing in pnictides.

Dynamically induced Fermi arcs and Fermi pockets in two dimensions: A model for underdoped cuprates

We investigate the effects of the dynamic bosonic fluctuations on the Fermi surface reconstruction in two dimensions as a model for the underdoped cuprates. At energies larger than the boson energy $\omega_b$, the dynamic nature of the fluctuations is not important and the quasi-particle dispersion exhibits the shadow feature like that induced by a static long range order. At lower energies, however, the shadow feature is pushed away by the finite $\omega_b$. The detailed low energy features are determined by the bare dispersion and the coupling of quasi-particles to the dynamic fluctuations. We present how these factors reconstruct the Fermi surface to produce the Fermi arcs or the Fermi pockets, or their coexistence. Our principal result is that the dynamic nature of the fluctuations, without invoking a yet-to-be-established translational symmetry breaking hidden order, can produce the Fermi pocket centered away from the $(\pi/2,\pi/2)$ towards the zone center which may coexist with the Fermi arcs. This is discussed in comparison with the experimental observations.

Origin of the high-energy kink in the photoemission spectrum of the high-temperature superconductorBi2Sr2CaCu2O8

The high-energy kink or the waterfall effect seen in the photoemission spectra of the cuprates is suggestive of the coupling of the quasiparticles to a high energy bosonic mode with implications for the mechanism of superconductivity. Recent experiments however indicate that this effect may be an artifact produced entirely by the matrix element effects, i.e. by the way the photoemitted electron couples to the incident photons in the emission process. In order to address this issue directly, we have carried out realistic computations of the photo-intensity in ${\rm Bi_2Sr_2CaCu_2O_8}$ (Bi2212) where the effects of the matrix element are included together with those of the corrections to the self-energy resulting from electronic excitations. Our results demonstrate that while the photoemission matrix element plays an important role in shaping the spectra, the waterfall effect is a clear signature of the presence of strong coupling of quasiparticles to electronic excitations.

Pairing within the self-consistent quasiparticle random-phase approximation at finite temperature

An approach to pairing in finite nuclei at nonzero temperature is proposed, which incorporates the effects due to the quasiparticle-number fluctuation (QNF) around Bardeen-Cooper-Schrieffer (BCS) mean field and dynamic coupling to quasiparticle-pair vibrations within the self-consistent quasiparticle random-phase approximation (SCQRPA). The numerical calculations of pairing gap, total energy, and heat capacity were carried out within a doubly folded multilevel model as well as realistic nuclei $^{56}\mathrm{Fe}$ and $^{120}\mathrm{Sn}$. The results obtained show that, under the effect of QNF, in the region of moderate and strong couplings, the sharp transition between the superconducting and normal phases is smoothed out, resulting in a thermal pairing gap, which does not collapse at the BCS critical temperature, but has a tail, which extends to high temperature. The dynamic coupling of quasiparticles to SCQRPA vibrations significantly improves the agreement with the results of exact calculations and those obtained within the finite-temperature quantal Monte Carlo method for the total energy and heat capacity. It also causes a deviation of the quasiparticle occupation numbers from the Fermi-Dirac distributions for free fermions.

Coupling of quasiparticles to phonons in high temperature superconductors

A detailed study of the quasiparticle excitations by angle resolved photoemission spectroscopy is reported in three different families of high temperature superconductors. The data show the coupling of quasiparticles to phonons in the range of 50–80 meV. The effect persists above the critical temperature and shows a slight decrease as we increase doping.

Effects of strong coupling on pairing fluctuations in layered superconductors

The influence of short inelastic lifetimes due to strong coupling of fermionic quasiparticles to bosons on superconducting fluctuation effects aboveTc is calculated. Considering a strong-coupling model for a layered superconducting metal, it is shown that pairing fluctuation corrections to the spin-lattice relaxation rate in weak coupling and very strong coupling are qualitatively different if the pairing fluctuation spectrum has s-wave symmetry. For weak coupling the corrections are positive, whereas for very strong coupling γ = 2ƒ d ω α2F(ω)/ω > 2 the corrections are negative. In contrast, the Pauli spin susceptibility is insensitive to strong-coupling corrections.