Formation Of Quasiparticles Research Articles

Evidence of Novel Quasiparticles in a Strongly Interacting Two-Dimensional Electron System: Giant Thermopower and Metallic Behaviour

We report thermopower ($S$) and electrical resistivity ($\rho_{2DES}$) measurements in low-density (10$^{14}$ m$^{-2}$), mesoscopic two-dimensional electron systems (2DESs) in GaAs/AlGaAs heterostructures at sub-Kelvin temperatures. We observe at temperatures $\lesssim$ 0.7 K a linearly growing $S$ as a function of temperature indicating metal-like behaviour. Interestingly this metallicity is not Drude-like, showing several unusual characteristics: i) the magnitude of $S$ exceeds the Mott prediction valid for non-interacting metallic 2DESs at similar carrier densities by over two orders of magnitude; and ii) $\rho_{2DES}$ in this regime is two orders of magnitude greater than the quantum of resistance $h/e^2$ and shows very little temperature-dependence. We provide evidence suggesting that these observations arise due to the formation of novel quasiparticles in the 2DES that are not electron-like. Finally, $\rho_{2DES}$ and $S$ show an intriguing decoupling in their density-dependence, the latter showing striking oscillations and even sign changes that are completely absent in the resistivity.

Resonance states, heavy quasiparticles, and the thermoelectric figure of merit of IV–VI materials

A high thermoelectric figure of merit ZT, with a maximum value ZT = 1.5 at 670–800 K for the composition Ge0.9Pb0.05Bi0.05Te, has been obtained for GeTe solid solutions with Bi and Pb impurities. This is conducive to a decrease in the hole concentration, an increase in the Seebeck coefficient, and a decrease in the lattice thermal conductivity. The main attention in interpretation of the experimental data is given to specific features of the energy spectrum of holes in the initial GeTe compound. The model of resonance states, formed with involvement of Ge atoms and metal vacancies, has been further developed. The types of defects and their transformation depending on temperature and the concentration of superstoichiometric Te have been considered. The experimental results give reason to believe that the interaction of localized and free charge carriers at elevated temperatures leads to a pronounced hybridization of their states and the formation of heavy quasiparticles, a situation in many ways similar to that observed in materials with heavy fermions.

Synthesis, magnetic, transport, and thermodynamic investigation of CeCo(Sb, Sn)3

CeCo(Sb, Sn)3 has been grown from Sn flux and characterized by single-crystal X-ray diffraction. The crystal structure of CeCo(Sb, Sn)3 adopts the LaPdSb3 structure-type and the structure is comprised of layers [CoX2]∞2 octahedra, where X=Sb and Sb, and nearly square nets of [X]∞2 which are separated by Ce atoms. CeCo(Sb, Sn)3 displays a metallic resistivity and the magnetic susceptibility data show paramagnetic behavior down to 2.25K. Specific heat measurements reveal a phase transition at approximately 2K, which is most likely magnetic. A Sommerfeld coefficient, γ=140mJ/K2mol-CeCo(Sb, Sn)3, was obtained by fitting the heat capacity data at low temperatures for CeCo(Sb, Sn)3 and is indicative of enhanced charge carrier mass; however, the low temperature resistivity and magnetoresistance data do not support the formation of an enhanced mass state for CeCo(Sb, Sn)3. Thus, the enhanced Sommerfeld coefficient likely results from low lying crystal field levels which give rise to Schottky contributions at low temperatures rather than the formation of heavy quasiparticles. Herein, we report the crystal growth, structure, and physical properties for CeCo(Sb, Sn)3.

Temperature-Dependent Development of the Magnetic-Field-Insensitive Heavy Fermion State in SmOs4Sb12

Analysis of previously published data of electrical resistivity and specific heat in SmOs 4 Sb 12 has been made combining with other physical quantities. The results demonstrate that the heavy-fermion state develops gradually in the temperature range of 10–100 K with decreasing temperature and to attain the Fermi-liquid characteristics below ∼2 K. In the same temperature (energy) range of 10–100 K, anomalies appear in several quantities corresponding to the rattling anharmonic ion oscillations and unidentified charge fluctuations, suggesting that such electric charge degrees of freedom are deeply involved in the heavy quasiparticle formation. We discuss a possible scenario in which those electric charge degrees of freedom are deeply involved in the heavy quasiparticle formation, whereby the “magnetic-field insensitive” feature is acquired.

Ideas by S.V. Vonsovsky and Modern Model Treatment of Magnetism

A review of fundamental works by Shubin and Vonsovsky on the formulation of the polar and s–d(f) exchange models is given. Their ideas are compared with subsequent developments in the theory of magnetism in d- and f-metals and their compounds. Modern approaches including various slave-boson and slave-fermion representations, formation of exotic quasiparticles etc. are discussed. Internal connections between different many-electron models (the Heisenberg, Hubbard, t–J, Anderson Hamiltonians) are presented. Description of anomalous rare-earth and actinide compounds (Kondo lattices, systems with heavy fermions and non-Fermi-liquid behavior) within the framework of the s–d(f) exchange model and related models is considered.

Non-Abelian states of matter

Quantum mechanics classifies all elementary particles as either fermions or bosons, and this classification is crucial to the understanding of a variety of physical systems, such as lasers, metals and superconductors. In certain two-dimensional systems, interactions between electrons or atoms lead to the formation of quasiparticles that break the fermion-boson dichotomy. A particularly interesting alternative is offered by 'non-Abelian' states of matter, in which the presence of quasiparticles makes the ground state degenerate, and interchanges of identical quasiparticles shift the system between different ground states. Present experimental studies attempt to identify non-Abelian states in systems that manifest the fractional quantum Hall effect. If such states can be identified, they may become useful for quantum computation.

Dynamics of Electron-Phonon Scattering: Crystal- and Angular-Momentum Transfer Probed by Resonant Inelastic X-Ray Scattering

Experimentally, we observe angular-momentum transfer in electron-phonon scattering, although it is commonly agreed that phonons transfer mostly linear momentum. Therefore, the incorporation of angular momentum to describe phonons is necessary already for simple semiconductors and bears significant implications for the formation of new quasiparticles in correlated functional materials. Separation of linear and angular-momentum transfer in electron-phonon scattering is achieved by highly selective excitations on the femtosecond time scale of resonant inelastic x-ray scattering.

Signature of Quantum Criticality in Photoemission Spectroscopy

A quantum phase transition in a heavy-fermion compound may destroy the Fermi-liquid ground state. However, the conditions for this breakdown have remained obscure. We report the first direct investigation of heavy quasiparticle formation and breakdown in the canonical system CeCu(6-x)Au(x) by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the single-ion Kondo energy scale T(K) exhibits an abrupt step near the quantum critical Au concentration of x(c) = 0.1. We show theoretically that this step is expected from a highly nonlinear renormalization of the local spin coupling at each Ce site, induced by spin fluctuations on neighboring sites. It provides a general high-temperature indicator for heavy-fermion quasiparticle breakdown at a quantum phase transition.

Optical properties of YbRh 2Si 2 and YbIr 2Si 2: A comparison

We report far-infrared optical properties of the Heavy Fermion compounds YbRh 2Si 2 and YbIr 2Si 2 for photon energies down to 2 meV and temperatures 0.4–300 K. Upon heavy quasiparticle formation the optical conductivities of both compounds develop a narrow Drude-like response above which a pseudogap structure appears. Compared to YbRh 2Si 2 the pseudogap of YbIr 2Si 2 manifests different structures reflecting differences in Kondo scale and low-energy electronic structure.

Correlation-Driven Heavy-Fermion Formation inLiV2O4

Optical reflectivity measurements were performed on a single crystal of the d-electron heavy-fermion (HF) metal LiV2O4. Our results evidence the highly incoherent charge dynamics above T* approximately 20 K and the redistribution of the spectral weight of the optical conductivity over broad energy scales ( approximately 5 eV) as the quantum coherence of the charge carriers is recovered. This reveals that LiV2O4 is close to a correlation-driven insulating state and indicates that, in sharp contrast to f-electron HF Kondo-lattice systems, strong electronic correlation effects dominate the heavy quasiparticle formation in LiV2O4.

Thermoelectric Response Near a Quantum Critical Point: The Case ofCeCoIn5

We present a study of thermoelectric coefficients in CeCoIn5 down to 0.1 K and up to 16 T in order to probe the thermoelectric signatures of quantum criticality. In the vicinity of the field-induced quantum critical point, the Nernst coefficient nu exhibits a dramatic enhancement without saturation down to the lowest measured temperature. The dimensionless ratio of the Seebeck coefficient to the electronic specific heat shows a minimum at a temperature close to threshold of the quasiparticle formation. Close to Tc(H), in the vortex-liquid state, the Nernst coefficient behaves anomalously in puzzling contrast with other superconductors and standard vortex dynamics.

Drude weight in quasi-two-dimensional organic conductors close to the Mott transition: Optical studies of the bandwidth, filling and temperature dependence

We explore the effects of electronic correlations in quasi-two-dimensional BEDT-TTF-based organic conductors with half, quarter and 1/5-filled bands. Close to the Mott transition but still on the metallic side, quasiparticles are observed in the 1/2-filled systems only at temperatures well below 100K, with a considerable growth of the Drude-like contribution. In contrast, the 1/4-filled conductors show a zero-frequency conduction peak already at room temperature which increases slightly upon cooling. Basically no change with temperature is observed for the Drude contribution of the 1/5-filled compound. Optical studies allow us to investigate the formation of quasiparticles at low temperature for systems of different filling. The strongest effect is observed in 1/2-filled compounds, it becomes weaker in 1/4 filled, while in the 1/5-filled compound no change of the Dude-peak on temperature is seen.

119Sn NMR on the correlated semi-metal [formula omitted

We report on Sn ( I = 1 2 ) 119 field-sweep NMR measurements on the tetragonal semimetal CeRu 4 Sn 6 and the metallic non-magnetic homologue LaRu 4 Sn 6 at a frequency of 47 MHz. In contrast to our previous investigations at 28.5 MHz, the spectra clearly show the signature of two different Sn sites. Furthermore, in spin-lattice relaxation measurements ( 1 / T 1 ) 119 , CeRu 4 Sn 6 exhibits semiconducting like behavior above 20 K, whereas below this temperature, ( 1 / T 1 ) 119 deviates towards higher values. This upturn could be either due to the formation of heavy quasiparticles out of a residual density of states or the result of low energy spin fluctuations claimed by recent μ SR investigations.

Nonvanishing Energy Scales at the Quantum Critical Point ofCeCoIn5

Heat and charge transport were used to probe the magnetic field-tuned quantum critical point in the heavy-fermion metal CeCoIn5. A comparison of electrical and thermal resistivities reveals three characteristic energy scales. A Fermi-liquid regime is observed below T(FL), with both transport coefficients diverging in parallel and T(FL) -->0 as H --> Hc, the critical field. The characteristic temperature of antiferromagnetic spin fluctuations, T(SF), is tuned to a minimum but finite value at Hc, which coincides with the end of the T-linear regime in the electrical resistivity. A third temperature scale, T(QP), signals the formation of quasiparticles, as fermions of charge e obeying the Wiedemann-Franz law. Unlike T(FL), it remains finite at Hc, so that the integrity of quasiparticles is preserved, even though the standard signature of Fermi-liquid theory fails.

Mott Transition in Kagomé Lattice Hubbard Model

We investigate the Mott transition in the kagomé lattice Hubbard model using a cluster extension of dynamical mean field theory. The calculation of the double occupancy, the density of states, and the static and dynamical spin correlation functions demonstrates that the system undergoes the first-order Mott transition at the Hubbard interaction U/W approximately 1.4 (W:bandwidth). In the metallic phase close to the Mott transition, we find the strong renormalization of three distinct bands, giving rise to the formation of heavy quasiparticles with strong frustrated interactions. It is elucidated that the quasiparticle states exhibit anomalous behavior in the temperature-dependent spin correlation functions.

ARPES on high-temperature superconductors: Simplicity vs. complexity (Review Article)

A notable role in understanding of the microscopic electronic properties of high-temperature superconductors (HTSC) belongs to angle-resolved photoemission spectroscopy (ARPES). This technique supplies a direct window into the reciprocal space of solids: the momentum-energy space where quasiparticles (electrons dressed in clouds of interactions) dwell. Any interaction in the electronic system, e.g., superconducting pairing, leads to modification of the quasiparticle spectrum—to redistribution of the spectral weight over the momentum-energy space probed by ARPES. Continued development of the technique had the effect that the picture seen through the ARPES window became clearer and sharper until the complexity of the electronic band structure of the cuprates had been resolved. Now, in the doping range optimal for superconductivity, the cuprates much resemble a normal metal with well-predicted electronic structure, though with rather strong electron-electron interaction. This principal disentanglement of the complex physics from complex structure reduced the mystery of HTSC to the tangible problem of the interaction responsible for quasiparticle formation. Here we present a short overview of recent ARPES results, which, we believe, suggest a way to resolve the HTSC puzzle.

Pressure-dependent infrared properties of the quasi-one-dimensional metal [formula omitted]- [formula omitted

The pressure-dependent mid-infrared properties of the quasi-one-dimensional metal β - Na 0.33 V 2 O 5 were studied at room temperature. The monotonic changes of the spectra below 12 GPa suggest a pressure-induced dimensional crossover and the relevance of electron–phonon coupling, leading to the formation of polaronic quasiparticles. The changes in the spectra above 12 GPa may be due to a redistribution of charge with a possible relation to structural changes. A possible relation between the interchain coupling and superconductivity is pointed out.

Crossover from divalent to valence fluctuating state of Eu in EuCu 2(Ge 1−x Si x) 2 probed by 63,65Cu - NMR

63,65Cu field sweep NMR investigations on a series of EuCu 2(Ge 1− x Si x ) 2 powder samples ( x = 0.3 , 0.7 , 1 ) are presented. EuCu 2(Ge, Si) 2 is unique among Eu-intermetallics because, upon tuning from a divalent antiferromagnetically ordered state for x = 0 to a valence fluctuating state for x = 1 , Heavy Fermion behaviour was found around x≈0.7. The Knight shift 63,65 K is negative for all stoichiometries. Hyperfine fields are largest for the antiferromagnetically ordered Ge-rich compound ( x = 0.3 ) and smallest for the valence fluctuating pure silicon compound ( x = 1 ). For the x = 0.3 compound, above T N≈19 K, 63 K( T) exhibits a Curie–Weiss like behaviour (≈ −10% at 20 K) whereas for EuCu 2Si 2 a temperature-independent shift of 63 K( T)≈−1.5% is found. Our results are discussed in terms of formation of heavy quasiparticles and/or presence of valence fluctuations of Eu.

Low-energy excitations of the Hubbard model on the Kagomé lattice

The Hubbard model on the Kagom\'e lattice is investigated in a metallic phase at half filling. By introducing anisotropic electron hopping on the lattice, we control geometrical frustration and clarify how the lattice geometry affects physical properties. By means of the fluctuation exchange approximation, we calculate the spin and charge susceptibilities, the one-particle spectral function, the quasiparticle renormalization factor, and the Fermi velocity. It is found that geometrical frustration of the Kagom\'e lattice suppresses the instability to various ordered states through the strong reduction of the wave-vector dependence of susceptibilities, thereby stabilizing the formation of quasiparticles due to the almost isotropic spin fluctuations in the Brillouin zone. These characteristic properties are discussed in connection with the effects of geometrical frustration in the strong-coupling regime.

The birth of a quasiparticle in silicon observed in time-frequency space.

The concept of quasiparticles in solid-state physics is an extremely powerful tool for describing complex many-body phenomena in terms of single-particle excitations. Introducing a simple particle, such as an electron, hole or phonon, deforms a many-body system through its interactions with other particles. In this way, the added particle is 'dressed' or 'renormalized' by a self-energy cloud that describes the response of the many-body system, so forming a new entity--the quasiparticle. Using ultrafast laser techniques, it is possible to impulsively generate bare particles and observe their subsequent dressing by the many-body interactions (that is, quasiparticle formation) on the time and energy scales governed by the Heisenberg uncertainty principle. Here we describe the coherent response of silicon to excitation with a 10-femtosecond (10(-14) s) laser pulse. The optical pulse interacts with the sample by way of the complex second-order nonlinear susceptibility to generate a force on the lattice driving coherent phonon excitation. Transforming the transient reflectivity signal into frequency-time space reveals interference effects leading to the coherent phonon generation and subsequent dressing of the phonon by electron-hole pair excitations.