Narrow Quasiparticle Bands Research Articles

The low and high temperature thermoelectric properties of Yb3Si5

Silicides have been of great interest for thermoelectric applications due to their abundant elements as well as thermal and chemical stability. In this paper, we examined the thermoelectric properties of Yb3Si5 polycrystalline samples in a wide temperature range from 10 to 800 K. The temperature dependence of the Seebeck coefficient was successfully analyzed by assuming a narrow 4f quasi-particle band, indicating the intermediate valence state of Yb2+-Yb3+ is responsible for the high power factor. A very large maximum power factor of ∼ 4.70 mWm−1K−2 was observed at 72 K and room temperature value ∼ 1.56 mWm−1K−2 for Yb3Si5. These results shows that Yb-Si compounds have large potential to be used as low temperature TE applications in the future. We also studied the Co-doping effect in Yb3Si5, namely, Yb3Co x Si5−x where x = 0, 0.1, 0.15, 0.20 and investigated their thermoelectric properties. While powder X-ray diffraction analysis confirmed all main peaks indexed to Yb3Si5 phase, SEM and EDX analyses revealed that Co is precipitated as metal particles, forming a composite material with Yb3Si5 phase. Thermoelectric properties of the Co-doped samples are also reported.

Doping Induced Spin State Transition inLaCoO3: Dynamical Mean-Field Study

Hole and electron doped LaCoO3 is studied using dynamical mean-field theory. The one-particle spectra are analyzed and compared to the available experimental data, in particular the x-ray absorption spectra. Analyzing the temporal spin-spin correlation functions we find the atomic intermediate spin state is not important for the observed Curie-Weiss susceptibility. Contrary to the commonly held view about the roles played by the t2g and eg electrons we find narrow quasiparticle bands of t2g character crossing the Fermi level accompanied by strongly damped eg excitations.

Low-temperature properties ofCeRu4Sn6from NMR and specific heat measurements: Heavy fermions emerging from a Kondo-insulating state

The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu$_4$Sn$_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $\Delta E_g1/k_B \simeq 30$K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat C detects these excitations in the form of an enhanced Sommerfeld coefficient $\gamma = C(T)/T$ : In zero field, $\gamma$ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at $\gamma = 0.6$ J/molK$^2$. This maximum is shifted to higher temperatures with increasing field suggesting a residual density of states at the Fermi level developing a spin gap $\Delta E_g2$. A simple model, based on two narrow quasiparticle bands located at the Fermi level - which cross the Fermi level in zero field at 0.022 states/meV f.u. - can account qualitatively as well as quantitatively for the measured observables. In particular, it is demonstrated that fitting our data of both specific heat and NMR to the model, incorporating a Ce magnetic moment of $\mu = \Delta E_g1/\mu_{0H} \simeq 1 \mu_B$, leads to the prediction of the field dependence of the gap. Our measurements rule out the presence of a quantum critical point as the origin for the enhanced $\gamma$ in CeRu$_4$Sn$_6$ and suggest that this arises rather from correlated, residual in-gap states at the Fermi level. This work provides a fundamental route for future investigations into the phenomenon of narrow-gap formation in the strongly correlated class of system

Pressure-induced valence change in the rare earth metals: The case of praseodymium

The rare earth metal praseodymium (Pr) transforms from the d-fcc crystal structure (Pr-III) to α -U one (Pr-IV) at 20 GPa with a large volume collapse ( Δ V / V = 0.16 ), which is associated with the valence change of the Pr ion. The two 4f electrons in the Pr ion is supposed to be itinerant in the Pr-IV phase. In order to investigate the electronic state of the phase IV, we performed the high-pressure electrical resistance measurement using the diamond anvil cell up to 32 GPa. In the Pr-IV phase, the temperature dependence of the resistance shows an upward negative curvature, which is similar to the itinerant 5f electron system in actinide metals and compounds. This suggests the narrow quasiparticle band of the 4f electrons near the Fermi energy. A new phase boundary is found at T 0 in the Pr-IV phase. From the temperature and magnetic field dependences of the resistance at 26 GPa, the ground state of the Pr-IV phase is suggested to be magnetic. Several possibilities for the origin of T 0 are discussed.

Elastic property of Kondo semiconductor CeOs 4Sb 12

Ultrasonic measurement was performed on Kondo semiconductor CeOs 4Sb 12 to investigate the elastic property. A distinct elastic softening toward low temperatures was observed in C 11 , whereas no softening was observed in C 44 . The obtained results were not explained reasonably based on the crystalline electric field effect, indicating that the softening is ascribed to another origin. Since CeOs 4Sb 12 is likely to form the narrow quasiparticle band, the coupling between elastic strain and the band may play a crucial role. We discuss the origin of the distinct softening in C 11 from the view of this coupling.

Magnetic and transport properties of UPd 2Sb

A new compound UPd 2Sb was prepared and studied by means of X-ray diffraction, magnetization, electrical resistivity, magnetoresistivity, thermoelectric power and specific heat measurements. The phase crystallizes with a cubic structure of the MnCu 2Al-type (s.g. F m 3 ¯ m ). It orders antiferromagnetically at T N=55 K and exhibits a modified Curie–Weiss behaviour with reduced effective magnetic moment at higher temperatures. The electrical resistivity behaves in a manner characteristic of systems with strong electronic correlations, showing Kondo effect in the paramagnetic region and Kondo-like response to the applied magnetic field. The Seebeck coefficient exhibits a behaviour expected for scattering of conduction electrons on a narrow quasiparticle band near the Fermi energy. The low-temperature electronic specific heat in UPd 2Sb is moderately enhanced being about 81 mJ/mol K 2.

Quasiparticles and the structure of orbital polarons in ferromagneticLaMnO3

We investigate the spectral functions and orbital-correlation functions relative to the position of an e g hole moving in the ferromagnetic plane of orbitally ordered LaMnO 3 . Solving this problem in a self-consistent Born approximation, we analyze a hole motion as a function of uniaxial pressure, the Jahn-Teller interaction, and the polarization of orbitals in the vicinity of the hole. We report a large redistribution of the spectral weight as compared with the free hole dispersion (t), strongly dependent on the type of the alternating orbital order. Polarization of orbitals around the hole (Δ) can lead to a very narrow quasiparticle band and to large incoherent spectral weight, indicating a confinement of holes in a lightly doped LaMnO 3 insulator. It thus strongly modifies the shape of the orbital polaron, and favors the localization of small orbital polarons in the regime of Δ > t.

Orbital polarons in LaMnO3

We investigate the spectral functions and orbital correlation functions relative to the position of an eg hole moving in the ferromagnetic plane of orbitally ordered LaMnO3. Polarization of orbitals around the hole leads to a very narrow quasiparticle band and to a confinement of the hole in an orbital polaron with the shape strongly depending on the type of the orbital order.

Hole dynamics and photoemission in at−Jmodel forSrCu2(BO3)2

The motion of a single hole in a $t\ensuremath{-}J$ model for the two-dimensional spin-gap compound ${\mathrm{SrCu}}_{2}({\mathrm{BO}}_{3}{)}_{2}$ is investigated. The undoped Heisenberg model for this system has an exact dimer eigenstate and shows a phase transition between a dimerized and a N\'eel phase at a certain ratio of the magnetic couplings. We calculate the photoemission spectrum in the disordered phase using a generalized spin-polaron picture. By varying the interdimer hopping parameters we find a crossover between a narrow quasiparticle band regime known from other strongly correlated systems and free-fermion behavior. The hole motion in the N\'eel-ordered phase is also briefly considered.

Intermediate valence in the ytterbium deuteride — the magnetic susceptibility evidence

Low-temperature magnetic properties of YbD x cubic deuteride phase are reported and interpreted in the model of intermediate valence system with the ground state consisting of two narrow quasiparticle bands located at the Fermi level. The estimated bandwidth, Γ=0.3 meV and the gap value, Δ=1.35 meV for YbD 2.67 will reproduce the shape of broad maximum in magnetization at around 4.5 K.

Intermediate valence in the ytterbium hydride—the specific heat evidence

Low temperature specific heat measurements of YbH x cubic hydride phase are reported and interpreted in the model of the intermediate valence system with ground state consisting of two narrow quasiparticle bands located at the Fermi level. The band width and the gap values obtained are a function of the hydrogen concentration and they reproduce well the shape of the broad hump observed around 4 K in the temperature dependence of the specific heat.

Hole-doping evolution of the quasiparticle band in models of strongly correlated electrons for the high-Tccuprates

Quantum Monte Carlo (QMC) and Maximum Entropy (ME) techniques are used to study the spectral function $A({\bf p},\omega)$ of the one band Hubbard model in strong coupling including a next-nearest-neighbor electronic hopping with amplitude $t'/t= -0.35$. These values of parameters are chosen to improve the comparison of the Hubbard model with angle-resolved photoemission (ARPES) data for $Sr_2 Cu O_2 Cl_2$. A narrow quasiparticle (q.p.) band is observed in the QMC analysis at the temperature of the simulation $T=t/3$, both at and away from half-filling. Such a narrow band produces a large accumulation of weight in the density of states at the top of the valence band. As the electronic density $< n >$ decreases further away from half-filling, the chemical potential travels through this energy window with a large number of states, and by $< n > \sim 0.70$ it has crossed it entirely. The region near momentum $(0,\pi)$ and $(\pi,0)$ in the spectral function is more sensitive to doping than momenta along the diagonal from $(0,0)$ to $(\pi,\pi)$. The evolution with hole density of the quasiparticle dispersion contains some of the features observed in recent ARPES data in the underdoped regime. For sufficiently large hole densities the ``flat'' bands at $(\pi,0)$ cross the Fermi energy, a prediction that could be tested with ARPES techniques applied to overdoped cuprates. The population of the q.p. band introduces a {\it hidden} density in the system which produces interesting consequences when the quasiparticles are assumed to interact through antiferromagnetic fluctuations and studied with the BCS gap equation formalism. In particular, a region of extended s-wave is found to compete with d-wave in the overdoped regime, i.e. when the chemical potential has almost entirely crossed the q.p.

Evolution of the single-particle spectral weight with doping

We have calculated the single-particle density of states N( ω) and the spectral weight A( p, ω) for the two-dimensional Hubbard model by combining quantum Monte Carlo simulations with the maximum-entropy analytic continuation technique. We present results for various values of the doping, temperature, and Coulomb repulsion U. At half-filling an insulating gap separates incoherent lower and upper Hubbard bands. In addition, narrow quasiparticle bands exist at the top of the lower Hubbard band and at the bottom of the upper Hubbard band. The bandwidth of the quasiparticle bands is of order 2 J, where J ≈ 4t 2 U . Upon small doping, spectral weight is transferred from states above the insulating gap to the top of the lower Hubbard band, and a narrow quasiparticle band of width ~ 4 J forms. We find that the dispersion of this narrow band is similar to the results of the recent angular resolved photoemission measurements of the hole doped cuprates. We argue that the generic nature of the quasiparticle dispersion relation observed in these materials arises from the strong Coulomb interaction and reflects the hole-spin correlations rather than the one-electron interactions which customarily determine the band structure.

Charge- and spin-density-wave ordering transitions in strongly correlated metals.

We study the quantum transition from a strongly correlated metal, with heavy fermionic quasiparticles, to a metal with commensurate charge or spin density wave order. To this end, we introduce and numerically analyze a large dimensionality model of Ising spins in a transverse field, coupled to two species of fermions; the analysis borrows heavily from recent progress in the solution of the Hubbard model in large dimensions. At low energies, the Ising order parameter fluctuations are characterized by the critical exponent $z \nu = 1$, while above an energy scale, $\Gamma$, there is a crossover to $z\nu = 1/2$ criticality. We show that $\Gamma$ is of the order of the width of the heavy quasiparticle band, and can be made arbitrarily small for a correlated metal close to a Mott-Hubbard insulator. Therefore, such a correlated metal has a significant intermediate energy range of $z\nu=1/2$ behavior, a single particle spectrum with a narrow quasiparticle band, and well-formed analogs of the lower and upper Hubbard bands; we suggest that these features are intimately related in general.

Electronic specific heat of heavy fermion systems

We study the electronic specific heat of the heavy-fermions and mixed-valence systems using the periodic Anderson model. A variational method has been used in these calculations. The electronic specific heat increases rapidly in the low temperature region, shows a maxima and then decreases. This maxima may well be associated to the formation of a narrow quasiparticle band near Fermi level and to the narrow gap in the energy spectrum evident from the density of states curves.

Quasiparticle spectrum in copper oxides

The results of electronic structure calculations of the CuO2 layer, performed by the generalized tight-binding method, are presented. The strong electron correlations on both the Cu and O ions are included explicitly. The resulting quasiparticle band structure contains an insulator gap for undoped compounds like La2CuO4. The valence band consists of a large number of narrow Hubbard quasiparticle bands. The dispersion law of quasiparticles at the top of the valence band has a minimum at the Γ point, indicating the existence of the multivalley Fermi surface in the doped La2-xSr x CuO4. The origin of the Fermi liquid behavior in the narrow quasiparticle bands is discussed.

Motion of a single hole in a quantum antiferromagnet.

We formulate a quasiparticle theory for a single hole in a quantum antiferromagnet in the limit that the Heisenberg exchange energy is much less than the hopping matrix element, J\ensuremath{\ll}t. We consider the ground state of the spins to be either a quantum N\'eel state or a d-wave resonating-valence-bond (RVB) state. We show in a self-consistent perturbation theory that the hole spectrum is strongly renormalized by the interactions with spin excitations. The hole can be described by a narrow quasiparticle band located at an energy of order -t with a quasiparticle residue of order J/t and a bandwidth of order J. Above the quasiparticle band is an incoherent band of width of order t. Our results indicate that the energy scale for any coherent phenomenon involving the holes is \ensuremath{\delta}J, where \ensuremath{\delta} is the doping concentration. In the N\'eel state we perform a spin-wave expansion on an anisotropic Heisenberg model. In the Ising limit we reproduce previously known results and then expand perturbatively about that limit. In this expansion we find that the holes have a quasiparticle residue of ${J}_{z}$/t and a bandwidth of ${J}_{\ensuremath{\perp}}$. In the Heisenberg limit we employ a ``dominant pole'' approximation in which we ignore contributions to the self-energy from the incoherent part of the hole spectrum. A similar technique is used to study the d-wave RVB state. The relevance of our results to recent optical experiments is discussed.

Temperature and field dependence of elastic properties in heavy fermion compounds

Heavy fermion compounds exhibit various elastic anomalies below the fluctuation temperature T ∗. Longitudinal elastic constant depression, anomalous thermal expansion and magnetostriction and dramatic high field effects in elastic constants have been observed in CeCu 6, CeRu 2Si 2 and UPt 3. These phenomena are due to a large Grüneisen parameter coupling to narrow quasiparticle bands and they will be quantitatively described by a scaling ansatz, a deformation potential model and a microscopic theory.

Narrow bands and magnetic properties of heavy fermions

Experimental results obtained for some heavy fermion compounds by far-infrared reflectivity and by point contact spectroscopy suggest a model for the ground state of these materials consisting of two narrow quasiparticle bands placed in a wide free carrier-like band and a gap in between. In a preceding work we applied the model to calculate the temperature dependence of the resistivity and of the specific heat. By using as parameters the band width of the quasiparticle bands and the gap, we obtained excellent fits with values of the parameters consistent with the experimentally determinated values. In this paper we extend the calculation to the magnetic properties of heavy fermions and we show that we can reproduce the correct shape of the susceptibility of UPt 3 and CeCu 6.

Narrow bands and magnetic properties of heavy fermions (abstract)

Far infrared measurements performed at low temperature on the heavy fermions UPt3 and CeCu6 give us evidence that an appropriate model describing these materials must consider two narrow quasiparticle bands in the vicinity of the Fermi level separated by a gap, or a pseudogap, in the order of a few meV. The bandwidth obtained by point contact spectroscopy is also in the meV range. With such a model and with the experimentally obtained values for the bandwidth and the gap we tried to calculated the temperature dependence of the resistivity and of the γ value of the specific heat and we found a very good agreement with the experiments. We want now to apply the model to calculate the magnetic properties of the heavy fermions. We show that a Pauli susceptibility including the effects of a strongly energy dependent density of states can account for the temperature dependence of the susceptibility in these materials. For temperatures above about 50 K we find a Curie law and for lower temperatures we compute a weak maximum. Furthermore, the magnetic saturation of UPt3 in very large fields (35 T) can be explained in this context as a splitting of the quasiparticle band at the Fermi level in two completely separated spin polarized subbands.