arXiv subjects

P. Fulde

Publications and source records attributed to P. Fulde.

At least 37 records · Page 2Linked to original sources

arXiv2006

Correlated Fermions on a Checkerboard Lattice

A model of strongly correlated spinless fermions hopping on a checkerboard lattice is mapped onto a quantum fully-packed loop model. We identify a large number of fluctuationless states specific to the fermionic case. We also show that for a class of fluctuating states, the fermionic sign problem can be gauged away. This claim is supported by numerically evaluating the energies of the low-lying states. Furthermore, we analyze in detail the excitations at the Rokhsar-Kivelson point of this model thereby using the relation to the height model and the single-mode approximation.

cond-mat.str-el↗

arXiv2006

Magnetic properties of doped GdI2

Motivated by the recent experimental studies on layered ferromagnetic metallic system GdI2 and its doped variant GdI2Hx we develop a model to understand their ground state magnetic phase diagram. Based on first principle electronic structure calculations we write down a phenomenological model and solve it under certain approximations to obtain the ground state energy. In the process we work out the phase diagram of the correlated double exchange model on a triangular lattice for the specific band structure at hand.

cond-mat.other↗

arXiv2006

On confined fractional charges: a simple model

We address the question whether features known from quantum chromodynamics (QCD) can possibly also show up in solid-state physics. It is shown that spinless fermions of charge $e$ on a checkerboard lattice with nearest-neighbor repulsion provide for a simple model of confined fractional charges. After defining a proper vacuum the system supports excitations with charges $\pm e/2$ attached to the ends of strings. There is a constant confining force acting between the fractional charges. It results from a reduction of vacuum fluctuations and a polarization of the vacuum in the vicinity of the connecting strings.

cond-mat.str-el↗

arXiv2006

Spectral functions and optical conductivity of spinless fermions on a checkerboard lattice

We study the dynamical properties of spinless fermions on the checkerboard lattice. Our main interest is the limit of large nearest-neighbor repulsion $V$ as compared with hopping $|t|$. The spectral functions show broad low-energy excitation which are due to the dynamics of fractionally charged excitations. Furthermore, it is shown that the fractional charges contribute to the electrical current density.

cond-mat.str-el↗

arXiv2005

Low temperature breakdown of coherent tunneling in amorphous solids induced by the nuclear quadrupole interaction

We consider the effect of the internal nuclear quadrupole interaction on quantum tunneling in complex multi-atomic two-level systems. Two distinct regimes of strong and weak interactions are found. The regimes depend on the relationship between a characteristic energy of the nuclear quadrupole interaction $λ_{\ast}$ and a bare tunneling coupling strength $Δ_{0}$. When $Δ_{0}>λ_{\ast}$, the internal interaction is negligible and tunneling remains coherent determined by $Δ_{0}$. When $Δ_{0}<λ_{\ast}$, coherent tunneling breaks down and an effective tunneling amplitude decreases by an exponentially small overlap factor $η^{\ast}\ll1$ between internal ground states of left and right wells of a tunneling system. This affects thermal and kinetic properties of tunneling systems at low temperatures $T<λ_{*}$. The theory is applied for interpreting the anomalous behavior of the resonant dielectric susceptibility in amorphous solids at low temperatures $T\leq 5$mK where the nuclear quadrupole interaction breaks down coherent tunneling. We suggest the experiments with external magnetic fields to test our predictions and to clarify the internal structure of tunneling systems in amorphous solids.

cond-mat.dis-nn↗

arXiv2005

Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses

The standard tunneling model describes quite satisfactorily the thermal properties of amorphous solids at temperatures $T<1K$ in terms of an ensemble of two-level systems possessing logarithmically uniform distribution over their tunneling amplitudes and uniform distribution over their asymmetry energies. In particular, this distribution explains the observable logarithmic temperature dependence of the dielectric constant. Yet, experiments have shown that at ultralow temperatures $T<5mK$ such a temperature behavior breaks down and the dielectric constant becomes temperature independent (plateau effect). In this letter we suggest an explanation of this behavior exploiting the effect of the nuclear quadrupole interaction on tunneling. We show that below a temperature corresponding to the characteristic energy of the nuclear quadrupole interaction the effective tunneling amplitude is reduced by a small overlap factor of the nuclear quadrupole ground states in the left and right potential wells of the tunneling system. It is just this reduction that explains the plateau effect . We predict that the application of a sufficiently large magnetic field $B>10T$ should restore the logarithmic dependence because of the suppression of the nuclear quadrupole interaction.

cond-mat.dis-nn↗

arXiv2005

Cluster approach study of intersite electron correlations in pyrochlore and checkerboard lattices

To treat effects of electron correlations in geometrically frustrated pyrochlore and checkerboard lattices, an extended single-orbital Hubbard model with nearest neighbor hopping $\sim t$ and Coulomb repulsion $\sim V$ is applied. Infinite on-site repulsion, $U\to\infty$, is assumed, thus double occupancies of sites are forbidden completely in the present study. A variational Gutzwiller type approach is extended to examine correlations due to short-range $V-$interaction and a cluster approximation is developed to evaluate a variational ground state energy of the system. Obtained analytically in a special case of quarter band filling appropriate to LiV$_2$O$_4$, the resulting simple expression describes the ground state energy in the regime of intermediate and strong coupling $V$. Like in the Brinkman-Rice theory based on the standard Gutzwiller approach to the Hubbard model, the mean value of the kinetic energy is shown to be reduced strongly as the coupling $V$ approaches a critical value $V_{c}$. This finding may contribute to explaining the observed heavy fermion behavior in LiV$_2$O$_4$.

cond-mat.str-el↗

arXiv2005

Effect of Nuclear Quadrupole Interaction on the Relaxation in Amorphous Solids

Recently it has been experimentally demonstrated that certain glasses display an unexpected magnetic field dependence of the dielectric constant. In particular, the echo technique experiments have shown that the echo amplitude depends on the magnetic field. The analysis of these experiments results in the conclusion that the effect seems to be related to the nuclear degrees of freedom of tunneling systems. The interactions of a nuclear quadrupole electrical moment with the crystal field and of a nuclear magnetic moment with magnetic field transform the two-level tunneling systems inherent in amorphous dielectrics into many-level tunneling systems. The fact that these features show up at temperatures $T<100mK$, where the properties of amorphous materials are governed by the long-range $R^{-3}$ interaction between tunneling systems, suggests that this interaction is responsible for the magnetic field dependent relaxation. We have developed a theory of many-body relaxation in an ensemble of interacting many-level tunneling systems and show that the relaxation rate is controlled by the magnetic field. The results obtained correlate with the available experimental data. Our approach strongly supports the idea that the nuclear quadrupole interaction is just the key for understanding the unusual behavior of glasses in a magnetic field.

cond-mat.dis-nn↗

arXiv2005

Charge ordering in the spinels AlV$_2$O$_4$ and LiV$_2$O$_4$

We develop a microscopic theory for the charge ordering (CO) transitions in the spinels AlV$_2$O$_4$ and LiV$_2$O$_4$ (under pressure). The high degeneracy of CO states is lifted by a coupling to the rhombohedral lattice deformations which favors transition to a CO state with inequivalent V(1) and V(2) sites forming Kagomé and trigonal planes respectively. We construct an extended Hubbard type model including a deformation potential which is treated in unrestricted Hartree Fock approximation and describes correctly the observed first-order CO transition. We also discuss the influence of associated orbital order. Furthermore we suggest that due to different band fillings AlV$_2$O$_4$ should remain metallic while LiV$_2$O$_4$ under pressure should become a semiconductor when charge disproportionation sets in.

cond-mat.str-el↗

arXiv2005

Marginal Fermi Liquid Theory in the Hubbard Model

We find Marginal Fermi Liquid (MFL) like behavior in the Hubbard model on a square lattice for a range of hole doping and on-site interaction parameter U. Thereby we use a self-consistent projection operator method. It enables us to compute the momentum and frequency dependence of the single-particle excitations with high resolution. The Fermi surface is found to be hole-like in the underdoped and electron-like in the overdoped regime. When a comparison is possible we find consistency with finite temperature quantum Monte Carlo results. We also find a discontinuous change with doping concentration from a MFL to Fermi liquid behavior resulting from a collapse of the lower Hubbard band. This renders Luttinger's theorem inapplicable in the underdoped regime.

cond-mat.str-el↗

arXiv2005

A Spin-Mechanical Device for Detection and Control of Spin Current by Nanomechanical Torque

We propose a spin-mechanical device to control and detect spin currents by mechanical torque. Our hybrid nano-electro-mechanical device, which contains a nanowire with a ferromagnetic-nonmagnetic interface, is designed to measure or induce spin polarized currents. Since spin carries angular momentum, a spin flip or spin transfer process involves a change in angular momentum--and hence, a torque--which enables mechanical measurement of spin flips. Conversely, an applied torque can result in spin polarization and spin current.

cond-mat.mes-hall↗

arXiv2004

Approximative treatment of 5f-systems with partial localization due to intra-atomic correlations

Increasing experimental and theoretical evidence points towards a dual nature of the 5$f$ electrons in actinide-based strongly correlated metallic compounds, with some 5$f$ electrons being localized and others delocalized. In a recent paper (PRB xxx, 2004), we suggested the interplay of intra-atomic correlations as described by Hund's rules and a weakly anisotropic hopping (hybridization) as a possible mechanism. The purpose of the present work is to provide a first step towards a microscopic description of partial localization in solids by analyzing how well various approximation schemes perform when applied to small clusters. It is found that many aspects of partial localization are described appropriately both by a variational wavefunction of Gutzwiller type and by a treatment which keeps only those interactions which are present in LDA+U calculations. In contrast, the energies and phase diagram calculated within the Hartree Fock approximation show little resemblence with the exact results. Enhancement of hopping anisotropy by Hund's rule correlations are found in all approximations.

cond-mat.str-el↗

arXiv2004

Strong-coupling theory of magnetic-exciton-mediated superconductivity in UPd$_2$Al$_3$

There is compelling evidence from inelastic-neutron-scattering and tunneling experiments that the heavy-fermion superconductor UPd$_2$Al$_3$ can be understood as a dual system consisting of magnetic excitons, arising from crystal-field-split U$^{4+}$ levels, coupled to delocalised f-electrons. We have computed the superconducting transition temperature and the mass renormalisation arising from a dual model with maximal spin anisotropy using a strong-coupling approach. We find an instability to two possible opposite-spin-pairing states with even- or odd-parity gap functions. Each has a line node perpendicular to the c-direction, in agreement with NMR relaxation-rate, specific-heat and thermal-conductivity measurements. In addition, both have total spin component $S_z$=0, compatible with the observation of a pronounced Knight shift and $H_{c2}$ Pauli limiting. For parameter values appropriate to UPd$_2$Al$_3$, we determine the dependence of the superconducting transition temperature $T_c$ on a phenomenological coupling constant $g$ and we investigate the associated mass enhancement and its anisotropy.

cond-mat.str-el↗

arXiv2003

On a dual model for 5f electrons

We study the effect of intra-atomic correlations on anisotropies of hopping of 5f electrons. It is shown that they may considerably enhance these anisotropies to the extend that electrons in some of the 5f orbitals remain localized. This provide a microscopic basis for a previously made assumption that some of the 5f electrons must be partially localized when Fermi surfaces and effective masses of the component are calculated. Calculations are performed on two- and three-sites clusters. They include phase diagrams in the presence and absence of an external magnetic field as function of anisotropic hopping.

cond-mat↗

arXiv2003

Fermi surface and heavy masses for UPd$_2$Al$_3$

We calculate the Fermi surface and the anisotropic heavy masses of UPd2Al3 by keeping two of the 5f electrons as localized. Good agreement with experiments is found. The theory contains essentially no adjustable parameter except for a small shift of the position of the Fermi energy of the order of a few meV. A discussion is given why localization of two f electrons is justified.

cond-mat.str-el↗

arXiv2002

The dual nature of 5f electrons and origin of heavy fermions in U compounds

We develop a theory for the electronic excitations in UPt$_3$ which is based on the localization of two of the $5f$ electrons. The remaining $f$ electron is delocalized and acquires a large effective mass by inducing intra-atomic excitations of the localized ones. The measured deHaas-vanAlphen frequencies of the heavy quasiparticles are explained as well as their anisotropic heavy mass. A model calculation for a small cluster reveals why only the largest of the different $5f$ hopping matrix elements is operative causing the electrons in other orbitals to localize.

cond-mat.str-el↗

arXiv2002

Quasiclassical Hamiltonians for large-spin systems

We propose a method for obtaining effective classical Hamiltonians \cal H for many-body quantum spin systems with large spins. This method uses the coherent-state representation of the partition function Z and the cumulant expansion in powers of 1/S. For the quantum Hamiltonian \hat H of a Heisenberg form, the 1/S corrections in \cal H have a non-Heisenberg many-spin form. The effective Hamiltonian \cal H can be treated by methods familiar for classical systems. The non-Heisenberg terms in \cal H may be responsible for such effects as spin-Peierls transition and uplifting of the classical degeneracy by quantum fluctuations.

cond-mat.stat-mech↗

arXiv2001

A microscopic model for the structural transition and spin gap formation in alpha'-NaV2O5

We present a microscopic model for alpha'-NaV2O5. Using an extended Hubbard model for the vanadium layers we derive an effective low-energy model consisting of pseudospin Ising chains and Heisenberg chains coupled to each other. We find a ``spin-Peierls-Ising'' phase transition which causes charge ordering on every second ladder and superexchange alternation on the other ladders. This transition can be identified with the first transition of the two closeby transitions observed in experiment. Due to charge ordering the effective coupling between the lattice and the superexchange is enhanced. This is demonstrated within a Slater-Koster approximation. It leads to a second instability with superexchange alternation on the charge-ordered ladders due to an alternating shift of the O sites on the rungs of that ladder. We can explain within our model the observed spin gap, the anomalous BCS ratio, and the anomalous shift of the critical temperature of the first transition in a magnetic field. To test the calculated superstructure we determine the low-energy magnon dispersion and find agreement with experiment.

cond-mat.str-el↗