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U. Eckern

Publications and source records attributed to U. Eckern.

At least 37 records · Page 2Linked to original sources
arXiv2004

Charge order, orbital order, and electron localization in the Magneli phase Ti4O7

The metal-insulator transition of the Magneli phase Ti4O7 is studied by means of augmented spherical wave (ASW) electronic structure calculations as based on density functional theory and the local density approximation. The results show that the metal-insulator transition arises from a complex interplay of charge order, orbital order, and singlet formation of those Ti 3d states which mediate metal-metal bonding inside the four-atom chains characteristic of the material. Ti4O7 thus combines important aspects of Fe3O4 and VO2. While the charge ordering closely resembles that observed at the Verwey transition, the orbital order and singlet formation appear to be identical to the mechanisms driving the metal-insulator transition of vanadium dioxide.

cond-mat.str-el↗
arXiv2004

Superconductor-Ferromagnet Bi-Layers: a Comparison of s-Wave and d-Wave Order Parameters

We study superconductor-ferromagnet bi-layers, not only for s-wave but also for d-wave superconductors. We observe oscillations of the critical temperature when varying the thickness of the ferromagnetic layer for both s-wave and d-wave superconductors. However, for a rotated d-wave order parameter the critical temperature differs considerably from that for the unrotated case. In addition we calculate the density of states for different thicknesses of the ferromagnetic layer; the results reflect the oscillatory behaviour of the superconducting correlations.

cond-mat.supr-con↗
arXiv2003

Disordered Josephson Junctions of d-Wave Superconductors

We study the Josephson effect between weakly coupled d-wave superconductors within the quasiclassical theory, in particular, the influence of interface roughness on the current-phase relation and the critical current of mirror junctions and $45^\circ$ asymmetric junctions. For mirror junctions the temperature dependence of the critical current is non-monotonic in the limit of low roughness, but monotonic for very rough interfaces. For $45^\circ$ asymmetric junctions with a linear dimension much larger than the superconducting coherence length we find a $\sin(2ϕ)$-like current-phase relation, whereas for contacts on the scale of the coherence length or smaller the usual $\sinϕ$-like behavior is observed. Our results compare well with recent experimental observations.

cond-mat.supr-con↗
arXiv2003

The Metal-Insulator Transition of the Magneli phase V_4O_7: Implications for V_2O_3

The metal-insulator transition (MIT) of the Magneli phase V_4O_7 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of all Magneli phase compounds V_nO_2n-1 (3 =< n =< 9) as well as of VO_2 and V_2O_3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. In addition, the relationship between the structural and electronic properties near the MIT of these oxides can be studied on an equal footing. For V_4O_7, a strong influence of metal-metal bonding across octahedral faces is found for states both parallel and perpendicular to the hexagonal c_hex axis of V_2O_3. Furthermore, the structural changes at the MIT cause localization of those states, which mediate in-plane metal-metal bonding via octahedral edges. This band narrowing opens the way to an increased influence of electronic correlations, which are regarded as playing a key role for the MIT of V_2O_3.

cond-mat↗
arXiv2003

Octahedral Tilting in ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd)

The perovskite-like compounds ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd) are studied by means of density functional theory based electronic structure calculations using the augmented spherical wave (ASW) method. The electronic properties are strongly influenced by covalent type bonding between transition metal d and oxygen p states. The characteristic tilting of the RuO6 octahedra arises mainly from the Cu--O bonding, allowing for optimal bond lengths between these two atoms. Our results provide a deeper understanding of octahedral tilting as a universal mechanism, applicable to a large variety of multinary compounds.

cond-mat.mtrl-sci↗
arXiv2002

Quantum spin chains with various defects

Using the density matrix renormalization group (DMRG) method, we study the quantum coherence in one-dimensional disordered Fermi systems. We consider in detail spinless fermions on a ring, and compare the influence of several kinds of impurities in a gapless and a dimerized, gapped system. In the translation-invariant system a so-called site-impurity, which can be realized by a local potential or a change of one link, increases for repulsive interaction and decreases for attractive interaction. The weakening of two neighboring bonds, which is a realization of a so-called bond-impurity, on the other hand, is healed for repulsive interaction, but enhanced for intermediate attractive interactions. This leads to strong suppression of the quantum coherence measured by the phase sensitivity, but not to localization. Adding a local distortion to a dimerized system we find that even the presence of a single site-impurity increases the metallic region found in the dimerized model. For a strong dimerization and a high barrier, an additional sharp maximum, even seen for systems with about 100 sites, occurs in the phase sensitivity. A bond-impurity in the dimerized system also opens a small metallic window in the otherwise isolating regime.

cond-mat.str-el↗
arXiv2002

From VO2 to V2O3: The Metal-Insulator Transition of the Magneli Phase V6O11

The metal-insulator transition (MIT) of V6O11 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of V6O11, VO2, and V2O3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. While the sesquioxide-like V 3d_yz states show rather weak changes on entering the low-temperature structure, some of the dioxide-like V 3d_x^2-y^2 states display splittings and shifts similar to those known from VO2. The MIT of V6O11 arises as a combination of changes appearing in both of these compounds. Our results shed new light onto the role of particular electronic states for the MIT of V2O3.

cond-mat.str-el↗
arXiv2002

Persistent Currents versus Phase Breaking in Mesoscopic Metallic Samples

Persistent currents in mesoscopic normal metal rings represent, even a decade after their first experimental observation, a challenge to both, theorists and experimentalists. After giving a brief review of the existing -- experimental and theoretical -- results, we concentrate on the (proposed) relationship of the size of the persistent current to the phase breaking rate. In particular, we consider effects induced by noise, scattering at two-level systems, and magnetic impurities.

cond-mat.mes-hall↗
arXiv2001

Two component interacting bosons: 1-d exact results

Motivated by the experiment of two-component Bose-Einstein condensates produced in magnetically trapped $^{87}Rb$, we study one dimensional Boson systems with repulsive $δ$-function interaction in the presence of SU(2) intrinsic degree of freedom by means of coordinate Bethe ansatz. The ground state and low-lying excitations are solved by both numerical and analytical methods. It is shown that the ground state is an isospin-ferromagnetic state and the excitations are composed of three elementary particles: holons, antiholons and isospinons. The isospinon is a triplet coupled to the ``ferromagnetic'' background antiparallelly.

cond-mat.str-el↗
arXiv2001

D-Wave Superconductors near Surfaces and Interfaces: A Scattering Matrix Approach within the Quasiclassical Technique

A recently developed method [A. Shelankov and M. Ozana, Phys. Rev. B 61, 7077 (2000)] is applied to investigate d-wave superconductors in the vicinity of (rough) surfaces. While this method allows the incorporation of arbitrary interfaces into the quasiclassical technique, we discuss, as examples, diffusive surfaces and boundaries with small tilted mirrors (facets). The properties of the surface enter via the scattering matrix in the boundary condition for the quasiclassical Green's function. The diffusive surface is described by an ensemble of random scattering matrices. We find that the fluctuations of the density of states around the average are small; the zero bias conductance peak broadens with increasing disorder. The faceted surface is described in the model where the scattering matrix couples m in- and m out-trajectories (m>=2). No zero bias conductance peak is found for [100] surfaces; the relation to the model of Fogelstrom et al. [Phys. Rev. Lett. 79, 281 (1997)] is discussed.

cond-mat.supr-con↗
arXiv2000

Exact solution of generalized Schulz-Shastry type models

A class of integrable one-dimensional models presented by Shastry and Schulz is consequently extended to the whole class of one-dimensional Hubbard- or XXZ-type models with correlated gauge-like hopping. A complete characterization concerning solvability by coordinate Bethe ansatz of this class of models is found.

cond-mat.str-el↗
arXiv2000

Theory of electrons with orbital degeneracy

The Hubbard model for electrons with orbital degeneracy is shown to have an underlying SU_d(4) symmetry of spin-orbital double. A hidden charge SU_c(4) symmetry is exposed and an extended Lieb-Mattis transformation which maps these two symmetries into each other is given. On the basis of elementary degenerate perturbative theory, it is shown that the system with strong repulsive coupling is equivalent to a SO(6) Heisenberg magnet at half-filling and a SU(4) one at quarter-filling. The band is half-filled at all temperature for $μ=3U/2$. The features of ground state and low-lying excitations in one dimension are indicated according to exact solutions. Various possibilities of symmetry breaking are also given.

cond-mat.str-el↗
arXiv1999

Bethe Ansatz solution of a new class of Hubbard-type models

We define one-dimensional particles with generalized exchange statistics. The exact solution of a Hubbard-type Hamiltonian constructed with such particles is achieved using the Coordinate Bethe Ansatz. The chosen deformation of the statistics is equivalent to the presence of a magnetic field produced by the particles themselves, which is present also in a ``free gas'' of these particles.

cond-mat.str-el↗
arXiv1998

Scalar potential effect in an integrable Kondo model

To study the impurity potential effect to the Kondo problem in a Luttinger liquid, we propose an integrable model of two interacting half-chains coupled with a single magnetic impurity ferromagnetically. It is shown that the scalar potential effectively reconciles the spin dynamics at low temperatures. Generally, there is a competition between the Kondo coupling $J$ and the impurity potential $V$. When the ferromagnetic Kondo coupling dominates over the impurity potential ($V<|SJ|$), the Furusaki-Nagaosa many-body singlet can be perfectly realized. However, when the impurity potential dominates over the Kondo coupling ($V\geq |SJ|$), the fixed point predicted by Furusaki and Nagaosa is unstable and the system must flow to a weak coupling fixed point. It is also found that the effective moment of the impurity measured from the susceptibility is considerably enlarged by the impurity potential. In addition, some quantum phase transitions driven by the impurity potential are found and the anomaly residual entropy is discussed.

cond-mat↗
arXiv1998

Ghost spins and novel quantum critical behavior in a spin chain with local bond-deformation

We study the boundary impurity-induced critical behavior in an integrable SU(2)-invariant model consisting of an open Heisenberg chain of arbitrary spin-$S$ (Takhatajian-Babujian model) interacting with an impurity of spin $\vec{S'}$ located at one of the boundaries. For $S=1/2$ or $S'=1/2$, the impurity interaction has a very simple form $J\vec{S}_1\cdot\vec{S'}$ which describes the deformed boundary bond between the impurity $\vec{S'}$ and the first bulk spin $\vec{S}_1$ with an arbitrary strength $J$. With a weak coupling $0 S'$ and $S<S'$ as in the usual Kondo problem. While for strong coupling $J\geq J_0/[(S+S')^2-1/4]$, the impurity spin is split into two ghost spins. Their cooperative effect leads to a variety of new critical behaviors with different values of $|S'-S|$.

cond-mat.str-el↗
arXiv1998

Transport in a Nearly Periodic Potential with a Magnetic Field

A two-dimensional gas of non-interacting quasiparticles in a nearly periodic potential and a perpendicular magnetic field is studied. The potential is a superposition of a periodic potential, induced e.g. by a charge density wave or a vortex lattice of a type--II superconductor, and a weak random potential due to impurities. Approximating this model by Dirac fermions with random mass, random energy, and random vector potential, we evaluate the density of states and the Hall conductivity using a self-consistent approximation. We obtain a singular average density of states $ρ(E)\approxρ_0 +|E|^α$, where $α$ decreases with the strength of randomness. The Hall conductivity has a plateau which is destroyed for strong tunneling through the saddle points of the nearly periodic potential.

cond-mat.mes-hall↗
arXiv1998

Non-linear conductivity of charge-density-wave systems

We consider the problem of sliding motion of a charge-density-wave subject to static disorder within an elastic medium model. Starting with a field-theoretical formulation, which allows exact disorder averaging, we propose a self-consistent approximation scheme to obtain results beyond the standard large-velocity expansion. Explicit calculations are carried out in three spatial dimensions. For the conductivity, we find a strong-coupling regime at electrical fields just above the pinning threshold. Phase and velocity correlation functions scale differently from the high-field regime, and static phase correlations converge to the pinned-phase behaviour. The sliding charge-density-wave is accompanied by narrow-band noise.

cond-mat.dis-nn↗