arXiv subjects

H. Rosner

Publications and source records attributed to H. Rosner.

At least 109 records · Page 6Linked to original sources

arXiv2008

Strong coupling between magnetic and structural order parameters in SrFe2As2

X-ray and Neutron diffraction as well as muon spin relaxation and Mössbauer experiments performed on SrFe$_2$As$_2$ polycrystalls confirm a sharp first order transition at $T_0 = 205$,K corresponding to an orthorhombic phase distortion and to a columnar antiferromagnetic Fe ordering with a propagation vector (1,0,1), and a larger distortion and larger size of the ordered moment than reported for BaFe$_2$As$_2$. The structural and the magnetic order parameters present an remarkable similarity in their temperature dependence from $T_0$ down to low temperatures, showing that both phenomena are intimately connected. Accordingly, the size of the ordered Fe moments scale with the lattice distortion when going from SrFe$_2$As$_2$ to BaFe$_2$As$_2$. Full-potential band structure calculations confirm that the columnar magnetic order and the orthorhombic lattice distortion are intrinsically tied to each other.

cond-mat.supr-con↗

arXiv2008

Electrical Resistivity and Specific Heat of EuFe2As2 Single Crystals: Magnetic homologue of SrFe2As2

We have grown single crystals of EuFe2As2 and investigated its electrical transport and thermodynamic properties. Electrical resistivity and specific heat measurements clearly establish the intrinsic nature of magnetic phase transitions at 20 K and 195 K. While the high temperature phase transition is associated with the itinerant moment of Fe, the low temperature phase transition is due to magnetic order of localized Eu-moments. Band structure calculations point out a very close similarity of the electronic structure with SrFe2As2. Magnetically, the Eu and Fe2As2 sublattice are nearly de-coupled.

cond-mat.supr-con↗

arXiv2008

Magnetic and structural transitions in layered FeAs systems: AFe2As2 versus RFeAsO compounds

Resistivity, specific heat and magnetic susceptibility measurements performed on SrFe2As2 samples evidence a behavior very similar to that observed in LaFeAsO and BaFe2As2 with the difference that the formation of the SDW and the lattice deformation occur in a pronounced first order transition at T_0=205K. Comparing further data evidences that the Fe-magnetism is stronger in SrFe2As2 and in EuFe2As2 than in the other layered FeAs systems investigated up to now. Full potential LDA band structure calculations confirm the large similarity between the compounds, especially for the relevant low energy Fe 3d states. The relation between structural details and magnetic order is analyzed.

cond-mat.supr-con↗

arXiv2008

Normal State Correlated Electronic Structure of Iron Pnictides

We describe the correlated electronic structure of a prototype Fe-pnictide superconductor, $SmO_{1-x}F_{x}FeAs$, using LDA+DMFT. Strong, multi-orbital electronic correlations generate a low-energy pseudogap in the undistorted phase, giving a bad, incoherent metal in qualitative agreement with observations. Very good semi-quantitative agreement with the experimental spectral functions is seen, and interpreted, within a correlated, multi-orbital picture. Our results show that Fe-pnictides should be understood as low-carrier density, incoherent metals, in resemblance to the underdoped cuprate superconductors.

cond-mat.str-el↗

arXiv2008

Electronic and magnetic properties of K$_{2}$CuP$_{2}$O$_{7}$ - a model S=1/2 Heisenberg chain system

The electronic and magnetic properties of K$_{2}$CuP$_{2}$O$_{7}$ were investigated by means of susceptibility, specific heat and $^{31}$P nuclear magnetic resonance (NMR) measurements and by LDA band structure calculations. The temperature dependence of the NMR shift $K(T)$ is well described by the $S=1/2$ Heisenberg antiferromagnetic chain (HAF) model with nearest neighbor exchange $J_{1}$ $\simeq $ $(141\pm 5)$ K. The corresponding mapping of an LDA-derived tight-binding model leads to $J_{1}^{LDA}$ $\simeq$ 196 K. The spin lattice relaxation rate $1/T_{1}$ decreases with temperature below 300 K but becomes nearly temperature independent between 30 K and 2 K as theoretically expected for an $S = {1/2}$ HAF chain. None of the investigated properties give any evidence for long range magnetic order above 2K, in agreement with the results of the band structure calculation, which yield extremely weak exchange to the next nearest neighbor (NNN) and a very small and frustrated inter-chain exchange. Thus, K$_{2}$CuP$_{2}$O$_{7}$ seems to be a better realization of a nearest neighbor $S = {1/2}$ HAF chain than the compounds reported so far.

cond-mat.str-el↗

arXiv2007

Superconductivity in the New Platinum Germanides MPt4Ge12 (M = Rare-earth and Alkaline-earth Metals) with Filled Skutterudite Structure

New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. The structure and composition were investigated by X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific heat, and electrical resistivity measurements evidence superconductivity in LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and superconducting states were established. Strong coupling and a crystal electric field singlet groundstate is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level. Similar behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12.

cond-mat.supr-con↗

arXiv2007

CeRuPO: A rare example of a Ferromagnetic Kondo lattice

We have determined the physical ground state properties of the compounds CeRuPO and CeOsPO by means of magnetic susceptibility chi(T), specific heat C(T), electrical resistivity rho(T), and thermopower S(T) measurements. chi(T) reveals a trivalent 4f1 cerium state in both compounds. For CeRuPO a pronounced decrease of rho(T) below 50K indicates the onset of coherent Kondo scattering which is confirmed by enhanced S(T). The temperature and magnetic field dependence of chi(T) and C(T) evidence ferromagnetic (FM) order at TC=15K. Thus, CeRuPO seems to be one of the rare example of a FM Kondo lattice. In contrast, CeOsPO shows antiferromagnetic order at TN=4.4K despite only minor changes in lattice parameters and electronic configuration. Additional 31P NMR results support these scenarios. LSDA+U calculations evidence a quasi two dimensional electronic band structure, reflecting a strong covalent bonding within the CeO and RuP layers and a weak ionic like bonding between the layers.

cond-mat.str-el↗

arXiv2007

The route of frustrated cuprates from antiferromagnetic to ferromagnetic spin-1/2 Heisenberg chains: Li2ZrCuO4 as a missing link near the quantum critical point

From thermodynamics, LSDA+U calculations and exact diagonalization studies of a five-band Hubbard (Cu 3d, O 2p) model on CuO2 stripes we found that Li2ZrCuO4 (Li2CuZrO4 in traditional notation) is close to a ferromagnetic critical point. Analyzing its measured susceptibility chi(T) and specific heat cp(T,H) within a Heisenberg model, we show that the ratio of the 2nd to the 1st neighbor exchange integrals alpha=-J2/J1 of the order of 0.3, i.e. it is close to the critical value alpha_c=1/4. Comparing with related chain cuprates we explain the rather strong field dependence of cp, the monotonous down shift of the peak of chi(T), and its increase for alpha approaching 1/4 + 0.

cond-mat.str-el↗

arXiv2006

Sr$_2$Cu(PO$_4$)$_2$: A real material realization of the 1D nearest neighbor Heisenberg chain

We present evidence that crystalline Sr_2Cu(PO_4)_2 is a nearly perfect one-dimensional (1D) spin-1/2 anti-ferromagnetic Heisenberg model (AHM) chain compound with nearest neighbor only exchange. We undertake a broad theoretical study of the magnetic properties of this compound using first principles (LDA, LDA+U calculations), exact diagonalization and Bethe-ansatz methodologies to decompose the individual magnetic contributions, quantify their effect, and fit to experimental data. We calculate that the conditions of one-dimensionality and short-ranged magnetic interactions are sufficiently fulfilled that Bethe's analytical solution should be applicable, opening up the possibility to explore effects beyond the infinite chain limit of the AHM Hamiltonian. We begin such an exploration by examining some extrinsic effects such as impurities and defects.

cond-mat.mtrl-sci↗

arXiv2006

Comparative investigation of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X=Cl, Br and Cu4Te5O12Cl4

We present a comparative study of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X=Cl, Br (Cu-2252(X)) and the newly synthesized Cu4Te5O12Cl4 (Cu-45124(Cl)) based on ab initio Density Functional Theory calculations. The magnetic behavior of Cu-45124(Cl) with a phase transition to an ordered state at a lower critical temperature T$_c$=13.6K than in Cu-2252(Cl) (T$_c$=18K) can be well understood in terms of the modified interaction paths. We identify the relevant structural changes between the two systems and discuss the hypothetical behavior of the not yet synthesized Cu-45124(Br) with an ab initio relaxed structure using Car-Parrinello Molecular Dynamics.

cond-mat.str-el↗

arXiv2006

Origin of Strong Coupling in Lithium under Pressure

In an attempt to provide a clearer understanding of the impressive increase in T_c under pressure in elemental Li, linear response calculation of the phonon dispersion curves, electron-phonon matrix elements, phonon linewidths and mode lambda's have been carried out on a finer mesh (24^3 in the Brillouin zone) than done previously, for the volume corresponding to 20 GPa pressure. The result illustrates the great need for a fine mesh (even finer than this) for converged results of lambda and the spectral function alpha^2 F. Although the initial pressure-induced transverse T_1 phonon instability (in harmonic approximation) near the symmetry point K has dominated attention, the current results show that the high value of T_c gets strong contributions from elsewhere in the zone, particularly from the longitudinal mode along (100).

cond-mat.supr-con↗

arXiv2005

High spin polarization in the ferromagnetic filled skutterudites KFe4Sb12 and NaFe4Sb12

The spin polarization of ferromagnetic alkali-metal iron antimonides KFe4Sb12 and NaFe4Sb12 is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization Pt of 67% and 60% for the K and Na compound, respectively, is inferred which establishes these materials as a new class of highly spin polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation (LSDA) that predict nearly 100% spin polarization in the density of states. We discuss the impact of calculated Fermi velocities and spin fluctuations on Pt.

cond-mat.mtrl-sci↗

arXiv2005

Superconductivity and Lattice Instability in Compressed Lithium from Fermi Surface Hot Spots

The highest superconducting temperature T$_c$ observed in any elemental metal (Li with T$_c$ ~ 20 K at pressure P ~ 40 GPa) is shown to arise from critical (formally divergent) electron-phonon coupling to the transverse T$_1$ phonon branch along intersections of Kohn anomaly surfaces with the Fermi surface. First principles linear response calculations of the phonon spectrum and spectral function $α^2 F(ω)$ reveal (harmonic) instability already at 25 GPa. Our results imply that the fcc phase is anharmonically stabilized in the 25-38 GPa range.

cond-mat.supr-con↗

arXiv2004

Comment on "Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2"

The microscopic origin of the recently observed helical structure in LiCu2O2 [1] is considered. It is shown that the frustrated antiferromagnetic double chain scenario adopted in Ref.1 is unrealistic. It should be replaced by a frustrated single-chain (CuO2) scenario proposed in Ref. 2 with ferromagnetic nearest neighbor and antiferromagnetic next nearest neighbor exchange integrals \. [1] T. Masuda et al. Phys. Rev. Lett. 92 (2004), 177201. [2] A. Gippius et al. Phys. Rev. B 70 (2004), R01426; cond-mat/0312576.

cond-mat.str-el↗

arXiv2004

Evidence for CuO conducting band splitting in the nodal direction of Bi-2212

Using angle-resolved photoemission spectroscopy with ultimate momentum resolution we have explicitly resolved the bilayer splitting in the nodal direction of Bi-2212. The splitting is observed in a wide doping range and, within the experimental uncertainty, its size does not depend on doping. The value of splitting derived from the experiment is in good agreement with that from band structure calculations which implies the absence of any electronic confinement to single planes within bilayers of Bi-2212. Other consequences of this finding are also discussed.

cond-mat.supr-con↗

arXiv2004

TiOCl, an orbital-ordered system?

We present first principles density functional calculations and downfolding studies of the electronic and magnetic properties of the layered quantum spin system TiOCl. We discuss explicitely the nature of the exchange pathes and attempt to clarify the concept of orbital ordering in this material. An analysis of the electronic structure of slightly distorted structures according to the phononic modes allowed in this material suggests that this system is subject to large orbital fluctuations driven by the electron-phonon coupling. Based on these results, we propose a microscopic explanation of the behavior of TiOCl near the phase transition to a spin-gapped system.

cond-mat.str-el↗

arXiv2004

Evidence for strong electron-phonon coupling in MgCNi_3

The title compound is investigated by specific heat measurements in the normal and superconducting state supplemented by upper critical field transport, susceptibility and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities and partial densities of states the presence of both strong electron-phonon interactions and considerable pair-breaking has been revealed. The specific heat and the upper critical field data can be described to first approximation by an effective single band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated s-wave superconductivity somewhat suppressed by sizeable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a van Hove type singularity in the electronic density of states near the Fermi energy is suggested.

cond-mat.supr-con↗

arXiv2003

NMR and LDA evidence for spiral magnetic order in the chain cuprate LiCu2O2

We report on {6,7}Li nuclear magnetic resonance measurements of the spin-chain compound LiCu2O2 in the paramagnetic and magnetically ordered states. Below T about 24 K the NMR lineshape presents a clear signature of incommensurate (IC) static modulation of the local magnetic field consistent with an IC spiral modulation of the magnetic moments. {7}Li NMR reveals strong phason-like dynamical fluctuations extending well below 24 K. We hypothesize that a series of phase transitions at 24.2, 22.5, and 9 K reflects a "Devil's staircase" type behavior generic for IC systems. LDA based calculations of exchange integrals reveal a large in-chain frustration leading to a magnetical spiral.

cond-mat.str-el↗