arXiv subjects

S. -L. Drechsler

Publications and source records attributed to S. -L. Drechsler.

At least 91 records · Page 5Linked to original sources

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

Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in Δ_[100]=1.5 meV for the a direction and Δ_[001]=2.3 meV along the c axis; however the critical temperature T_c in PC in all cases is near to that in the bulk sample. The value 2Δ_[001]/kT_c=3.6 is close to the BCS value of 3.52, and the temperature dependence Δ_[001](T) is BCS-like, while the for small gap Δ_[100](T) is below BCS behavior at T>T_c/2 similarly as in the two-gap superconductor MgB2. It is supposed that the directional variation Δcan be attributed to a multiband nature of the SC state in YNi2B2C.

cond-mat.supr-con↗

arXiv2006

Exact one- and two-particle excitation spectra of acute-angle helimagnets above their saturation magnetic field

The two-magnon problem for the frustrated XXZ spin-1/2 Heisenberg Hamiltonian and external magnetic fields exceeding the saturation field Bs is considered. We show that the problem can be exactly mapped onto an effective tight-binding impurity problem. It allows to obtain explicit exact expressions for the two-magnon Green's functions for arbitrary dimension and number of interactions. We apply this theory to a quasi-one dimensional helimagnet with ferromagnetic nearest neighbor J1 < 0 and antiferromagnetic next-nearest neighbor J2 > 0 interactions. An outstanding feature of the excitation spectrum is the existence of two-magnon bound states. This leads to deviations of the saturation field Bs from its classical value Bs(classical) which coincides with the one-magnon instability. For the refined frustration ratio |J2/J1|> 0.374661 the minimum of the two-magnon spectrum occurs at the boundary of the Brillouin zone. Based on the two-magnon approach, we propose general analytic expressions for the saturation field Bs, confirming known previous results for one-dimensional isotropic systems, but explore also the role of interchain and long-ranged intrachain interactions as well as of the exchange anisotropy.

cond-mat.str-el↗

arXiv2006

Point-contact spectroscopy of the borocarbide superconductor YNi2B2C

Point-contact (PC) spectroscopy measurements on YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) function with a dominant maximum around 12meV and a further weak structure (kink or shallow broad maximum) at higher energy at about 50meV. Other phonon maxima at 20, 24 and 32meV specified in the phonon DOS of YNi2B2C by neutron measurements [PRB, V.55, 9058 (1997)] are not resolved in the PC spectra pointing out to the main role of the low energy phonon modes in EPI. Directional study of the SC gap results in Δ_[100]=1.5meV for the a- direction and Δ_[001]=2.4meV along the c-axis which may point to anisotropic and/or multiband behavior. Noteworthy, the critical temperature T_c in all cases corresponds to that of bulk samples. The value 2Δ_[001]/kT_c=3.6 is close to the BCS one of 3.52, and the temperature dependence Δ(T) is BCS-like, while for the a-direction Δ(T) deviates from mean-field BCS behavior above T_c/2. The directional variation in Δcan be attributed to the multiband nature of the SC state in YNi2B2C predicted 10 years ago (PRL, V.80, 1730 (1998)).

cond-mat.supr-con↗

arXiv2006

Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C

The point-contact (PC) spectroscopy study of the electron-phonon(boson) interaction (EP(B)I) spectral function in HoNi2B2C reveals phonon maxima at 16 and 22meV and 34meV. For the first time the pronounced high energy maxima at about 50meV and 100meV were resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitation with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the CEF peak in EP(B)I constant λ_{PC} is evaluated as 20-30%, while contribution of the high frequency modes at 50 and 100meV amounts about 10% for each maxima. PC Andreev reflection measurements below the critical temperature T_c=8.5K reveals two different superconducting (SC) states separated by T_c^*=5.6K which is close to the Neel temperature T_N=5.3K. Below T_c^* the gap Δin HoNi2B2C exhibits a standard single-band BCS-like behavior with 2Δ/kT_c^*=3.9. Above T_c^* the gap features in PC spectra are strongly suppressed pointing to a peculiar SC state between T_c^* and T_c, where incommensurate magnetic order develops.

cond-mat.supr-con↗

arXiv2006

Point-contact spectroscopy of the nickel borocarbide superconductors RNi2B2C (R=Y, Dy, Ho, Er, Tm, Lu)

An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi2B2C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron- boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R=Dy, Ho, Er and Tm, while for R=Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R=Ho and near 6 meV in R=Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R=Y, Lu. At low temperature the SC gap in R=Ho exhibits a standard single-band BCS-like dependence, which vanishes above T_c^*= 5.6K< T_c=8.5K, where a specifc magnetic ordering starts to play a role. For R=Tm (T_c=10.5 K) a decrease of the SC gap is observed below 5 K.

cond-mat.supr-con↗

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

Coexistence of metallicity and superconductivity in adjacent bilayers of a high-Tc superconductor

Experimental studies of the electronic structure remain the basic means for understanding the nature of high-temperature superconductivity (HTSC) and testing relevant theoretical models. Appreciable contributions to establish the overall picture in HTSC have recently been made by investigations on the charge dynamics in BSCCO (ref. 1) and the spin dynamics in YBCO, using Angle Resolved Photoemission Spectroscopy (ARPES) and Inelastic Neutron Scattering (INS), respectively. Concentration of each of these techniques on a "suitable" compound turns out to be a barrier on the way to a crucial quantitative test allowing to support or discard spin fluctuations (interaction between the charge and spin degrees of freedom) as a possible origin for the pairing in doped cuprates. Here we solve the long-standing puzzle of ARPES on YBCO by showing that the photoelectron spectrum of YBCO generally includes two components: One from the topmost anomalously overdoped metallic CuO2 bilayer and the other from the next superconducting bilayer that retains the bulk properties. Our findings clearly show the opening of a large superconducting gap and, for the first time, demonstrate the anisotropic renormalization in the bulk component of YBCO, supporting the universality of these effects for different cuprate families. With our study we re-open this cuprate family for new systematic ARPES investigations.

cond-mat.supr-con↗

arXiv2006

High-field ESR studies of the quantum spin magnet CaCu2O3

We report an electron spin resonance (ESR) study of the s=1/2-Heisenberg pseudo-ladder magnet CaCu2O3 in pulsed magnetic fields up to 40 T. At sub-Terahertz frequencies we observe an ESR signal originating from a small amount of uncompensated spins residing presumably at the imperfections of the strongly antiferromagnetically correlated host spin lattice. The data give evidence that these few percent of ''extra'' spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic ordering at T_N = 25 K. By mapping the frequency/resonance field diagram we have determined a small gap for magnetic excitations below T_N of the order of 0.3 - 0.8 meV. Such a small value of the gap explains the occurrence of the spin-flop transition in CaCu2O 3 at weak magnetic fields H_{sf} ~ 3 T. Qualitative changes of the ESR response with increasing the field strength give indications that strong magnetic fields reduce the antiferromagnetic correlations and may even suppress the long-range magnetic order in CaCu2O3. ESR data support scenarios with a significant role of the ''extra'' spin states for the properties of low-dimensional quantum magnets.

cond-mat.str-el↗

arXiv2005

Weak inter-band coupling in Mg$^{10}$B$_{2}$: a specific heat analysis

The superconducting state of Mg$^{10}$B$_{2}$ is investigated by specific heat measurements in detail. The specific heat in the normal state is analyzed using a recently developed computer code. This allows for an extraction of the electronic specific heat in the superconducting state with high accuracy and a fair determination of the main lattice features. One of the two investigated samples shows a hump in the specific heat at low temperatures within the superconducting state, accompanied by an unusual low value of the small gap, $Δ_π(0)=1.32 meV$, pointing to a very weak inter-band coupling. This sample allows for a detailed analysis of the contribution from the $π$-band to the electronic specific heat in the superconducting state. Therefore the usual analysis method is modified, to include the individual conservation of entropy of both bands. From analyzing the deviation function $D(t)$ of MgB$_{2}$, the theoretically predicted weak inter-band coupling scenario is confirmed.

cond-mat.supr-con↗

arXiv2005

The role of carbon for superconductivity in MgC$_{x}$Ni$_{3}$ from specific heat

The influence of carbon deficiency on superconductivity of MgCNi$_{3}$ is investigated by specific heat measurements in the normal and superconducting state. In order to perform a detailed analysis of the normal state specific heat, a computer code is developed which allows for an instantaneous estimate of the main features of the lattice dynamics. By analyzing the evolution of the lattice vibrations within the series and simultaneously considering the visible mass enhancement, the loss in the electron-phonon coupling can be attributed to significant changes of the prominent Ni vibrations. The present data well supports the recently established picture of strong electron-phonon coupling and ferromagnetic spin fluctuations in this compound.

cond-mat.supr-con↗

arXiv2005

Charge Transfer Excitations in Insulating Copper Oxides

A semi-quantitative cluster approach is developed to describe the charge transfer (CT) electron-hole excitations in insulating cuprates in a rather wide energy range up to 10- 15 eV. It generalizes the Zhang-Ng (ZN) model of CT excitons by considering the complete set of Cu3d and O2p orbitals within the CuO4 embedded molecular cluster method and by introducing one-center (intra-center) Frenkel-like and two-center (inter-center) excitons. Special attention is paid to the transition matrix element effects both in optical and electron energy loss spectra (EELS). In the latter case we obtain the momentum dependence of matrix elements both for intra-center and inter-center transitions. We are able to give a semi-quantitative description of the optical and EELS spectra for a large number of 0D (like CuB2O4), 1D (Sr2CuO3) and 2D (like Sr2CuO2Cl2) insulating cuprates in a unifying manner. By comparing our analysis with the experimental data we find that the CT gap in insulating cuprates is determined by nearly degenerate intra-center localized excitations and inter-center CT excitons. The corresponding EELS intensity is found to be strongly k-dependent: even for isolated exciton it sharply decreases by approaching the Brillouin zone (BZ) boundary. It is shown that the interaction of two-center excitons can result in destructive interference effects with an intensity compensation point.

cond-mat.str-el↗

arXiv2005

Distribution of the superconducting gap in an YNi2B2C film studied by point contact spectroscopy

The differential resistances $R_d=dV/dI(V)$ of point contacts between a normal metal and a c axis oriented YNi2B2C film ($T_c$ = 15.2K) in the superconducting (SC) state have been investigated. $R_d(V)$ contains clear "gap" features connected with processes of Andreev reflection at the boundary between normal metal and superconductor that allow the determination of the SC gap $Δ$ and its temperature and magnetic field dependence. A distribution of $Δ$ from $Δ_min\approx$ 1.5 meV to $Δ_max\approx$ 2.4 meV is revealed; however the critical temperature $T_c$ in all cases corresponded to that of the film. The value 2$Δ_max/k_BT_c\approx$3.66 is close to the BCS value of 3.52, and the temperature dependence $Δ(T)$ is BCS-like, irrespective of the actual $Δ$ value. It is supposed that the distribution of $Δ$ can be attributed to a gap anisotropy or to a multiband nature of the SC state in YNi2B2C, rather than to the presence of nodes in the gap.

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

Point-contact investigations of challenging superconductors: two-band MgB2, antiferromagnetic HoNi2B2C, heavy fermion UPd2Al3, paramagnetic MgCNi3

An overview on recent efforts in point-contact (PC) spectroscopy of title superconductors is given. Distinct phonon features and crystalline-electric-field effects are observed in PC spectra of HoNi2B2C. Results of study of superconducting (SC) gap and excess current versus temperature and magnetic field reflecting specific multi-band electronic structure in MgB2 are presented. The nature of the extremely nonlinear I(V) curves in the antiferromagnetic (AF) and SC state are elucidated for UPd2Al3 break-junctions and MgCNi3 point contacts.

cond-mat.supr-con↗

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

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↗