arXiv subjects

C. Geibel

Publications and source records attributed to C. Geibel.

At least 127 records · Page 7Linked to original sources

arXiv2008

High-Temperature Superconductivity in Eu0.5K0.5Fe2As2

Subsequent to our recent report of SDW type transition at 190 K and antiferromagnetic order below 20 K in EuFe2As2, we have studied the effect of K-doping on the SDW transition at high temperature and AF order at low temperature. 50% K doping suppresses the SDW transition and in turn gives rise to high-temperature superconductivity below T_c = 32 K, as observed in the electrical resistivity, AC susceptibility as well as magnetization. A well defined anomaly in the specific heat provides additional evidence for bulk superconductivity.

cond-mat.supr-con↗

arXiv2008

Low-temperature thermopower study of YbRh2Si2

The heavy-fermion compound YbRh2Si2 exhibits an antiferromagnetic (AFM) phase transition at an extremely low temperature of TN = 70 mK. Upon applying a tiny magnetic field of Bc = 60 mT the AFM ordering is suppressed and the system is driven toward a field-induced quantum critical point (QCP). Here, we present low-temperature thermopower S(T) measurements of high-quality YbRh2Si2 single crystals down to 30 mK. S(T) is found negative with comparably large values in the paramagnetic state. In zero field no Landau-Fermi-liquid (LFL) like behavior is observed within the magnetically ordered phase. However, a sign change from negative to positive appears at lowest temperatures on the magnetic side of the QCP. For higher fields B > Bc a linear extrapolation of S to zero clearly evidences the recovery of LFL regime. The crossover temperature is sharply determined and coincides perfectly with the one derived from resistivity and specific heat investigations.

cond-mat.str-el↗

arXiv2008

Strong frustration due to competing ferromagnetic and antiferromagnetic interactions: magnetic properties of M(VO)2(PO4)2 (M = Ca and Sr)

We present a detailed investigation of the magnetic properties of complex vanadium phosphates M(VO)2(PO4)2 (M = Ca, Sr) by means of magnetization, specific heat, 31P NMR measurements, and band structure calculations. Experimental data evidence the presence of ferro- and antiferromagnetic interactions in M(VO)2(PO4)2 resulting in a nearly vanishing Curie-Weiss temperature θ_{CW} < 1 K that contrasts with the maximum of magnetic susceptibility at 3 K. Specific heat and NMR measurements also reveal weak exchange couplings with the thermodynamic energy scale J_c = 10-15 K. Additionally, the reduced maximum of the magnetic specific heat indicates strong frustration of the spin system. Band structure calculations show that the spin systems of the M(VO)2(PO4)2 compounds are essentially three-dimensional with the frustration caused by competing ferro- and antiferromagnetic interactions. Both calcium and strontium compounds undergo antiferromagnetic long-range ordering at T_N = 1.5 K and 1.9 K, respectively. The spin model reveals an unusual example of controllable frustration in three-dimensional magnetic systems.

cond-mat.str-el↗

arXiv2008

Columnar magnetic structure coupled with orthorhombic distortion in the antiferromagnetic iron arsenide SrFe$_2$As$_2$

Neutron powder diffraction experiments were carried out on polycrystalline SrFe$_2$As$_2$ in order to determine the magnetic structure and its relationship with the crystallographic one. Below $T_0$=205 K, magnetic reflections appear simultaneously with the onset of the orthorhombic distortion. From a detailed Rietveld analysis, the magnetic propagation vector of SrFe$_2$As$_2$ is determined to be {\textit{\textbf{q}}}=(1 0 1); the coupling of Fe moments is antiferromagnetic along the longer $a$ direction within the Fe-As layer, and the interlayer coupling is antiferromagnetic as well. The size of the Fe magnetic moment is deduced to be 1.01(3) $μ_{\rm B}$ with an orientation parallel to the a-axis. The temperature dependence of the magnetic moment shows excellent agreement with not only that of the muon precession frequency but also with that of the structural distortion, revealing the strong coupling of the columnar magnetic order and the structural distortion in SrFe$_2$As$_2$.

cond-mat.supr-con↗

arXiv2008

Superconductivity in SrFe_(2-x)Co_xAs_2: Internal Doping of the Iron Arsenide Layers

In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination, magnetic susceptibility, electrical resistivity, and specific heat are reported. The observation of bulk superconductivity in all thermodynamic properties -- despite strong disorder in the Fe-As layer -- favors an itinerant picture in contrast to the cuprates and renders a p- or d-wave scenario unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3) leads to the suppression of the magnetic ordering present in SrFe_2As_2 due to a rigid down-shift of the Fe-3d_(x^2-y^2) related band edge in the density of states.

cond-mat.supr-con↗

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

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

Single crystal growth and anisotropy of CeRuPO

We report on the single crystal growth of the ferromagnetic Kondo lattice system CeRuPO using a Sn flux method. Magnetic susceptibility and electrical resistivity measurements indicate strong anisotropy of this structurally layered compound. They evidence that the magnetic moments order ferromagnetically along the c-direction of the tetragonal unit cell, whereas the crystal electric field (CEF) anisotropy favors the ab-plane. Therefore, CeRuPO presents the unusual case within rare earth systems, where the anisotropy of the interionic exchange interaction overcomes the single ion anisotropy due to the CEF interaction.

cond-mat.str-el↗

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

ESR of YbRh2Si2 and 174YbRh2Si2 : local and itinerant properties

Below the Kondo temperature the heavy Fermion compound YbRh$_{2}$Si$_{2}$ shows a well defined Electron Spin Resonance (ESR) with local Yb$^{3+}$ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb$^{3+}$ in YPd$_3$:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic $^{174}$YbRh$_{2}$Si$_{2}$ we could exclude unresolved hyperfine contributions to the lineshape.

cond-mat.str-el↗

arXiv2007

Structure and transport in multi-orbital Kondo systems

We consider Kondo impurity systems with multiple local orbitals, such as rare earth ions in a metallic host or multi--level quantum dots coupled to metallic leads. It is shown that the multiplet structure of the local orbitals leads to multiple Kondo peaks above the Fermi energy $E_F$, and to ``shadow'' peaks below $E_F$. We use a slave boson mean field theory, which recovers the strong coupling Fermi liquid fixed point, to calculate the Kondo peak positions, widths, and heights analytically at T=0, and NCA calculations to fit the temperature dependence of high--resolution photoemission spectra of Ce compounds. In addition, an approximate conductance quantization for transport through multi--level quantum dots or single--atom transistors in the Kondo regime due to a generalized Friedel sum rule is demonstrated.

cond-mat.str-el↗

arXiv2007

High Resolution Photoemission Study on Low-T_K Ce Systems: Kondo Resonance, Crystal Field Structures, and their Temperature Dependence

We present a high-resolution photoemission study on the strongly correlated Ce-compounds CeCu_6, CeCu_2Si_2, CeRu_2Si_2, CeNi_2Ge_2, and CeSi_2. Using a normalization procedure based on a division by the Fermi-Dirac distribution we get access to the spectral density of states up to an energy of 5 k_BT above the Fermi energy E_F. Thus we can resolve the Kondo resonance and the crystal field (CF) fine-structure for different temperatures above and around the Kondo temperature T_K. The CF peaks are identified with multiple Kondo resonances within the multiorbital Anderson impurity model. Our theoretical 4f spectra, calculated from an extended non-crossing approximation (NCA), describe consistently the observed photoemission features and their temperature dependence. By fitting the NCA spectra to the experimental data and extrapolating the former to low temperatures, T_K can be extracted quantitatively. The resulting values for T_K and the crystal field energies are in excellent agreement with the results from bulk sensitive measurements, e.g. inelastic neutron scattering.

cond-mat.str-el↗

arXiv2007

Multiple energy scales at a quantum critical point

We report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh$_2$Si$_2$. The data provide evidence for an energy scale in the equilibrium excitation spectrum, that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality.

cond-mat.str-el↗

arXiv2006

Quantum phase transition in the heavy-fermion compound YbIr$_2$Si$_2$

We investigate the pressure-temperature phase diagram of YbIr$_2$Si$_2$ by measuring the electrical resistivity $ρ(T)$. In contrast to the widely investigated YbRh$_2$Si$_2$, YbIr$_2$Si$_2$ is a paramagnetic metal below $p_c\simeq 8$ GPa. Interestingly, a first-order, presumably ferromagnetic, transition develops at $p_c$. Similar magnetic properties were also observed in YbRh$_2$Si$_2$ and YbCu$_2$Si$_2$ at sufficiently high pressures, suggesting a uniform picture for these Yb compounds. The ground state of YbIr$_2$Si$_2$ under pressure can be described by Landau Fermi-liquid (LFL) theory, in agreement with the nearly ferromagnetic Fermi-liquid (NFFL) model. Moreover, evidence of a weak valence transition, characterized by a jump of the Kadowaki-Woods (KW) ratio as well as an enhancement of the residual resistivity $ρ_0$ and of the quasiparticle-quasiparticle scattering cross section, is observed around 6 GPa.

cond-mat.str-el↗

arXiv2006

Spin dynamics of $Yb Rh_2 Si_2$ observed by Electron Spin Resonance

Below the Kondo temperature $T_{\rm K}$ electron spin resonance (ESR) usually is not observable from the Kondo-ion itself because the characteristic spin fluctuation energy results in a huge width of the ESR line. The heavy fermion metal YbRh$_{2}$Si$_{2}$ seems to be an exceptional case where definite ESR spectra show characteristic properties of the Kondo-ion Yb$^{3+}$ well \textit{below} $T_{\rm K}$. We found that the spin dynamics of YbRh$_{2}$Si$_{2}$, as determined by its ESR relaxation, is spatially characterized by an anisotropy of the zero temperature residual relaxation only.

cond-mat.str-el↗

arXiv2006

Observation of an optical non-Fermi-liquid behavior in the heavy fermion state of YbRh$_{2}$Si$_{2}$

We report far-infrared optical properties of YbRh$_{2}$Si$_{2}$ for photon energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy quasiparticle state, a linear dependence of the low-energy scattering rate on both temperature and photon energy was found. We relate this distinct dynamical behavior different from that of Fermi liquid materials to the non-Fermi liquid nature of YbRh$_{2}$Si$_{2}$ which is due to its close vicinity to an antiferromagnetic quantum critical point.

cond-mat.str-el↗