arXiv subjects

H. -H. Klauss

Publications and source records attributed to H. -H. Klauss.

At least 73 records · Page 4Linked to original sources

arXiv2009

Field-induced coupled superconductivity and spin density wave order in the Heavy Fermion compound CeCoIn5

The high field superconducting state in CeCoIn5 has been studied by transverse field muon spin rotation measurements with an applied field parallel to the crystallographic c-axis close to the upper critical field Hc2 = 4.97 T. At magnetic fields >= 4.8 T the muon Knight shift is enhanced and the superconducting transition changes from second order towards first order as predicted for Pauli-limited superconductors. The field and temperature dependence of the transverse muon spin relaxation rate sigma reveal paramagnetic spin fluctuations in the field regime from 2 T < H < 4.8 T. In the normal state close to Hc2 correlated spin fluctuations as described by the self consistent renormalization theory are observed. The results support the formation of a mode-coupled superconducting and antiferromagnetically ordered phase in CeCoIn5 for H directed parallel to the c-axis.

cond-mat.str-el↗

arXiv2009

Interplay of Rare Earth and Iron magnetism in RFeAsO with R = La, Ce, Pr, and Sm: A muon spin relaxation study and symmetry analysis

We report zero field muon spin relaxation (muSR) measurements on RFeAsO with R = La, Ce, Pr, and Sm. We study the interaction of the FeAs and R (rare earth) electronic systems in the non superconducting magnetically ordered parent compounds of RFeAsO{1-x}Fx superconductors via a detailed comparison of the local hyperfine fields at the muon site with available Moessbauer spectroscopy and neutron scattering data. These studies provide microscopic evidence of long range commensurate magnetic Fe order with the Fe moments not varying by more than 15 % within the series RFeAsO with R = La, Ce, Pr, and Sm. At low temperatures, long range R magnetic order is also observed. Different combined Fe and R magnetic structures are proposed for all compounds using the muon site in the crystal structure obtained by electronic potential calculations. Our data point to a strong effect of R order on the iron subsystem in the case of different symmetry of Fe and R order parameters resulting in a Fe spin reorientation in the R ordered phase in PrFeAsO. Our symmetry analysis proves the absence of collinear Fe--R Heisenberg interactions in RFeAsO. A strong Fe--Ce coupling due to non--Heisenberg anisotropic exchange is found in CeFeAsO which results in a large staggered Ce magnetization induced by the magnetically ordered Fe sublattice far above T_N{Ce}. Finally, we argue that the magnetic R--Fe interaction is probably not crucial for the observed enhanced superconductivity in RFeAsO{1-x}Fx with a magnetic R ion.

cond-mat.supr-con↗

arXiv2009

Coexistence of incommensurate magnetism and superconductivity in Fe_{1+y}Se_xTe_{1-x}

We report an investigation into the superconducting and magnetic properties of Fe_{1+y}Se_{x}Te_{1-x} single crystals by magnetic susceptibility, muon spin rotation, and neutron diffraction. We find three regimes of behavior in the phase diagram for 0\leq x\leq 0.5: (i) commensurate magnetic order for x< 0.1, (ii) bulk superconductivity for $x\lesssim 0.1$, and (iii) a range \sim 0.25\leq x\leq 0.45 in which superconductivity coexists with static incommensurate magnetic order. The results are qualitatively consistent with a two-band mean-field model in which itinerant magnetism and extended s-wave superconductivity are competing order parameters.

cond-mat.supr-con↗

arXiv2009

Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near T_c in several disordered FeAs based Superconductors

We report resistivity and upper critical field B_c2(T) data for disordered (As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field range up to 60 T. These samples exhibit a slightly enhanced superconducting transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4 T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0) approximately 63 T. We compare our results with B_c2(T)-data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced flattening of B_c2(T) for applied fields H II ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively.

cond-mat.supr-con↗

arXiv2009

Quantum critical dynamics of a S = 1/2 antiferromagnetic Heisenberg chain studied by 13C-NMR spectroscopy

We present a 13C-NMR study of the magnetic field driven transition to complete polarization of the S=1/2 antiferromagnetic Heisenberg chain system copper pyrazine dinitrate Cu(C_4H_4N_2)(NO_3)_2 (CuPzN). The static local magnetization as well as the low-frequency spin dynamics, probed via the nuclear spin-lattice relaxation rate 1/T_1, were explored from the low to the high field limit and at temperatures from the quantum regime (k_B T << J) up to the classical regime (k_B T >> J). The experimental data show very good agreement with quantum Monte Carlo calculations over the complete range of parameters investigated. Close to the critical field, as derived from static experiments, a pronounced maximum in 1/T_1 is found which we interpret as the finite-temperature manifestation of a diverging density of zero-energy magnetic excitations at the field-driven quantum critical point.

cond-mat.str-el↗

arXiv2009

Orbital and spin effects for the upper critical field in As deficient disordered Fe pnictide superconductors

We report B_c2 data for LaO_{0.9}F_{0.1}FeAs_{1-delta} in a wide T and field range up to 60 Tesla. The large slope of B_c2 approx ~ -6 Tesla/K near an improved T_c = 28.5 K of the in-plane B_c2(T) contrasts with a flattening starting at 23 K above 30 Tesla we regard as the onset of Pauli-limited behavior (PLB) with B_c2(0) about 65 Tesla. We interpret a similar hitherto unexplained flattening of the B_c2(T) curves reported for at least three other disordered closely related systems as also as a manifestation of PLB. Their Maki parameters have been estimated analyzing their B_c2(T) data within the WHH approach. The pronounced PLB of (Ba,K)Fe_2As_2 single crystals from a tin-flux is attributed also to a significant As deficiency. Consequences of our results are discussed in terms of disorder effects within conventional (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), can be discarded for our samples. The increase of dB_c2/dT|_{T_c} by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity driven transition from s_{+-} USC to CSC of an extended s_{++}-wave state or to a stabilized s_{+-}-state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with B_c2 data from the literature with no PLB for fields below 60 to 70 Tesla probed so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.

cond-mat.supr-con↗

arXiv2009

Two-gap superconductivity in Ba_1-xK_xFe_2As_2: A complementary study of the magnetic penetration depth by \muSR and ARPES

We investigate the magnetic penetration depth λin superconducting Ba_1-xK_xFe_2As_2 (T_c\simeq32K) with muon-spin rotation (\muSR) and angle-resolved photoemission (ARPES). Using \muSR, we find the penetration-depth anisotropy γ_λ=λ_c/λ_{ab} and the second-critical-field anisotropy γ_{H_c2} to show an opposite T-evolution below T_c. This dichotomy resembles the situation in the two-gap superconductor MgB_2. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth λ_ab around 7K. The complementarity of \muSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T-evolution of λ_ab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate.

cond-mat.supr-con↗

arXiv2009

Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR

We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x = 0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for all nuclei also scale with one another, despite the fact that the form factors for each site sample different regions of q-space. This result suggests a lack of any q-space structure in the dynamical spin susceptibility that might be expected in the presence of antiferromagnetic correlations. Rather, our results are more compatible with simple quasi-particle scattering. Furthermore, we find that the increase in the electric field gradient at the As cannot be accounted for by LDA calculations, suggesting that structural changes, in particular the position of the As in the unit cell, dominate the NQR response.

cond-mat.supr-con↗

arXiv2009

Momentum-resolved superconducting gap in the bulk of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ from combined ARPES and $μ$SR measurements

Here we present a calculation of the temperature-dependent London penetration depth, $λ(T)$, in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ (BKFA) on the basis of the electronic band structure [1,2] and momentum-dependent superconducting gap [3] extracted from angle-resolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of $λ(T)$ by muon spin rotation ($μ$SR) [4]. The value of $λ(T=0)$, calculated with \emph{no} adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence $λ(T)$ is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new iron-based superconductors in general allows us to state that all hole-doped of them bear two nearly isotropic gaps with coupling constants $2Δ/k_{\rm B}T_{\rm c}=2.5\pm1.5$ and $7\pm2$.

cond-mat.supr-con↗

arXiv2009

The electronic phase diagram of the LaO1-xFxFeAs superconductor

The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, e.g. in high-transition-temperature cuprates 1, heavy fermions 2 and organic superconductors3. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, e.g. charge carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently high-temperature superconductivity has been discovered in iron-pnictides providing a new class of unconventional superconductors4,5,6. Similar to other unconventional superconductors the parent compounds of the pnictides exhibit a magnetic ground state7,8 and superconductivity is induced upon charge carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of x-ray scattering, MuSR and Moessbauer spectroscopy on the series LaO1-xFxFeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and of the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in ironpnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

cond-mat.supr-con↗

arXiv2009

Superconductivity and Field-Induced Magnetism in SrFe_1.75Co_0.25As_2

Using muon-spin rotation, we studied the in-plane (λ_ab) and the out of plane (λ_c) magnetic field penetration depth in SrFe_1.75Co_0.25As_2 (T_c=13.3 K). Both λ_ab(T) and λ_c(T) are consistent with the presence of two superconducting gaps with the gap to T_c ratios 2Δ/k_BT_c=7.2 and 2.7. The penetration depth anisotropy γ_λ=λ_c/λ_ab increases from γ_λ=2.1 at T_c to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.

cond-mat.supr-con↗

arXiv2009

Probing the Ground State Properties of Iron-based Superconducting Pnictides and Related Systems by Muon-Spin Spectroscopy

In this short review, we attempt to give a comprehensive discussion of studies performed to date by muon-spin spectroscopy (more precisely the relaxation and rotation technique, also know as \muSR technique) on the recently discovered layered iron-based superconductors. On one side, \muSR has been used to characterized the magnetic state of different families of layered iron-based systems. Similarly the subtle interplay of the magnetic state and the structural transition present in some families has been investigated. We will also discuss the information provided by this technique on the interaction between the magnetic state and the superconducting phase. Finally the \muSR technique has been used to investigate the magnetic penetration depth of the superconducting ground state. The study of its absolute value, temperature and magnetic field dependence provides crucial tests for investigating possible unconventional superconducting states in such systems.

cond-mat.supr-con↗

arXiv2008

Muon-spin rotation study of the in-plane magnetic penetration depth of FeSe_0.85: evidence for nodeless superconductivity

The in-plane magnetic penetration depth λ_{ab} of the iron selenide superconductor with the nominal composition FeSe_0.85 was studied by means of muon-spin rotation. The measurements of λ_{ab}^{-2}(T) are inconsistent with the presence of nodes in the gap as well as with a simple isotropic s-wave type of the order parameter, but can be equally well described within a two-gap (s+s) and an anisotropic s-wave gap picture. This implies that the superconducting energy gap in FeSe_0.85 contains no nodes.

cond-mat.supr-con↗

arXiv2008

Commensurate Spin Density Wave in LaOFeAs: A Local Probe Study

We present a detailed study on the magnetic order in the undoped mother compound LaOFeAs of the recently discovered Fe-based superconductor LaO$_{1-x}$F$_x$FeAs. In particular, we present local probe measurements of the magnetic properties of LaOFeAs by means of $^{57}$Fe Mössbauer spectroscopy and muon spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5) $μ_B$ at the iron site below T_N = 138 K, well separated from a structural phase transition at T_N = 156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below T_N are reproduced.

cond-mat.supr-con↗

arXiv2008

Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho

We report muon spin relaxation measurements on the magnetic structures of RBaCo_2O_5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co^3+ of CoO_6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.

cond-mat.str-el↗

arXiv2008

Field and Temperature Dependence of the Superfluid Density in LaO_{1-x}F_xFeAs Superconductors: A Muon Spin Relaxation Study

We present zero field and transverse field \muSR experiments on the recently discovered electron doped Fe-based superconductor LaO_{1-x}F_xFeAs. The zero field experiments on underdoped (x=0.075) and optimally doped (x=0.1) samples rule out any static magnetic order above 1.6 K in these superconducting samples. From transverse field experiments in the vortex phase we deduce the temperature and field dependence of the superfluid density. Whereas the temperature dependence is consistent with a weak coupling BCS s-wave or a dirty d-wave gap function scenario, the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of λ_{ab} (0) = 254(2)nm for LaO_{0.9}F_{0.1}FeAs and λ_{ab} (0) = 364(8)nm for LaO_{0.925}F_{0.075}FeAs. Further evidence for unconventional superconductivity is provided by the ratio of T_c versus the superfluid density, which is close to the Uemura line of hole doped high-T_c cuprates.

cond-mat.supr-con↗

arXiv2007

Electronic structure of the molecule based magnet Cu PM(NO3)2 (H2O)2

We present density functional calculations on the molecule based S=1/2 antiferromagnetic chain compound Cu PM(NO3)2 (H2O)2; PM = pyrimidine. The properties of the ferro- and antiferromagnetic state are investigated at the level of the local density approximation and with the hybrid functional B3LYP. Spin density maps illustrate the exchange path via the pyrimidine molecule which mediates the magnetism in the one-dimensional chain. The computed exchange coupling is antiferromagnetic and in reasonable agreement with the experiment. It is suggested that the antiferromagnetic coupling is due to the possibility of stronger delocalization of the charges on the nitrogen atoms, compared to the ferromagnetic case. In addition, computed isotropic and anisotropic hyperfine interaction parameters are compared with recent NMR experiments.

cond-mat.mtrl-sci↗

arXiv2007

High field level crossing studies on spin dimers in the low dimensional quantum spin system Na$_2$T$_2$(C$_2$O$_4$)$_3$(H$_2$O)$_2$ with T=Ni,Co,Fe,Mn

In this paper we demonstrate the application of high magnetic fields to study the magnetic properties of low dimensional spin systems. We present a case study on the series of 2-leg spin-ladder compounds Na$_2$T$_2$(C$_2$O$_4$)$_3$(H$_2$O)$_2$ with T = Ni, Co, Fe and Mn. In all compounds the transition metal is in the $T^{2+}$ high spin configuation. The localized spin varies from S=1 to 3/2, 2 and 5/2 within this series. The magnetic properties were examined experimentally by magnetic susceptibility, pulsed high field magnetization and specific heat measurements. The data are analysed using a spin hamiltonian description. Although the transition metal ions form structurally a 2-leg ladder, an isolated dimer model consistently describes the observations very well. This behaviour can be understood in terms of the different coordination and superexchange angles of the oxalate ligands along the rungs and legs of the 2-leg spin ladder. All compounds exhibit magnetic field driven ground state changes which at very low temperatures lead to a multistep behaviour in the magnetization curves. In the Co and Fe compounds a strong axial anisotropy induced by the orbital magnetism leads to a nearly degenerate ground state and a strongly reduced critical field. We find a monotonous decrease of the intradimer magnetic exchange if the spin quantum number is increased.

cond-mat.mtrl-sci↗