arXiv subjects

B. Büchner

Publications and source records attributed to B. Büchner.

At least 199 records · Page 11Linked to original sources

arXiv2012

Evidence for Triplet Superconductivity in a Superconductor-Ferromagnet Spin Valve

We have studied the dependence of the superconducting (SC) transition temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers in the spin valve system CoO_x/Fe1/Cu/Fe2/Pb. We find that this dependence is nonmonotonic when passing from the parallel to the antiparallel case and reveals a distinct minimum near the orthogonal configuration. The analysis of the data in the framework of the SC triplet spin valve theory gives direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the two magnetizations.

cond-mat.supr-con↗

arXiv2012

Low temperature ballistic spin transport in the S=1/2 antiferromagnetic Heisenberg chain compound SrCuO2

We report zero and longitudinal magnetic field muon spin relaxation measurements of the spin S=1/2 antiferromagnetic Heisenberg chain material SrCuO2. We find that in a weak applied magnetic field B the spin-lattice relaxation rate follows a power law B^n with n=-0.9(3). This result is temperature independent for 5K < T < 300 K. Within conformal field theory and using the Müller ansatz we conclude ballistic spin transport in SrCuO2.

cond-mat.str-el↗

arXiv2012

Pr magnetism and its interplay with the Fe spin density wave in PrFeAsO

We have studied the magnetism of the Pr3+ ions in PrFeAsO_1-xF_x (x = 0; 0.15) and its interaction with the Fe magnetic order (for x = 0). Specific heat data confirm the presence of a first excited crystal electric field (CEF) level around 3.5 meV in the undoped compound PrFeAsO. This finding is in agreement with recent neutron scattering experiments. The doped compound is found to have a much lower first CEF splitting of about 2.0 meV. The Pr ordering in PrFeAsO gives rise to large anomalies in the specific heat and the thermal expansion coefficient. In addition, a field-induced transition is found at low temperatures that is most pronounced for the magnetostriction coefficient. This transition, which is absent in the doped compound, is attributed to a reversal of the Fe spin canting as the antiferromagnetic Pr order is destroyed by the external magnetic field.

cond-mat.supr-con↗

arXiv2012

Flux jumps and vortex pinning in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ single crystals

In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$. The isothermal magnetic field ($H||c$ axis) dependent magnetization ($M$) loops exhibit a second peak (SP) or `fishtail effect', as well as remarkable flux jumps at low temperatures. The critical current density $J_c$ obtained from the $M(H)$ loops is rather high, of the order of 10$^6$ A/cm$^2$. The analysis of the temperature and field dependent $J_c$ implies that high $J_c$ is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well on the field sweep rate. The field for first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggests that SP in M(H) is likely related to crossover in creep dynamics from elastic to plastic mechanism. We have constructed the vortex phase diagram on field-temperature plane.

cond-mat.supr-con↗

arXiv2012

$^{139}$La NMR investigation in underdoped La$_{1.93}$Sr$_{0.07}$CuO$_4$

We report $^{139}$La and $^{63}$Cu nuclear magnetic and quadrupole resonance (NMR/NQR) studies in an underdoped La$_{1.93}$Sr$_{0.07}$CuO$_4$ single crystal, focusing on the $^{139}$La NMR in the normal state. We demonstrate that the local structural distortions in the low-temperature orthorhombic structure cause the tilting of the direction of the electric field gradient (EFG) at the nuclei from the c axis, resulting in two NMR central transition spectra at both the $^{139}$La and $^{63}$Cu nuclei in an external field. Taking into account the tilt angle of the EFG, the temperature dependence of the $^{139}$La spectra allowed us to determine the $^{139}$La Knight shift and the structural order parameter. The angle and temperature dependence of the $^{139}$La spectrum is in perfect agreement with the macroscopic average structure and proves a displacive transition. The $^{139}$La nuclear spin-lattice relaxation rates, $T_1^{-1}$, suggest that La$_{1.93}$Sr$_{0.07}$CuO$_4$ undergoes a gradual change to a temperature-independent paramagnetic regime in the high temperature region. Both the spectra and $T_1^{-1}$ of the $^{139}$La as a function of temperature reveal a sharp anomaly around $T_S=387(1)$ K, implying a first-order-like structural transition, and a dramatic change below ~70 K arising from collective glassy spin freezing.

cond-mat.supr-con↗

arXiv2012

Strong pairing at iron $3d_{xz,yz}$ orbitals in hole-doped BaFe$_2$As$_2$

Among numerous hypotheses, recently proposed to explain superconductivity in iron-based superconductors [1-9], many consider Fermi surface (FS) nesting [2, 4, 8, 10] and dimensionality [4, 9] as important contributors. Precise determination of the electronic spectrum and its modification by superconductivity, crucial for further theoretical advance, were hindered by a rich structure of the FS [11-17]. Here, using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we disentangle the electronic structure of hole-doped BaFe2As2, and show that nesting and dimensionality of FS sheets have no immediate relation to the superconducting pairing. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: the magnitude of the superconducting gap maximizes at 10.5 meV exclusively for iron 3dxz;yz orbitals, while for others drops to 3.5 meV. Presented results reveal similarities of electronic response to superconducting and magneto-structural transitions [18, 19], implying that relation between these two phases is more intimate than just competition for FS, and demonstrate importance of orbital physics in iron superconductors.

cond-mat.supr-con↗

arXiv2012

"Cigar" Fermi surface as a possible requisite for superconductivity in iron-based superconductors

Recently discovered A-Fe-Se (A - alkali metal) materials have questioned the most popular theories of iron-based superconductors because of their unusual electronic structure [1]. Controversial photoemission data taken in the superconducting state [2-7] are in conflict with highly magnetic state seen by neutron-, muSR-spectroscopies and transport/thermodynamic probes [8-10]. These results lead to suggestions to consider all iron-based materials as originating from Mott-insulators or semiconductors, thus once again raising the question of close relation between the cuprates and Fe-based superconductors [e.g. 2]. Here we study electronic and magnetic properties of Rb0.77Fe1.61Se2 (Tc = 32.6 K) in normal and superconducting states by means of photoemission and muSR spectroscopies as well as band structure calculations. We demonstrate that the puzzling behavior of these novel materials is the result of separation into metallic (~12%) and insulating (~ 88%) phases. Only the former becomes superconducting and has a usual electronic structure of electron-doped FeSe-slabs. Our results thus imply that the antiferromagnetic insulating phase is just a byproduct of Rb-intercalation and its magnetic properties have hardly any relation to the superconductivity. Instead, we find that also in this, already third class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a van Hove singularity to the Fermi level. These findings set the direction for effective search of new superconducting materials.

cond-mat.supr-con↗

arXiv2012

Electronic confinement and ordering instabilities in colossal magnetoresistive bilayer manganites

We present angle-resolved photoemission studies of (La1-zPrz)2-2xSr1+2xMn2O7 with x=0.4 and z=0.1,0.2 and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO2-layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ~400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, cause the observed renormalisation of the spectral features.

cond-mat.str-el↗

arXiv2012

Probing the Unconventional Superconducting State of LiFeAs by Quasiparticle Interference

A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc~18K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van-Hove singularity at the center of the Brillouin zone which is in stark contrast with other pnictide superconductors where nesting is crucial for both scattering and s+- superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the hole-like bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s++, s+-, and d-wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary p-wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g. s+id-wave symmetry).

cond-mat.supr-con↗

arXiv2012

Hole-doping in BaFe$_2$As$_2$: The case of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals

Single crystals of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ with $x$ = 0, 0.25, 0.35, 0.4 were grown using a self-flux high temperature solution growth technique. The superconducting and normal state properties were studied by temperature dependent magnetic susceptibility, electrical resistivity and specific heat revealing that the magnetic and structural transition is rapidly suppressed upon Na-substitution at the Ba-site in BaFe$_2$As$_2$, giving rise to superconductivity. A superconducting transition as high as 34 K is reached for a Na-content of $x$=0.4. The positive Hall coefficient confirms that the substitution of Ba by Na results in hole-doping similarly to the substitution of Ba by K. Angle resolved photoemission spectroscopy was performed on all Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ crystals. The Fermi surface of hole-doped Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ is to high extent the same as the Fermi surface found for the K-doped sister compounds, suggesting a similar impact of the substitution of Ba by either K or Na on the electronic band dispersion at the Fermi level

cond-mat.supr-con↗

arXiv2012

Spinon heat transport and spin-phonon interaction in the antiferromagnetic spin-1/2 Heisenberg chain cuprates Sr2CuO3 and SrCuO2

We have investigated the thermal conductivity κ_mag of high-purity single crystals of the spin chain compound Sr2CuO3 which is considered an excellent realization of the one-dimensional spin-1/2 antiferromagnetic Heisenberg model. We find that the spinon heat conductivity κ_mag is strongly enhanced as compared to previous results obtained on samples with lower chemical purity. The analysis of κ_mag allows to compute the spinon mean free path l_mag as a function of temperature. At low-temperature we find l_mag\sim0.5\mum, corresponding to more than 1200 chain unit cells. Upon increasing the temperature, the mean free path decreases strongly and approaches an exponential decay ~1/T*exp(T*/T) which is characteristic for umklapp processes with the energy scale k_B T*. Based on Matthiesen's rule we decompose l_mag into a temperature-independent spinon-defect scattering length l0 and a temperature dependent spinon-phonon scattering length l_sp(T). By comparing l_mag(T) of Sr2CuO3 with that of SrCuO2, we show that the spin-phonon interaction, as expressed by l_sp is practically the same in both systems. The comparison of the empirically derived l_sp with model calculations for the spin-phonon interaction of the one-dimensional spin-1/2 XY model yields reasonable agreement with the experimental data.

cond-mat.str-el↗

arXiv2012

Weak ferrimagnetism and multiple magnetization reversal in α-Cr3(PO4)2

The chromium(II) orthophosphate α-Cr3(PO4)2 is a weak ferrimagnet with the Curie temperature TC = 29 K confirmed by a λ-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature Θ = - 96 K, indicating an intermediate frustration ratio |Θ|/TC ~ 3. In its magnetically ordered states α-Cr3(PO4)2 exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e. either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 μB/Cr2+, as compared to the spin-only moment 4.9 μB/Cr2+, cannot be ascribed to the occurence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d4 Cr2+ ions. X-ray absorption spectra at the K-edge indicate divalent chromium and unravel the high-spin state of these ions at the L2,3-edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr2+ in its six crystallographically inequivalent positions.

cond-mat.mtrl-sci↗

arXiv2012

Local magnetic anisotropy in BaFe$_2$As$_2$: a polarized inelastic neutron scattering study

The anisotropy of the magnetic excitations in BaFe$_2$As$_2$ was studied by polarized inelastic neutron scattering which allows one to separate the components of the magnetic response. Despite the in-plane orientation of the static ordered moment we find the in-plane polarized magnons to exhibit a larger gap than the out-of-plane polarized ones indicating very strong single-ion anisotropy within the layers. It costs more energy to rotate a spin within the orthorhombic {\it a-b} plane than rotating it perpendicular to the FeAs layers.

cond-mat.supr-con↗

arXiv2011

AC susceptibility investigation of vortex dynamics in nearly-optimally doped REFeAsO$_{1-x}$F$_{x}$ superconductors (RE = La, Ce, Sm)

Ac susceptibility and static magnetization measurements were performed in the nearly-optimally doped LaFeAsO$_{0.9}$F$_{0.1}$ and CeFeAsO$_{0.92}$F$_{0.08}$ superconductors, complementing earlier results on SmFeAsO$_{0.8}$F$_{0.2}$ [Phys. Rev. {\bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizeable reduction of the liquid phase upon increasing $T_{c}$ in the range of applied fields ($H \leq 5$ T). This result indicates that SmFeAsO$_{0.8}$F$_{0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intra-grain depinning energy $U_{0}$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\lesssim H_{cr} \lesssim 5000$ Oe) separating regions where single and collective depinning processes are at work. Analysis of the data in terms of a simple two-fluid picture for slightly anisotropic materials allows to estimate the zero-temperature penetration depth $λ_{ab}(0)$ and the anisotropy parameter $γ$ for the three materials. Finally, a sizeable suppression of the superfluid density is deduced in a $s^{\pm}$ two-gap scenario.

cond-mat.supr-con↗

arXiv2011

Investigation of the In-Gap Electronic Structure of LaAlO3 - SrTiO3 Heterointerfaces by Soft X-ray Spectroscopy

We investigated \LAO\ - \STO\ heterointerfaces grown either in oxygen rich or poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the Ti L$_{2,3}$ absorption edge of the valence band and Ti 2p core level spectroscopy directly monitor the impact of oxygen treatment upon the electronic structure. Two types of Ti$^{3+}$ related charge carriers are identified. One is located at the Fermi energy and related to the filling of the \STO\ conduction band. It appears for low oxygen pressure only. The other one is centered at E$_{B}$ $\approx$ 1 eV and independent of the oxygen pressure during growth. It is probably due to defects. The magnitude of both excitations is comparable. It is shown that low oxygen pressure is detrimental for the Ti - O bonding. Our results shed light on the nature of the charge carriers in the vicinity of the \LAO\ - \STO\ interface.

cond-mat.str-el↗

arXiv2011

Bond disorder and breakdown of ballistic heat transport in the spin-1/2 antiferromagnetic Heisenberg chain as seen in Ca-doped SrCuO2

We study the impact of a weak bond disorder on the spinon heat transport in the S=1/2 antiferromagnetic (AFM) Heisenberg chain material Sr_{1-x}Ca_xCuO_2. We observe a drastic suppression in the magnetic heat conductivity kappa_mag even at tiny disorder levels (i.e., Ca-doping levels), in stark contrast to previous findings for kappa_mag of S=1/2 two-dimensional square lattice and two-leg spin-ladder systems, where a similar bond disorder has no effect on kappa_mag. Hence, our results underpin the exceptional role of integrability of the S=1/2 AFM Heisenberg chain model and suggest that the bond disorder effectively destroys the ballistic nature of its heat transport. We further show that the suppression of kappa_mag is captured by an effective spinon-impurity scattering length, which exhibits the same doping dependence as the long-distance exponential decay length of the spin-spin correlation as determined by density-matrix renormalization group calculations.

cond-mat.str-el↗

arXiv2011

Raman Evidence for Superconducting Gap and Spin-Phonon Coupling in Superconductor Ca(Fe0.95Co0.05)2As2

Inelastic light scattering studies on single crystal of electron-doped Ca(Fe0.95Co0.05)2As2 superconductor, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at TSM ~ 140 K and superconducting transition temperature Tc ~ 23 K, reveal evidence for superconductivity-induced phonon renormalization; in particular the phonon mode near 260 cm-1 shows hardening below Tc, signaling its coupling with the superconducting gap. All the three Raman active phonon modes show anomalous temperature dependence between room temperature and Tc i.e phonon frequency decreases with lowering temperature. Further, frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory-based calculations, we show that the low temperature phase (Tc < T < TSM) exhibits short-ranged stripe anti-ferromagnetic ordering, and estimate the spin-phonon couplings that are responsible for these phonon anomalies.

cond-mat.supr-con↗

arXiv2011

Pseudogap in the chain states of YBCO

As established by scanning tunneling microscopy (STM) cleaved surfaces of the high temperature superconductor YBa$_2$Cu$_2$O$_{7-δ}$ develop charge density wave (CDW) modulations in the one-dimensional (1D) CuO chains. At the same time, no signatures of the CDW have been reported in the spectral function of the chain band previously studied by photoemission. We use soft X-ray angle resolved photoemission (SX-ARPES) to detect a chain-derived surface band that had not been detected in previous work. The $2k_\textup{F}$ for the new surface band is found to be 0.55\,Å$^{-1}$, which matches the wave vector of the CDW observed in direct space by STM. This reveals the relevance of the Fermi surface nesting for the formation of CDWs in the CuO chains in YBa$_2$Cu$_2$O$_{7-δ}$. In agreement with the short range nature of the CDW order the newly detected surface band exhibits a pseudogap, whose energy scale also corresponds to that observed by STM.

cond-mat.supr-con↗