arXiv subjects

Th. Wolf

Publications and source records attributed to Th. Wolf.

At least 37 records · Page 2Linked to original sources

arXiv2013

Solution calorimetry investigations of new phase BaCe0.6Y0.3In0.1O2.8

The preparation of BaCeO3 doped by yttrium and indium oxides (BaCe0.6Y0.3In0.1O2.8) has been performed by solid-state reaction from BaCO3, CeO2, Y2O3, In2O3. The compound BaCe0.6Y0.3In0.1O2.8 has been synthesized for the first time. The X-ray measurements have showed that BaCe0.6Y0.3In0.1O2.8 has an orthorhombic structure (space group Pmcn). The standard formation enthalpies of BaCe0.6Y0.3In0.1O2.8 have been determined by solution calorimetry combining the solution enthalpies of BaCe0.6Y0.3In0.1O2.8 and BaCl2 + 0.6CeCl3 + 0.3YCl3 + 0.1InCl3 mixtures in 1 M HCl with 0.1 M KI at 298.15 K and literature data. It has been obtained that above-mentioned mixed oxide is thermodynamically stable with respect to their decomposition into binary oxides at room temperatures. It has been also shown that BaCe0.6Y0.3In0.1O2.8 has been more thermodynamically favored than BaCe0.9In0.1O2.95.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic anisotropic energy gap and low energy spin wave excitation in antiferromagnetic block phase of K2Fe4Se5

Neutron scattering experiments were performed to investigate magnetic order and magnetic excitations in ternary iron chalcogenide K2Fe4Se5. The formation of a superlattice structure below 580 K together with the decoupling between the Fe-vacancy order-disorder transition and the antiferromagnetic order transition appears to be a common feature in the A2Fe4Se5 family. The study of spin dynamics of K2Fe4Se5 reveals two distinct energy gaps at the magnetic Brillouin zone center, which indicates the presence of magnetic anisotropy and the decrease of local symmetry due to electronic and orbital anisotropy. The low-energy spin wave excitations of K2Fe4Se5 can be properly described by linear spin wave theory within a Heisenberg model. Compared to iron pnictides, K2Fe4Se5 exhibits a more two-dimensional magnetism as characterized by large differences not only between out-of-plane and in-plane spin wave velocities, but also between out-of-plane and in-plane exchange interactions.

cond-mat.supr-con↗

arXiv2012

Formation enthalpy of BaCe0.7Nd0.2In0.1O2.85

In this paper for the first time we synthesized the compound BaCe0.7Nd0.2In0.1O2.85 by solid-state reaction. The phase has orthorhombic structure (space group Pmcn). We also measured the standard formation enthalpies of BaCe0.7Nd0.2In0.1O2.85 by solution calorimetry in 1 M HCl with 0.1 M KI. We determined the stability of Nd(In)-doped barium cerate with respect to mixtures of binary oxides. On the basis of these data we established that above-mentioned mixed oxide is thermodynamically stable with respect to their decomposition into binary oxides at room temperatures. We also established that BaCe0.7Nd0.2In0.1O2.85 oxide is thermodynamically favoured than BaCe0.8Nd0.2O2.9.

cond-mat.mtrl-sci↗

arXiv2012

Electronic structure of single-crystalline Mg$_x$Al$_{1-x}$B$_2$ probed by x-ray diffraction multipole refinements and polarization-dependent x-ray absorption spectroscopy

X-ray diffraction multipole refinements of single-crystalline Mg$_x$Al$_{1-x}$B$_2$ and polarization-dependent near-edge x-ray absorption fine structure at the B 1$s$ edge reveal a strongly anisotropic electronic structure. Comparing the data for superconducting compounds ($x= 0.8$, 1.0) with those for the non-superconductor ($x=0$) gives direct evidence for a rearrangement of the hybridizations of the boron $p_z$ bonds and underline the importance of holes in the $σ$-bonded covalent $sp^2$ states for the superconducting properties of the diborides. The data indicate that Mg is approximately divalent in MgB$_2$ and suggest predominantly ionic bonds between the Mg ions and the two-dimensional B rings. For AlB$_2$ ($x=0$), on the other hand, about 1.5 electrons per Al atom are transferred to the B sheets while the residual 1.5 electrons remain at the Al site which suggests significant covalent bonding between the Al ions and the B sheets. This finding together with the static electron deformation density points to almost equivalent electron counts on B sheets of MgB$_2$ and AlB$_2$\@, yet with a completely different electron/hole distribution between the $σ$ and $π$ bonds.

cond-mat.supr-con↗

arXiv2012

Electronic structure of single-crystalline Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ probed by x-ray absorption spectroscopy: Evidence for isovalent substitution of Fe$^{2+}$ by Co$^{2+}$

The substitutional dependence of valence and spin-state configurations of Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x =$ 0, 0.05, 0.11, 0.17, and 0.38) is investigated with near-edge x-ray absorption fine structure at the $L_{2,3}$ edges of Fe, Co, and As. The present data provide direct spectroscopic evidence for an effectively isovalent substitution of Fe$^{2+}$ by Co$^{2+}$, which is in contrast to the widely assumed Co-induced electron-doping effect. Moreover, the data reveal that not only does the Fe valency remain completely unaffected across the entire doping range, but so do the Co and As valencies as well. The data underline a prominent role of the hybridization between (Fe,Co) 3$d_{xy}$, $d_{xz}$, $d_{yz}$ orbitals and As $4s/4p$ states for the band structure in $A$(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and suggest that the covalency of the (Fe,Co)-As bond is a key parameter for the interplay between magnetism and superconductivity.

cond-mat.supr-con↗

arXiv2012

Competition between commensurate and incommensurate magnetic ordering in Fe(1+y)Te

The Fe1+y Te1-x Sex compounds belong to the family of iron-based high temperature superconductors, in which superconductivity often appears upon doping antiferromagnetic parent compounds. Unlike other Fe-based superconductors (in which the antiferromagnetic order is at the Fermi surface nesting wavevector [1/2,1/2,1]), the Fe1+y Te1-x Sex parent compound Fe1+y Te orders at a different wavevector, [1/2, 0, 1/2]. Furthermore, the ordering wavevector depends on y, the occupation of interstitial sites with excess iron; the origin of this behavior is controversial. Using inelastic neutron scattering on Fe1.08 Te, we find incommensurate magnetic fluctuations above the Neel temperature, even though the ordered state is bicollinear and commensurate with gapped spin waves. This behavior can be understood in terms of a competition between commensurate and incommensurate order, which we explain as a lock-in transition caused by the magnetic anisotropy.

cond-mat.supr-con↗

arXiv2011

Preparation and ionic conductivity of new Bi12.5Lu1.5ReO24.5 phase

The substitution of Re into Bi2O3 allows stabilization of the d- Bi2O3 structure by additional substitution of lutetium ion to give phase of composition Bi12.5Lu1.5ReO24.5. The phase was synthesized for the first time. Structural analysis performed by neutron diffraction showed that space group was Fm3m with lattice parameter a = 5.5591(2) Å. The phase has been found to show high ion conductivity at moderate temperature. The conductivity was measured in the temperature range of 600-800 K. The conductivity of Bi12.5Lu1.5ReO24.5 at 800 K is the same as the conductivity of BiCuVOx, Bi12.5Ln1.5ReO24.5 (Ln = Eu, La, Nd) phases. In this connection the Bi12.5Lu1.5ReO24.5 phase offers excellent potential for moderate temperature application.

cond-mat.mtrl-sci↗

arXiv2011

Magnetic Blue Phase in the Chiral Itinerant Magnet MnSi

Chiral nematic liquid crystals sometimes form blue phases characterized by spirals twisting in different directions. By combining model calculations with neutron-scattering experiments, we show that the magnetic analogue of blue phases does form in the chiral itinerant magnet MnSi in a large part of the phase diagram. The properties of this blue phase explain a number of previously reported puzzling features of MnSi such as partial magnetic order and a two-component specific-heat and thermal-expansion anomaly at the magnetic transition.

cond-mat.str-el↗

arXiv2011

Complex magneto-elastic properties in the frustrated kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$

High resolution heat capacity and thermal expansion experiments performed on single crystalline kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ are presented. The parent compounds Co$_3$V$_2$O$_8$ and Ni$_3$V$_2$O$_8$ undergo a complex sequence of first- and second-order magnetic phase transitions. The low-temperature ($T<40$ K) magnetic entropy evolves monotonously with the doping content $x$, from the full S=1 Ni$^{2+}$ magnetic entropy in Ni$_3$V$_2$O$_8$ to half of the S=3/2 Co$^{2+}$ magnetic entropy in Co$_3$V$_2$O$_8$. Thermal expansion coefficients $α_i$ ($i = a$, $b$ and $c$) show a strong anisotropy for all (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ compounds. The low-temperature magnetic distortion indicates that Co-doping (Ni-doping) has similar effects to applying a uniaxial pressures along $a$ or $b$ ($c$). Linear Grüneisen parameters $Γ_i$ are extracted for the three main axes $i$ and exhibit a complex behavior with both temperature and doping. For each axis, $Γ_i$ and $α_i$ exhibit a sign change (at low temperature) at the critical concentration $x_c\simeq0.25$, at which the incommensurate magnetic propagation vector changes. Beyond our study, an understanding of the multiple and complex parameters (magnetic frustration, magnetic anisotropy, mixture of S=1 and S=3/2 ions, etc.) is now necessarily to bring light to the rich magneto-elastic properties of (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$.

cond-mat.str-el↗

arXiv2011

Chiral criticality in doped Mn$_{1-y}$Fe$_y$Si compounds

The critical spin fluctuations in doped compounds Mn$_{1-y}$Fe$_y$Si have been studied by means of ac-susceptibility measurements, polarized neutron small angle scattering and spin echo spectroscopy. It is shown that these compounds undergo the transition from the paramagnetic to helimagnetic phase through continuous, yet well distinguishable crossovers: (i) from paramagnetic to partially chiral, (ii) from partially chiral to highly chiral fluctuating state. The crossover points are identified on the basis of combined analysis of the temperature dependence of ac-susceptibility and polarized SANS data. The whole transition is marked by two inflection point of the temperature dependence of ac-susceptibility: the upper one corresponds to the crossover to partially chiral state at $T^*$, where the inverse correlation length $κ\approx 2 k$, the lower one corresponds to the transition to the spin helix structure. The intermediate crossover to the highly chiral phase is observed at the inflection point $T_k$ of the first derivative of ac-susceptibility, where $κ\approx k$. The temperature crossovers to the highly chiral fluctuating state is associated with the enhancing influence of the Dzyaloshinskii-Moria interaction close to $T_c$.

cond-mat.mtrl-sci↗

arXiv2010

Evidence of precursor superconductivity as high as 180 K from infrared spectroscopy

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O7-d (R=Y, Gd, Eu) provides important new information about the anomalous normal state properties of underdoped cuprate high temperature superconductors. Besides competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of ~180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions.

cond-mat.supr-con↗

arXiv2010

Electric field control of multiferroic domains in Ni$_3$V$_2$O$_8$ imaged by X-ray polarization enhanced topography

The magnetic structure of multiferroic Ni$_3$V$_2$O$_8$ has been investigated using non-resonant X-ray magnetic scattering. Incident circularly polarized X-rays combined with full polarization analysis of the scattered beam is shown to yield high sensitivity to the components of the cycloidal magnetic order, including their relative phases. New information on the magnetic structure in the ferroelectric phase is obtained, where it is found that the magnetic moments on the "cross-tie" sites are quenched relative to those on the "spine" sites. This implies that the onset of ferroelectricity is associated mainly with spine site magnetic order. We also demonstrate that our technique enables the imaging of multiferroic domains through polarization enhanced topography. This approach is used to image the domains as the sample is cycled by an electric field through its hysteresis loop, revealing the gradual switching of domains without nucleation.

cond-mat.str-el↗

arXiv2009

Resistivity of Mn$_{1-x}$Fe$_x$Si single crystals: Evidence for quantum critical behavior

Resistivity measurements have been made on Mn$_{1-x}$Fe$_x$Si single crystals between 2 and 300K for $x$ = 0, 0.05, 0.08, 0.12 and 0.15. Fe doping is found to depress the magnetic ordering temperature from 30K for $x$ = 0 to below 2K for $x$ = 0.15. Although Fe doping results in a large increase of the low-temperature residual resistivity, the temperature dependence of the resistivity above the magnetic transition remains practically unaffected by increasing Fe content. An analysis of the temperature derivative of the resistivity provides strong evidence for the existence of a non-Fermi-liquid ground state near $x$ = 0.15 and thus for a quantum critical point tuned by Fe content.

cond-mat.str-el↗

arXiv2009

Muon spin rotation study of magnetism and superconductivity in BaFe2-xCoxAs2 and Pr1-xSrxFeAsO

We present a muon spin rotation (\muSR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe2-xCoxAs2 with x=0.08, 0.20, and 0.25 (Tc=9, 25 and 20K) and of polycrystalline hole-doped Pr1-xSrxFeAsO with x=0 and 0.2 (Tc=15 K). In the former series we observe some interesting parallels with the electron doped SmFeAsO1-xFx 1111-type system [A.J. Drew et al., to appear in Nature Materials 2009 and arXiv:0807.4876]. In particular, we obtain evidence that strongly disordered static magnetism coexists with superconductivity on a microscopic scale in underdoped samples and even at optimum doping there is a slowing down (or enhancement) of dynamic magnetic correlations below Tc\approx25K. To the contrary, for the hole-doped Pr1-xSrxFeAsO samples we obtain evidence for a mesoscopic phase segregation into regions with nearly unperturbed AF order and others that are non magnetic and most likely superconducting. The observed trend resembles the one that was previously reported for hole-doped Ba1-xKxFe2As2 [A.A. Aczel et al., Phys. Rev. B 78, 214503 (2008); J.T. Park et al., arXiv:0811.2224] and thus seems to be fairly common in these hole doped systems.

cond-mat.supr-con↗

arXiv2008

Neutron diffraction on antiferromagnetic ordering in single-crystal BaFe2As2

Neutron diffraction experiments have been carried out on a Sn-flux grown BaFe2As2 single crystal, the parent compound of the A-122 family of FeAs-based high-Tc superconductors. A tetragonal to orthorhombic structural phase transition and a three dimensional long-range antiferromagnetic ordering of the iron moment, with a unique magnetic propagation wavevector k = (1, 0, 1), have been found to take place at ~90 K. The magnetic moments of iron are aligned along the long a axis in the low temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of BaFe2As2 single crystal is the same as those in other A-122 iron pnictides compounds. We argue that the tin incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower Neel temperature T_N observed in the experiment.

cond-mat.supr-con↗

arXiv2008

Experimental magnetic form factors in Co3V2O8: A combined study of ab initio calculations, magnetic Compton scattering and polarized neutron diffraction

We present a combination of ab initio calculations, magnetic Compton scattering and polarized neutron experiments, which elucidate the density distribution of unpaired electrons in the kagome staircase system Co3V2O8. Ab initio wave functions were used to calculate the spin densities in real and momentum space, which show good agreement with the respective experiments. It has been found that the spin polarized orbitals are equally distributed between the t2g and the eg levels for the spine (s) Co ions, while the eg orbitals of the cross-tie (c) Co ions only represent 30% of the atomic spin density. Furthermore, the results reveal that the magnetic moments of the cross-tie Co ions, which are significantly smaller than those of the spine Co ions in the zero-field ferromagnetic structure, do not saturate by applying an external magnetic field of 2 T along the easy axis a, but that the increasing bulk magnetization originates from induced magnetic moments on the O and V sites. The refined individual magnetic moments are mu(Co_c)=1.54(4) mu_B, mu(Co_s)=2.87(3) mu_B, mu(V)=0.41(4) mu_B, mu(O1)=0.05(5) mu_B, mu(O2)=0.35(5) mu_B, and; mu(O3)=0.36(5) mu_B combining to the same macroscopic magnetization value, which was previously only attributed to the Co ions.

cond-mat.mtrl-sci↗

arXiv2008

$T_c$ Dependence of Energy Gap and Asymmetry of Coherence Peaks in NdBa$_2$Cu$_3$O$_{7-δ}$

We report here the results of scanning tunneling spectroscopic experiments performed on hole doped NdBa$_2$Cu$_3$O$_{7-δ}$ single crystals of $T_c$ values of 76 K, 93.5 K and 95.5 K. The energy gaps are observed to be increasing with decreasing $T_c$ values. The coherence peaks are asymmetric with the peaks at the filled states being larger than those at the empty ones. The asymmetry increases with decreasing $T_c$. The observed asymmetry and its $T_c$ dependence can be explained by considering the Mott insulating nature of the material at the undoped state.

cond-mat.supr-con↗

arXiv2008

Excitation of a bosonic mode by electron tunneling into a cuprate superconductor

We performed scanning tunneling spectroscopic experiments on hole-doped NdBa$_2$Cu$_3$O$_{7-δ}$. The d$I$/d$V$ curves obtained at 4.2 K are asymmetric with clear peak-dip and hump structures. Energy derivatives of these curves show peaks at energies beyond the dip features. Highly precise full potential bandstructure calculations confirm a featureless electronic density of states in that energy region. Our results indicate that tunneling electrons couple to a collective mode in the CuO$_2$ plane.

cond-mat.supr-con↗