arXiv subjects

B. Roessli

Publications and source records attributed to B. Roessli.

At least 55 records · Page 3Linked to original sources

arXiv2009

Diffuse scattering from the lead-based relaxor ferroelectric PbMg_1/3Ta_2/3O_3

The relaxor ferroelectric PbMg_1/3Ta_2/3O_3 was studied by single-crystal neutron and synchrotron x-ray diffraction and its detailed atomic structure has been modeled in terms of static Pb-displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb-based relaxor family like PbMg_1/3Nb_2/3O_3 or PbZn_1/3Nb_2/3O_3 the diffuse scattering in the [H,0,0]/[0,K,0] scattering plane has a butterfly-shape around the (h,0,0) Bragg reflections and is transverse to the scattering vector for (h,h,0) peaks. In the [H,H,0]/[0,0,L] plane the diffuse scattering is elongated along the <1,1,2> directions and is transverse to the scattering vector for (h,h,h) reflections. We find that a model consisting of correlated Pb-displacements along the <1,1,1>-directions reproduces the main features of the diffuse scattering in PbMg_1/3Ta_2/3O_3 adequately when the correlation lengths between the Pb-ion displacement vectors are longest along the <1,1,1> and <1,-1,0> and shortest along <1,1,-2> directions.

cond-mat.mtrl-sci↗

arXiv2009

Synthesis and structural characterization of 2Dioxane.2H2O.CuCl2: metal-organic compound with Heisenberg antiferromagnetic S=1/2 chains

A novel organometallic compound 2Dioxane.CuCl2.2H2O has been synthesized and structurally characterized by X-ray crystallography. Magnetic susceptibility and zero-field inelastic neutron scattering have also been used to study its magnetic properties. It turns out that this material is a weakly coupled one-dimensional S=1/2 Heisenberg antiferromagnetic chain system with chain direction along the crystallographic c axis and the nearest-neighbor intra-chain exchange constant J=0.85(4) meV. The next-nearest-neighbor inter-chain exchange constant J' is also estimated to be 0.05 meV. The observed magnetic excitation spectrum from inelastic neutron scattering is in excellent agreement with numerical calculations based on the Muller ansatz.

cond-mat.str-el↗

arXiv2009

Excitations from a chiral magnetized state of a frustrated quantum spin liquid

We study excitations in weakly interacting pairs of quantum spin ladders coupled through geometrically frustrated bonds. The ground state is a disordered spin liquid, that at high fields is replaced by an ordered chiral helimagnetic phase. The spectra observed by high-field inelastic neutron scattering experiments on the prototype compound Sul-Cu2Cl4 are qualitatively different from those in the previously studied frustration-free spin liquids. Beyond the critical field Hc=3.7 T, the soft mode that drives the quantum phase transition spawns two separate excitations: a gapless Goldstone mode and a massive magnon. Additional massive quasiparticles are clearly visible below Hc, but are destroyed in the ordered phase. In their place one observes a sharply bound excitation continuum.

cond-mat.str-el↗

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

Spin-glass state and long-range magnetic order in Pb(Fe1/2Nb1/2)O3

We have investigated the magnetic ground-state of the multiferroic relaxor ferroelectric \pfn with $μ$SR spectroscopy and neutron scattering. We find that a transition to a partially disordered phase occurs below T=20 K that coexists with long-range antiferromagnetic ordering. The disordered phase resembles a spin-glass. No clustering of magnetic ions could be evidenced by $μ$SR so that the coexistence appears homogeneous in the sample.

cond-mat.mtrl-sci↗

arXiv2008

Soft Modes and Relaxor Ferroelectrics

Relaxor ferroelectrics are difficult to study and understand. The experiment shows that at low energy scattering there is an acoustic mode, an optic mode, dynamic quasi-elastic scattering and strictly elastic scattering as well as Bragg peaks at the zone centre. We have studied the scattering using the TASP spectrometer at PSI and have analysed the data using a model with interactions between the different components particularly to determine the properties of the elastic scattering. The quasi-elastic scattering begins to become significant at the Burns temperature of 620 K. It steadily increases in intensity on cooling reaching a maximum at ~400 K. Below this temperature the strictly elastic scattering begins to increase and shows a broadened line shape characteristic of crystals in a random applied field. We show that all the results obtained from PMN for the elastic scattering are consistent with the crystal having a random field transition at ~400 K. We have obtained similar results for PMN-PT and PZN-PT suggesting that random fields of the nano-regions also play an important role in these materials.

cond-mat.mtrl-sci↗

arXiv2008

Noncentrosymmetric Commensurate Magnetic Ordering of Multiferroic ErMn2O5

The non-centrosymmetric magnetic structure of ErMn$_2$O$_5$ has been shown to be very similar to that of HoMn$_2$O$_5$ (Vecchini {\it et al.}, 2008 Phys. Rev. B {\bf 77} 134434). The magnetic modulation at 25 K has propagation vector $\vec k$=(1/2,0,1/4) and the symmetry imposes very few constraints on the magnetic configurations allowed. Only by combining the results of bulk magnetisation measurements, powder and single crystal neutron diffraction and spherical neutron polarization analysis was it possible to distinguish clearly between different models. The susceptibility measurements show that the erbium magnetic moments are aligned parallel to the c-axis indicating strong single-ion anisotropy. Spherical neutron polarimetry demonstrates the presence of two unequally populated chirality domains in ErMn$_2$O$_5$ single crystals. X-ray diffraction measurements on an ErMn$_2$O$_5$ powder using synchrotron radiation show that the buckling angles of the Mn-O-Mn bond change below the transition to the ferroelectric phase.

cond-mat.mtrl-sci↗

arXiv2008

Evolution of the neutron quasi-elastic scattering through the ferroelectric phase transition in 93%PbZn$_{1/3}$Nb$_{2/3}$O$_3$ - 7% PbTiO$_3$

We show that the neutron diffuse scattering in relaxor ferroelectric (1-x)PbZn$_{1/3}$Nb$_{2/3}$O$_{3}$ - x PbTiO$_{3}$ (x=0.07) consists of two components. The first component is strictly elastic but extended in q-space and grows below 600 K. The second component, that was not reported before for the (1-x)PbZn$_{1/3}$Nb$_{2/3}$O$_{3}$ - x PbTiO$_{3}$ (x=0.07) relaxor ferroelectrics, is quasi-elastic with a line-width that has a similar temperature dependence as the width of the central peak observed by Brillouin spectroscopy. The temperature dependence of the susceptibility of the quasi-elastic scattering has a maximum at the ferroelectric transition.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic and orbital ordering in the spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states; the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

cond-mat.str-el↗

arXiv2007

Magnetism in Geometrically Frustrated YMnO3 Under Hydrostatic Pressure Studied with Implanted Muons

The rare-earth ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T_N ~ 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no relaxing signal resolvable (which would be expected for different coexisting spin distributions). The transition temperature T_N increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.

cond-mat.str-el↗

arXiv2006

Simultaneous Antiferromagnetic Fe3+ and Nd3+ Ordering in NdFe3(11BO3)4

By means of magnetic susceptibility and specific heat measurements, x-ray and unpolarised neutron diffraction investigations on powder and single-crystal samples, simultaneous long-range antiferromagnetic Fe and Nd ordering in NdFe3(11BO3)4 with R 3 2 chemical structure has been found at temperatures below TN = 30.5(5) K down to 1.6 K. At temperatures down to 20 K to the propagation vector is khex = [0,0,3/2] and becomes slightly incommensurate at lower temperatures. Symmetry analysis yields magnetic spiral configurations with the magnetic moments oriented parallel to hexagonal basal plane according to the irreducible representations tau_3 in the commensurate case. This is in agreement with the easy directions of magnetisation perpendicular to the c-axis as determined by magnetic susceptibility measurements. At 1.6 K the magnetic Fe moment amounts to 4.9 muB close to the free ion moment of Fe3+. The magnetic Nd3+ moment saturates presumably due to crystal-field effects at 2.7 muB.

cond-mat.mtrl-sci↗

arXiv2006

The Structural Phase Transition of the Relaxor Ferroelectric 68%PbMg1/3Nb2/3O3-32%PbTiO3

Neutron scattering techniques have been used to study the relaxor ferroelectric 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 denoted in this paper as 0.68PMN-0.32PT. On cooling, these relaxor ferroelectrics have a long-range ordered ferroelectric phase and the composition is close to that at which the ferroelectric structure changes from rhombohedral to tetragonal. It was found that above the Burns temperature of about 600K, the transverse optic mode and the transverse acoustic mode are strongly coupled and a model was used to describe this coupling that gave similar parameters to those obtained for the coupling in PMN. Below the Burns temperature additional quasi-elastic scattering was found which increased in intensity as the sample was cooled down to the ferroelectric transition temperature but then decreased in intensity. This behaviour is similar to that found in PMN. This scattering is associated with the dynamic polar nano-regions that occur below the Burns temperature. In addition to this scattering a strictly elastic resolution limited peak was observed that was much weaker than the corresponding peak in pure PMN and which decreased in intensity on cooling below the ferroelectric phase whereas for PMN, which does not have a long-range ordered ferroelectric phase, the intensity of this component increased monotonically as the sample was cooled. The results of our study are compared with the recent measurements of Stock et al. [PRB 73 064107] who studied 0.4PMN-0.6PT. The results are qualitatively consistent with the random field model developed to describe the scattering from PMN.

cond-mat.mtrl-sci↗

arXiv2006

Pressure Induced Changes in the Antiferromagnetic Superconductor YbPd2Sn

Low temperature ac magnetic susceptibility measurements of the coexistent antiferromagnetic superconductor YbPd2Sn have been made in hydrostatic pressures < 74 kbar in moissanite anvil cells. The superconducting transition temperature is forced to T(SC) = 0 K at a pressure of 58 kbar. The initial suppression of the superconducting transition temperature is corroborated by lower hydrostatic pressure (p < 16 kbar) four point resisitivity measurements, made in a piston cylinder pressure cell. At ambient pressure, in a modest magnetic field of ~ 500 G, this compound displays reentrant superconducting behaviour. This reentrant superconductivity is suppressed to lower temperature and lower magnetic field as pressure is increased. The antiferromagnetic ordering temperature, which was measured at T(N) = 0.12 K at ambient pressure is enhanced, to reach T(N) = 0.58 K at p = 74 kbar. The reasons for the coexistence of superconductivity and antiferromagnetism is discussed in the light of these and previous findings. Also considered is why superconductivity on the border of long range magnetic order is so much rarer in Yb compounds than in Ce compounds. The presence of a new transition visible by ac magnetic susceptibility under pressure and in magnetic fields greater than 1.5 kG is suggested.

cond-mat.str-el↗

arXiv2006

Spin dynamics and magnetic order in magnetically frustrated Tb2Sn2O7

We report a study of the geometrically frustrated magnetic material Tb2Sn2O7 by the positive muon spin relaxation technique. No signature of a static magnetically ordered state is detected while neutron magnetic reflections are observed in agreement with a published report. This is explained by the dynamical nature of the ground state of Tb2Sn2O7: the Tb3+ magnetic moment characteristic fluctuation time is ~ 10^{-10} s. The strong effect of the magnetic field on the muon spin-lattice relaxation rate at low fields indicates a large field-induced increase of the magnetic density of states of the collective excitations at low energy.

cond-mat.str-el↗

arXiv2006

Anomalous pressure dependence of the atomic displacements in the relaxor ferroelectric PbMg$_{1/3}$Ta$_{2/3}$O$_3$

The crystal structure of the PbMg$_{1/3}$Ta$_{2/3}$O$_3$ (PMT) relaxor ferroelectric was studied under hydrostatic pressure up to $\sim 7$ GPa by means of powder neutron diffraction. We find a drastic pressure-induced decrease of the lead displacement from the inversion centre which correlates with an increase by $\sim$ 50 % of the anisotropy of the oxygen temperature factor. The vibrations of the Mg/Ta are, in contrast, rather pressure insensitive. We attribute these changes being responsible for the previously reported pressure-induced suppression of the anomalous dielectric permittivity and diffuse scattering in relaxor ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2005

Reduction of the ordered magnetic moment in YMnO3 with hydrostatic pressure

YMnO3 exhibits a ferroelectric transition at high temperature (~ 900 K) and magnetic ordering at T_N ~ 70 K where the dielectric constant shows an anomaly indicative of the magneto-dielectric effect. Here we report powder neutron diffraction experiments in this compound that show that the magnetic moment at saturation is reduced by application of hydrostatic pressure. Our results yield further insight about the nature of the spin-lattice interaction in ferroic materials.

cond-mat.str-el↗

arXiv2005

Spin fluctuations in the stacked-triangular antiferromagnet YMnO3

The spectrum of spin fluctuations in the stacked-triangular antiferromagnet YMnO3 was studied above the Neel temperature using both unpolarized and polarized inelastic neutron scattering. We find an in-plane and an out-of-plane excitation. The in-plane mode has two components just above TN, a resolution-limited central peak and a Debye-like contribution. The quasi-elastic fluctuations have a line-width that increases with q like Dq^z and the dynamical exponent z=2.3. The out-of-plane fluctuations have a gap at the magnetic zone center and do not show any appreciable q-dependence at small wave-vectors.

cond-mat.str-el↗

arXiv2004

Spin waves and magnetic interactions in LiCu2O2

The quasi-one-dimensional helimagnet LiCu2O2 was studied by single crystal inelastic neutron scattering. The dispersion relation of spin wave excitations was measured in the vicinity of the principal magnetic Bragg reflection. A spin wave theoretical analysis of the data yields an estimate of the relevant exchange constants and explains the mechanism of geometric frustration that leads to helimagnetism. It is found that the simple antiferromagnetic J_1-J_2 model that was previously proposed is inadequate for LiCu2O2. The experimental findings are generally in a qualitative agreement with first principal calculations of [A. A. Gippius et al., Phys. Rev. B {\bf 70}, 020406 (2004)], though certain important discrepancies remain to be explained.

cond-mat.str-el↗