arXiv subjects

Th. Proffen

Publications and source records attributed to Th. Proffen.

At least 19 recordsLinked to original sources

arXiv2024

Quantum magnetism in the frustrated square lattice oxyhalides YbBi2IO4 and YbBi2ClO4

Square-lattice systems offer a direct route for realizing 2D quantum magnetism with frustration induced by competing interactions. In this work, the square-lattice materials YbBi2IO4 and YbBi2ClO4 were investigated using a combination of magnetization and specific heat measurements on polycrystalline samples. Specific heat measurements provide evidence for long-range magnetic order below TN = 0.21 K (0.25 K) for YbBi2IO4 (YbBi2ClO4). On the other hand, a rather broad maximum is found in the temperature-dependent magnetic susceptibility, located at Tmax = 0.33 K (0.38 K) in YbBi2IO4 (YbBi2ClO4), consistent with the quasi-2D magnetism expected for the large separation between the magnetic layers. Estimation of the magnetic entropy supports the expected Kramers' doublet ground state for Yb3+ and the observed paramagnetic behavior is consistent with a well-isolated doublet. Roughly two-thirds of the entropy is consumed above TN, due to a combination of the quasi-2D behavior and magnetic frustration. The impact of frustration is examined from the viewpoint of a simplified J1-J2 square lattice model, which is frustrated for antiferromagnetic interactions. Specifically, a high-temperature series expansion analysis of the temperature-dependent specific heat and magnetization data yields J2/J1 = 0.30 (0.23) for YbBi2IO4 (YbBi2ClO4). This simplified analysis suggests strong frustration that should promote significant quantum fluctuations in these compounds, and thus motivates future work on the static and dynamic magnetic properties of these materials.

cond-mat.str-el↗

arXiv2013

Lightly stuffed pyrochlore structure of single-crystalline Yb2Ti2O7 grown by the optical floating zone technique

Recent neutron scattering and specific heat studies on the pyrochlore Yb2Ti2O7 have revealed variations in its magnetic behavior below 265mK. In the best samples, a sharp anomaly in the specific heat is observed at T=265mK. Other samples, especially single crystals, have broad features in the specific heat which vary in sharpness and temperature depending on the sample, indicating that the magnetic ground state may be qualitatively different in such samples. We performed detailed comparisons of the chemical structure of a pulverised single crystal of Yb2Ti2O7, grown by the floating zone technique, to a sintered powder sample of Yb2Ti2O7. Rietveld refinements of neutron powder diffraction data on these samples reveal that the crushed single crystal is best described as a "stuffed" pyrochlore, Yb2(Ti(2-x)Yb(x))O7-(x/2) with x = 0.046(4), despite perfectly stoichiometric starting material. Substituting magnetic Yb3+ on the non-magnetic Ti4+ sublattice would introduce random exchange bonds and local lattice deformations. These are expected to be the mechanism leading to of the variation of the delicate magnetic ground state of Yb2Ti2O7. Determination of the cubic cell length, a, could be useful as a method for characterizing the stoichiometry of non- pulverised single crystals at room temperature.

cond-mat.str-el↗

arXiv2010

Atomic displacements in the "charge-ice" pyrochlore Bi2Ti2O7 studied by neutron total scattering

The oxide pyrochlore Bi$_2$Ti$_2$O$_6$O$^\prime$ is known to be associated with large displacements of Bi and O$^\prime$ atoms from their ideal crystallographic positions. Neutron total scattering, analyzed in both reciprocal and real space, is employed here to understand the nature of these displacements. Rietveld analysis and maximum entropy methods are used to produce an average picture of the structural non-ideality. Local structure is modeled via large-box reverse Monte Carlo simulations constrained simultaneously by the Bragg profile and real-space pair distribution function. Direct visualization and statistical analyses of these models show the precise nature of the static Bi and O$^\prime$ displacements. Correlations between neighboring Bi displacements are analyzed using coordinates from the large-box simulations. The framework of continuous symmetry measures has been applied to distributions of O$^\prime$Bi$_4$ tetrahedra to examine deviations from ideality. Bi displacements from ideal positions appear correlated over local length scales. The results are consistent with the idea that these nonmagnetic lone-pair containing pyrochlore compounds can be regarded as highly structurally frustrated systems.

cond-mat.mtrl-sci↗

arXiv2009

Local Structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies

The combined local structure techniques, extended x-ray absorption fine structure (EXAFS) and neutron pair distribution function analysis, have been used for temperatures 4 <= T <= 330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co sites (x <= 0.35), indicating few, if any, JT-active, singly occupied e_g Co sites exist.

cond-mat.str-el↗

arXiv2008

An Experimental and Theoretical Study of the Variation of 4f Hybridization Across the La1-xCexIn3 Series

Crystal structures of a series of La1-xCexIn3 (x = 0.02, 0.2, 0.5, or 0.8) intermetallic compounds have been investigated by both neutron and X-ray diffraction, and their physical properties have been characterized by magnetic susceptibility and specific heat measurements. Our results emphasize atypical atomic displacement parameters (ADP) for the In and the rare-earth sites. Depending on the x value, the In ADP presents either an "ellipsoidal" elongation (La-rich compounds) or a "butterfly-like" distortion (Ce-rich compounds). These deformations have been understood by theoretical techniques based on the band theory and are the result of hybridization between conduction electrons and 4f-electrons.

cond-mat.str-el↗

arXiv2006

Understanding the insulating phase in CMR manganites: Shortening of the Jahn-Teller long-bond across the phase diagram of La_{1-x} Ca_x Mn O_3

The detailed evolution of the magnitude of the local Jahn-Teller (JT) distortion in La_{1-x} Ca_x Mn O_3 is obtained across the phase diagram for 0 < x < 0.5 from high quality neutron diffraction data using the atomic pair distribution function (PDF) method. A local JT distortion is observed in the insulating phase for all Ca concentrations studied. However, in contrast with earlier local structure studies its magnitude is not constant, but decreases continuously with increasing Ca content. This observation is at odds with a simple small-polaron picture for the insulating state.

cond-mat.str-el↗

arXiv2006

Dielectric anomalies and spiral magnetic order in CoCr2O4

We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr$_2$O$_4$, an insulating spinel exhibiting both ferrimagnetic and spiral magnetic structures. Below $T_c$ = 94 K the sample develops long-range ferrimagnetic order, and we attribute a sharp phase transition at $T_N$ $\approx$ 25 K with the onset of long-range spiral magnetic order. Neutron measurements confirm that while the structure remains cubic at 80 K and at 11 K; there is complex magnetic ordering by 11 K. Density functional theory supports the view of a ferrimagnetic semiconductor with magnetic interactions consistent with non-collinear ordering. Capacitance measurements on CoCr$_2$O$_4$, show a sharp decrease in the dielectric constant at $T_N$, but also an anomaly showing thermal hysteresis falling between approximately $T$ = 50 K and $T$ = 57 K. We tentatively attribute the appearance of this higher temperature dielectric anomaly to the development of \textit{short-range} spiral magnetic order, and discuss these results in the context of utilizing dielectric spectroscopy to investigate non-collinear short-range magnetic structures.

cond-mat.mtrl-sci↗

arXiv2006

Study of temperature dependent atomic correlations in MgB$_{2}$

We have studied the evolution with temperature of the local as well as the average crystal structure of MgB$_2$ using the real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction. We have investigated the correlations of the B-B and B-Mg nearest neighbor pair motion by comparing, in the wide temperature range from T=10 K up to T=600 K, the mean-square displacements (MSD) of single atoms with the mean-square relative displacements (MSRD) obtained from the PDF peak linewidths. The results show that the single atom B and Mg vibrations are mostly decoupled from each other, with a small predominance of positive (in phase) correlation factor for both the B-B and B-Mg pairs. The small positive correlation is almost temperature independent, in contrast with our theoretical calculations; this can be a direct consequence of the strong decay processes of the $E_{2g}$ anharmonic phonons.

cond-mat.supr-con↗

arXiv2005

2D Ferromagnetism in the High-Tc Analogue Cs_2AgF_4

Although the precise mechanism of high-Tc superconductivity in the layered cuprates remains unknown, it is generally thought that strong 2D Heisenberg antiferromagnetism combined with disruptive hole doping is an essential aspect of the phenomenon. Intensive studies of other layered 3d transition metal systems have greatly extended our understanding of strongly correlated electron states, but to date have failed to show strong 2D antiferromagnetism or high-Tc superconductivity. For this reason the largely unexplored 4d^9 Ag^II fluorides, which are structurally and perhaps magnetically similar to the 3d^9 Cu^II cuprates, merit close study. Here we present a comprehensive study of magnetism in the layered Ag^II fluoride Cs_2AgF_4, using magnetic susceptometry, neutron diffraction and inelastic neutron scattering techniques. We find that this material is well described as a 2D Heisenberg ferromagnet, in sharp contrast to the high-Tc cuprates. The exchange constant J is the largest known for any material of this type. We suggest that orbital ordering may be the origin of the ferromagnetism we observe in this material.

cond-mat.str-el↗

arXiv2004

Direct observation of the formation of polar nanoregions in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ using neutron pair distribution function analysis

Using neutron pair distribution function (PDF) analysis over the temperature range from 1000 K to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0 % to a maximum of $\sim$ 30% as the temperature decreases from 650 K to 15 K. Below T$\sim$200 K the PNRs start to overlap as their volume fraction reaches the percolation threshold. We propose that percolating PNRs and their concomitant overlap play a significant role in the relaxor behavior of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.

cond-mat.dis-nn↗

arXiv2004

Neutron diffraction study of average and local structure in La0.5Ca0.5MnO3

We used neutron powder diffraction to obtain the local and long-range structure of La0.5Ca0.5MnO3 at room temperature and 20K. By combining Rietveld and pair distribution function analysis of the total neutron scattering data, we have analyzed the structure of the compound using two competing models describing the low temperature phase: first the charge ordered/orbital ordered model and second the Mn-Mn dimer model. These structural models fit the 20K neutron powder diffraction pattern equally well using Rietveld analysis. Therefore, pair distribution function analysis is used to probe the local and medium-range structure revealing a system with two distinctly distorted Mn octahedra and Mn ions with non-integral valence states. The distorted octahedra differ with the structural model for the Zener polaron type Mn-Mn dimer picture proposed for Pr0.6Ca0.4MnO3 and order in a similar checkerboard configuration associated with the CE-type anti-ferromagnetic structure. Therefore, locally the charge difference and structural ordering between the two Mn is appreciable enough to describe the system at 20K as "partially charge ordered".

cond-mat.str-el↗

arXiv2004

Orbital correlations in the pseudo-cubic \emph{O} and rhombohedral ${R}$-phases of LaMnO$_3$

The local and intermediate structure of stoichiometric LaMnO$_3$ has been studied in the pseudocubic and rhombohedral phases at high temperatures (300 to 1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis carried out. The nature of the Jahn-Teller (JT) transition around 750 K is confirmed to be orbital order to disorder. In the high temperature orthorhombic ($O$) and rhombohedral ($R$) phases the MnO$_6$ octahedra are still fully distorted locally. The data suggest the presence of local orbitally ordered clusters of diameter $\sim 16$ Å($\sim$four MnO$_6$ octahedra) implying strong nearest neighbor JT anti-ferrodistortive coupling.

cond-mat.str-el↗

arXiv2003

The role of the lattice in the $γ\to α$ phase transition of Ce: a high pressure neutron and x-ray diffraction study

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the $γ\to α$ phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the $γ\to α$ phase transition, $ΔS^{γ- α}_{\rm vib} \approx (0.75 \pm 0.15)$k$_{\rm B}$, is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence which is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the $γ\to α$ phase transition in elemental Ce.

cond-mat.str-el↗

arXiv2002

Little helpers for your experiment - NOBUGS2002/013

The World Wide Web (WWW) is a wonderful tool to provide users with documentation and tools that make the preparation and running of an experiment a little easier. We present a set of tools that allow one to estimate the activation of a sample in the neutron beam, or to calculate the absorption of a sample. Ever tried to figure out the time for the next sample change - here our scheduler can help. In addition there is a simple database that can be used to keep track of all the data files collected at various facilities. We present these little tools, but also demonstrate how easy it is to create new WWW based tools that make the life of users easier.

cond-mat.mtrl-sci↗

arXiv2002

Sharing data between facilities: using the NeXus time-of-flight powder diffractometer file format

NeXus is an international standard data format intended to reduce the need for redundant software development efforts in the neutron and x-ray scattering communities. As the NeXus standard matures it is starting to be used at laboratories for storing raw data. The Manuel Lujan Jr. Neutron Scattering Center (MLNSC) at Los Alamos National Laboratory and the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory have been working with NeXus in an effort to share data and software. MLNSC is now writing files compliant with NeXus and the Integrated Spectral Analysis Workbench (ISAW) software from IPNS is being used with this data. Problems can arise if the standard is interpreted in different ways and information that belongs in the file is not accounted for in the standard. This paper will discuss an inter-laboratory collaboration in relation to a maturing data standard.

cond-mat.mtrl-sci↗

arXiv2002

A New face on old code

In science we often use established code that was developed many years ago. Frequently the documentation is lost and many of us know the feeling of putting a number in the wrong column and trying to understand why the program is not working. However, these old codes are usually very useful, well tested and established. Rather than writing new code from scratch, which must first be tested against the established standard, it is desirable to put a new front end on the standard, commonly referred to as 'wrapping'. This paper will discuss wrapping of FORTRAN code using a selection of other languages to provide simpler user interface.

cond-mat↗

arXiv2002

Chemical short range order obtained from the atomic pair distribution function

Many crystalline materials show chemical short range order and relaxation of neighboring atoms. Local structural information can be obtained by analyzing the atomic pair distribution function (PDF) obtained from powder diffraction data. In this paper, we present the successful extraction of chemical short range order parameters from the x-ray PDF of a quenched Cu_3Au sample.

cond-mat.mtrl-sci↗

arXiv2001

Local Atomic Strain in ZnSe(1-x)Te(x) from High Real Space Resolution Neutron Pair Distribution Function Measurements

High real-space resolution atomic pair distribution functions (PDFs) have been obtained from ZnSe(1-x)Te(x) using neutron powder diffraction. Distinct Zn-Se and Zn-Te nearest neighbor (nn) bonds, differing in length by delta_r= 0.14Angstroms, are resolved in the measured PDF allowing the evolution with composition of the individual bond-lengths to be studied. The local bond-lengths change much more slowly with doping than the average bond-length obtained crystallographically. The nn bond-length distributions are constant with doping but higher-neighbor pair distributions broaden significantly indicating that most of the strain from the alloying is accommodated by bond-bending forces in the alloy. The PDFs of alloys across the whole doping range are well fit using a model based on the Kirkwood potential. The resulting PDFs give excellent agreement with the measured PDFs over the entire alloy range with no adjustable parameters.

cond-mat.mtrl-sci↗