Electric field control of multiferroic domains in Ni3_3V2_2O8_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.Comment: 9 pages, 6 figure

Magnetically driven ferroelectric order in Ni3_3V2_2O8_8

We show that for Ni$_3$V$_2$O$_8$ long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition. The temperature and magnetic field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry of this system by constraining the data to be consistent with Landau theory for continuous phase transitions. This phenomenological theory explains our observation the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.Comment: 11 pages, 3 figure

Competing Magnetic Phases on a "Kagome Staircase"

We present thermodynamic and neutron data on Ni_3V_2O_8, a spin-1 system on a kagome staircase. The extreme degeneracy of the kagome antiferromagnet is lifted to produce two incommensurate phases at finite T - one amplitude modulated, the other helical - plus a commensurate canted antiferromagnet for T ->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni_3V_2O_8 thus provides an elegant example of order from sub leading interactions in a highly frustrated systemComment: 4 pages, 3 figure

beta-Cu3V2O8: Magnetic ordering in a spin-1/2 kagome-staircase lattice

The spin-1/2 Cu2+ ions in beta-Cu3V2O8 occupy the sites of a Kagome-staircase lattice, an anisotropic variant of the Kagome net: buckled layers and imbedded plaquettes of three edge-shared CuO4 squares break the ideal Kagome symmetry. Susceptibility and heat capacity measurements show the onset of short-range ordering at approximately 75 K, and a magnetic phase transition with the characteristics of antiferromagnetism at ~29 K. Comparison to the Curie Weiss theta (theta,CW = -135 K) indicates that the geometric frustration is largely relieved by the anisotropy. A ferromagnetic contribution to the magnetization below the ordering temperature and negative magnetization in zero-field cooled measurements at low fields are attributed to uncompensated spins at grain boundaries or defects.Comment: 9 pages, 5 figure

Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8

We present powder and single-crystal neutron diffraction and bulk measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to 8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This system contains two types of Ni ions, which we call "spine" and "cross-tie". Our neutron measurements can be described with the paramagnetic space group Cmca for T < 15K and each observed magnetically ordered phase is characterized by the appropriate irreducible representation(s). Our zero-field measurements show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order which is dominated by a longitudinally-modulated structure with the spine spins mainly parallel to the a-axis. Upon further cooling, a transition is induced at T_HL=6.3K to an elliptically polarized incommensurate structure with both spine and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a first-order phase transition, below which the magnetic structure is a commensurate antiferromagnet with the staggered magnetization primarily along the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat peak at T_CC'=2.3K indicates an additional transition, which we were however not able to relate to a change of the magnetic structure. Neutron, specific heat, and magnetization measurements produce a comprehensive temperature-field phase diagram. The symmetries of the two incommensurate magnetic phases are consistent with the observation that only one phase has a spontaneous ferroelectric polarization. All the observed magnetic structures are explained theoretically using a simplified model Hamiltonian, involving competing nearest- and next-nearest-neighbor exchange interactions, spin anisotropy, Dzyaloshinskii-Moriya and pseudo-dipolar interactions.Comment: 25 pages, 19 figure

Strongly linked current flow in polycrystalline forms of the new superconductor MgB2

The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One of the central questions is whether this new superconductor resembles a high-temperature-cuprate superconductor or a low-temperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks[2]. Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive[3,4]. These studies of MgB2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains[3]. Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determined by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered

Prospective analysis of psychological differences between adult and elderly cancer patients during postoperaritve adjuvant chemotherapy

PURPOSE: Despite the burgeoning geriatric population with cancer and the importance of understanding how age may be related to mental adjustment and quality of life so far, differences in coping strategies and psychological harm between the elderly and adults are hardly being taken into account to modify the approach to this population. The aim of this prospective study is to describe the differences in psychological characteristics between older and adult cancer patients and examine dissimilarities in their psychological evolution during adjuvant chemotherapy. METHODS: Adults (18-69 years old) and older patients (≥ 70) with newly diagnosed non-metastatic resected cancer admitted to receive adjuvant chemotherapy were recruited. Patients completed the following questionnaires: mini-mental adjustment to cancer, brief symptom inventory, shared decision-making questionnaire-patient's version, multidimensional scale of perceived social support, EORTC quality-of-life instrument, life orientation test-revised, and satisfaction with life scale. RESULTS: 500 cancer patients (394 adults and 106 older) were evaluated. The impact of the diagnosis was less negative among older patients, with no differences in coping strategies, quality of life, or search for support. Regarding psychological changes from the beginning to the end of the adjuvant treatment, both age groups reported more somatic symptoms, increased psychological difficulty, reduced coping strategies, and a significant decrease in quality of life at the end of postoperative chemotherapy. CONCLUSION: Although there were clear psychological differences between adults and senior cancer patients, their evolution during adjuvant chemotherapy was similar, with deterioration in quality of life and coping. This negative psychological impact of adjuvant chemotherapy should be taken into account when considering interventions

Giant anharmonicity and non-linear electron-phonon coupling in MgB2_{2}; A combined first-principles calculations and neutron scattering study

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB$_{2}$. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E$_{2g}$ symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B $\sigma$ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to explaining the observed high T$_{c}$ and boron isotope effect in MgB$_{2}$Comment: In this revised version (to appear in PRL) we also discuss the boron isotope effect. Please visit http://www.ncnr.nist.gov/staff/taner/mgb2 for detail

Coupled magnetic and ferroelectric domains in multiferroic Ni3V2O8

Electric control of multiferroic domains is demonstrated through polarized magnetic neutron diffraction. Cooling to the cycloidal multiferroic phase of Ni3V2O8 in an electric field (E) causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with E. Quantitative analysis indicates the E-treated sample has handedness that can be reversed by E. We further show close association between cycloidal and ferroelectric domains through E-driven spin and electric polarization hysteresis. We suggest that definite cycloidal handedness is achieved through magneto-elastically induced Dzyaloshinskii-Moriya interactions.Comment: 4 pages, 5 figures. Updated affiliations and references. Minor changes to text and figure

Investigation of the Spin Density Wave in NaxCoO2

Magnetic susceptibility, transport and heat capacity measurements of single crystal NaxCoO2 (x=0.71) are reported. A transition to a spin density wave (SDW) state at Tmag = 22 K is observable in all measurements, except chi(ac) data in which a cusp is observed at 4 K and attributed to a low temperature glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and low temperature hysteresis are only visible along the c-axis. The system also exhibits a substantial (~40%) positive magnetoresistance below this temperature. Calculations of the electronic heat capacity gamma above and below Tmag and the size of the jump in C indicate that the onset of the SDW brings about the opening of gap and the removal of part of the Fermi surface. Reduced in-plane electron-electron scattering counteracts the loss of carriers below the transition and as a result we see a net reduction in resistivity below Tmag. Sodium ordering transitions at higher temperatures are observable as peaks in the heat capacity with a corresponding increase in resistivity.Comment: 14 pages, 6 figure