Colossal Magnetoresistance in a Mott Insulator via Magnetic Field-Driven Insulator-Metal Transition.

We present a new type of colossal magnetoresistance (CMR) arising from an anomalous collapse of the Mott insulating state via a modest magnetic field in a bilayer ruthenate, Ti-doped Ca_{3}Ru_{2}O_{7}. Such an insulator-metal transition is accompanied by changes in both lattice and magnetic structures. Our findings have important implications because a magnetic field usually stabilizes the insulating ground state in a Mott-Hubbard system, thus calling for a deeper theoretical study to reexamine the magnetic field tuning of Mott systems with magnetic and electronic instabilities and spin-lattice-charge coupling. This study further provides a model approach to search for CMR systems other than manganites, such as Mott insulators in the vicinity of the boundary between competing phases.

From (pi,0) magnetic order to superconductivity with (pi,pi) magnetic resonance in Fe(1.02)Te(1-x)Se(x).

The iron chalcogenide Fe(1+y)(Te(1-x)Se(x)) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe(1+y)Te exhibits antiferromagnetic order with an in-plane magnetic wave vector (pi,0) (ref. 6). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (pi,pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (pi,pi) (refs 9, 10, 11). A central question in this burgeoning field is therefore how (pi,pi) superconductivity can emerge from a (pi,0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (pi,0) are suppressed and the mode at (pi, pi) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.

The effect of uniaxial pressure on the antiferromagnetic structure of UNiAl studied using single-crystal neutron diffraction.

The effect of uniaxial pressure on the magnetic ordering in a single-crystalline sample of UNiAl has been studied by means of neutron diffraction. The crystal and magnetic structures remain in a first approximation unaffected for the pressure applied along the c axis. For perpendicularly applied pressure, severe changes to the magnetic ordering are found. From the original six magnetic reflections that are associated with each nuclear reflection, two disappear and the remaining four rotate in the reciprocal plane, marking modification of the propagation vector. The U magnetic moments are reduced upon application of the pressure along the a axis, by about 0.11 µ(B) kbar(-1). In contrast, the magnetic phase transition temperature increases by about 0.6 K kbar(-1). The results are explained qualitatively in terms of pressure-induced changes of exchange interactions.

Field-induced ferromagnetic structure in Er(2)Ni(2)Pb.

We have studied the effect of magnetic fields up to 4.5 T on the ground-state structure in Er(2)Ni(2)Pb using powder neutron diffraction measurements at low temperatures. The zero-field magnetic state that itself is not uniform and consists of different magnetic phases is rather unstable against the magnetic field. As the field is increased, the magnetic reflections of the zero-field structure disappear and a new magnetic phase with commensurate propagation vector is clearly observed in a field of 0.5 T. At higher fields a ferromagnetic state is established in Er(2)Ni(2)Pb, which can be fully described only by a model that combines at least two irreducible representations. The refined Er magnetic moment magnitude of 9.10 ± 0.07 µ(B) is very close to the Er(3+) free ion value of 9.0 µ(B).

Anomalous shift of magnetic diffuse scattering studied by neutron diffraction.

Neutron diffraction results, in the vicinity of the magnetic phase transition of USb and MnF(2), are reported. The thermal evolution of the magnetic diffuse signal and nuclear Bragg reflections demonstrate that the centre of gravity of the magnetic signals does not lie at the predicted position as calculated from nuclear reflections. This phenomenon, called the q-shift, was first found using resonance x-ray scattering (RXS). The present results show that, (i) the effect is not an artefact of RXS and is also found with neutrons (ii) that the effect arises from the bulk of the sample and is not restricted to the near surface layer (∼2000 Å) associated with the RXS probe in actinide systems, (iii) the effect is not restricted to actinide compounds.

Magnetic-field effects on the fragile antiferromagnetism in YbBiPt.

We present neutron-diffraction data for the cubic-heavy-fermion YbBiPt that show broad magnetic diffraction peaks due to the fragile short-range antiferromagnetic (AFM) order persist under an applied magnetic-field H . Our results for H ⊥ [ 1 ¯ 1 0 ] and a temperature of T = 0.14 1 K show that 1 2 , 1 2 , 3 2 ) magnetic diffraction peak can be described by the same two-peak line shape found for µ 0 H = 0 T below the Néel temperature of T N = 0.4 K . Both components of the peak exist for µ 0 H ≲ 1.4 T , which is well past the AFM phase boundary determined from our new resistivity data. Using neutron-diffraction data taken at T = 0.13 ( 2 ) K for H ∥ 0 0 1 taken at or 1 1 0 , we show that domains of short-range AFM order change size throughout the previously determined AFM and non-Fermi liquid regions of the phase diagram, and that the appearance of a magnetic diffraction peak at 1 2 , 1 2 , 1 2 at µ 0 H ≈ 0.4 T signals canting of the ordered magnetic moment away from 1 1 1 . The continued broadness of the magnetic diffraction peaks under a magnetic field and their persistence across the AFM phase boundary established by detailed transport and thermodynamic experiments present an interesting quandary concerning the nature of YbBiPt's electronic ground state.

Neutron scattering study of the field-induced soliton lattice in CuGeO3

CuGeO3 undergoes a transition from a spin-Peierls phase to an incommensurate phase at a critical field of H(c) approximately 12.5 T. In the high-field phase a lattice of solitons forms, with both structural and magnetic components, and these have been studied using neutron scattering techniques. Our results provide direct evidence for a long-ranged magnetic soliton structure which has both staggered and uniform magnetizations with amplitudes that are broadly in accord with theoretical estimates. The magnetic soliton width gamma(m) and the field dependence of the incommensurability deltak(SP) are found to agree well with theoretical predictions.

NSD and cell survival.

The relation between the NSD concept and cell survival in fractionated irradiation is established; the slope of Strandquist's lines is related to the shape of survival curves for the given type of cells and radiation. A new parameter, CBE, derived from TDF for determining cumulative biological effect is proposed.

Strandquist's graphs and the new mathematical formula for cumulative biological effect.

The new formula for cumulative biological effect (CBE) derived on the basis of main characteristics of Strandquist's graphs has been used for an analysis of experimental fractionation data. Some new aspects concerning fractionated irradiation have been mentioned.

Perspectives of tumour radiotherapy in the Czech Republic and "Oncology 2000" Foundation.

The incidence of tumours has still an increasing tendency. A great effort is being devoted to prevention as well as to improving early diagnosis and treatment. It is the radiotherapy which seems to bring new treatment possibilities at the present. Its goal consists in destroying all tumour cells in the target volume while all normal cells being damaged unsubstantially only. That is being achieved with standard available means only with rather great difficulties. However, in the last time two radiotherapy methods have been developed which are based on the use of the so called hadrons (i.e. particles exhibiting strong mutual interactions) and come nearer to the mentioned ideal goal of any tumour therapy. The first method consists in the application of beams of charged hadrons having a suitable characteristic of energy transfer; the most energy being transferred to the matter in the end of hadron tracks. Thus, by a suitable choice of initial energy of beam particles this maximum may lie in a tumour volume, while all other tissues obtain a smaller dose, the difference being greater when a multiple-field technique is used. The other approach (neutron capture therapy-NCT) makes use of epithermal (or thermal) neutrons captured by special isotopes accumulated in tumour cells with the help of suitable chemical carriers. Both the methods are now being introduced into clinical use in many countries. The "Oncology 2000" Foundation was grounded to introduce these methods into clinical use also in the Czech Republic. It proceeds with activities started by an interdisciplinary group of specialists formed approximately 20 years ago and having prepared some necessary conditions. As to the NCT a corresponding facility is now being finished and clinical tests are expected to be started in 1997. The main attention is now devoted to building up an oncology center based on the use of hadron beams.

Inactivation of individual cells by divers ions at different LET values.

A new formula linking the shape of survival curve to the inactivation probabilities after different numbers of cell hits has been derived. It has been used in analyzing recent experimental data obtained with monolayer cells irradiated at definite values of LET (in different parts of Bragg peaks). The new approach allows not only deriving the values of inactivation probabilities at given LET values; unexpected consequences seem to follow especially for inactivation characteristics of carbon ions in different parts of the Bragg peak, too.

(3 + 1)-dimensional crystal and antiferromagnetic structures in CeRuSn.

At 320 K, the crystal structure of CeRuSn is commensurate with the related CeCoAl-type of structure by the doubling of the c lattice parameter. However, with lowering the temperature it becomes incommensurate with x and z position parameters at all three elemental sites being modulated as one moves along the c-axis. The resulting crystal structure can be conveniently described within the superspace formalism in (3 + 1) dimensions. The modulation vector, after initially strong temperature dependence, approaches a value close to qnuc = (0 0 0.35). Below TN = 2.8 (1) K, CeRuSn orders antiferromagnetically with a propagation vector qmag = (0 0 0.175), i.e. with the magnetic unit cell doubled along the c-axis direction with respect to the incommensurate crystal structure. Ce moments appear to be nearly collinear, confined to the a-c plane, forming ferromagnetically coupled pairs. Their magnitudes are modulated between 0.11 and 0.95 µB as one moves along the c-axis.

Magnetic structure of La2O3FeMnSe2: neutron diffraction and physical property measurements.

We report on the characterization of the mixed layered lanthanum iron manganese oxyselenide La(2)O(3)FeMnSe(2), where Fe and Mn share the same crystallographic position. The susceptibility data show a magnetic transition temperature of 76 K and a strong difference between field cooled and zero field cooled (ZFC) data at low fields. While the ZFC magnetization curve exhibits negative values below about 45 K, hysteresis measurement reveals, after an initial negative magnetic moment, a hysteresis loop typical for ferromagnetic material, pointing to competing ferromagnetic and antiferromagnetic interactions. Resistivity and dielectric permittivity measurements indicate that La(2)O(3)FeMnSe(2) is a semiconductor. We performed x-ray diffraction at 295 K and neutron diffraction at 90 and 1.7 K. The nuclear and magnetic structure was refined in the space group I4/mmm with a = 4.11031 (3) Å and c = 18.7613 (2) Å at 295 K. We did not detect a structural distortion and the Fe and Mn atoms were randomly distributed. The magnetic order was found to be antiferromagnetic, with a propagation vector q = (0,0,0) and magnetic moments of 3.44 (5) µ(B) per Fe/Mn atom aligned within the a-b plane. This magnetic order is different with respect to the pure Fe or Mn compositions reported in other studies.

Neutron study of the magnetism in NiCl2·4SC(NH2)2.

We study the strongly anisotropic quasi-one-dimensional S = 1 quantum magnet NiCl2·4SC(NH2)2 using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field Hc1. For Hc1 < H < Hc2, where Hc2 indicates the transition to a fully magnetized state, three-dimensional magnetic order is established with the AF moment perpendicular to the magnetic field. We mapped the temperature/magnetic field phase diagram, and we find that the total ordered magnetic moment reaches m(tot) = 2.1 µB at the field µ(0)H = 6 T and is thus close to the saturation value of the fully ordered moment. We study the magnetic spin dynamics in the fully magnetized state for H > Hc2, and we demonstrate the presence of an AF interaction between Ni(2+) on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin-waves that develop from the lower-energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices.

Transverse magnetism in uniaxial antiferromagnet UNiGa.

UNiGa crystallizes in the hexagonal ZrNiAl structure and orders antiferromagnetically below T(N)=39.3 K with the U moments oriented along the c-axis (easy magnetization axis). There are four different antiferromagnetic phases in zero magnetic field and two field induced magnetic phases in UNiGa. For all of them, a strong uniaxial anisotropy is encountered. All the magnetic phases are described by propagation vectors of (0 0 q(i))-type, where q(i) describes the stacking of the ferromagnetic basal planes along the hexagonal c-axis. However, recently (0 0 L)-type Bragg reflections associated with the magnetic ordering have been observed by neutron diffraction. Based on unpolarized and polarized neutron diffraction and non-resonant and resonant synchrotron x-ray scattering experiments combined with polarization analysis we conclude that small amounts of magnetic moments oriented perpendicular to the c-axis exist in UNiGa. Whether these moments reside on Ni atoms outside the U-Ni planes or at interstitial regions could not be determined.