A molecular dynamics simulation of tje uniaxial phase of N2 adsorbed on graphite

A computer simulation study of a nitrogen monolayer adsorbed on a graphite substrate at low temperatures is reported. The adsorbed phase was slightly compressed relative to the commensurate √3 × √3 phase. The compression was taken to be uniaxial; i.e. a 5% change in the large intermolecular spacing along a glide line of the oriented herringbone structure (UXI phase). Thermodynamic properties were evaluated together with orientational and translational ordering parameters at two temperatures, one above (≈ 40 K) and one below (≈ 15 K) the in-plane disordering transition. For purposes of comparison, simulations of the commensurate phase at these temperatures are reported, together with a 15 K simulation of a phase that has been uniaxially compressed in the direction perpendicular to that of the UXI phase (UYI phase). The simulations indicate that the UXI phase is stable but the UYI phase tends to transform into domains of UXI; it is also concluded that the compression necessary to form the UXI phase from the commensurate does not produce a significant change in the out-of-plane ordering, at least at the lower temperature, but does bring about changes in the in-plane ordering of these molecules.     

Measurements of the anisotropic in-plane resistivity of underdoped FeAs-based pnictide superconductors

We systematically investigated the in-plane resistivity anisotropy of electron-underdoped EuFe(2-x)Co(x)As(2) and BaFe(2-x)Co(x)As(2) and hole-underdoped Ba(1-x)K(x)Fe(2)As(2). Large in-plane resistivity anisotropy was found in the former samples, while tiny in-plane resistivity anisotropy was detected in the latter ones. When it is detected, the anisotropy starts above the structural transition temperature and increases smoothly through it. As the temperature is lowered further, the anisotropy takes a dramatic enhancement through the magnetic transition temperature. We found that the anisotropy is universally tied to the presence of T-linear behavior of resistivity. Our results demonstrate that the nematic state is caused by electronic degrees of freedom, and the microscopic orbital involvement in the magnetically ordered state must be fundamentally different between the hole- and electron-doped materials.     

Reorientation of magnetization with temperature in 2D ferromagnets

We investigated 2D Heisenberg ferromagnet (monolayer) with the account of dipolar forces and uniaxial anisotropy and found a reorientation phase transition in temperature from out-of-plane to in-plane phase. This phase transition is of the first order with hysteresis. We estimated the temperatures of switching both analytically and numerically.     

Magnetic domain structure and magnetic anisotropy in Ga1-xMn(x)As

Large, well-defined magnetic domains, on the scale of hundreds of micrometers, are observed in Ga1-xMn(x)As epilayers using a high-resolution magneto-optical imaging technique. The orientations of the magnetic moments in the domains clearly show in-plane magnetic anisotropy, which changes through a second-order transition from a biaxial mode (easy axes nearly along [100] and [010]) at low temperatures to an unusual uniaxial mode (easy axis along [110]) as the temperature increases above about T(c)/2. This transition is a result of the interplay between the natural cubic anisotropy of the GaMnAs zinc-blende structure and a uniaxial anisotropy which attribute to the effects of surface reconstruction.     

Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C

We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron diffraction. The ordering vector is Q(A) = (0.48,0,0) and the phase appears above a critical in-plane magnetic field of 0.9 T. The field was applied in order to test the assumption that the zero-field magnetic structure at Q(F) = (0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity were destroyed. We present theoretical calculations which show that two effects are important: a suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase and a reduction of the superconducting condensation energy due to the periodic modulation of the moments at Q(A).     

Structure, magnetic and transport properties of La1−xBixMnO3

We have investigated the structural, magnetic and transport properties of La_1−xBi_xMnO₃ samples. As the Bi content increases, a structural transition from rhombohedral to pseudocubic and a magnetic phase transition from ferromagnetic ordering to cluster glass are identified. Metal–insulator (MI) transitions and large magnetoresistance (MR) effects are observed at low Bi doping levels, while insulating behavior of resistivity is found in the whole measured temperature range at high-doping levels. Two distinct ferromagnetic insulating (FI) states are found at low temperatures in this system. One can be suppressed and the other can be enhanced by applying magnetic fields. Possible reasons for the observed structural, magnetic phase transitions and changes of resistivity behavior with Bi doping are discussed.     

Interface biquadratic coupling and magnon scattering in exchange-biased ferromagnetic thin films grown on epitaxial FeF2

The magnetic anisotropy of ferromagnetic (FM) Ni, Co, and Fe polycrystalline thin films grown on antiferromagnetic (AF) FeF(2)(110) epitaxial layers was studied, as a function of temperature, using ferromagnetic resonance. In addition to an in-plane anisotropy in the FM induced by fluctuations in the AF short-range order, a perpendicular (biquadratic) magnetic anisotropy, with an out-of-plane component, was found which increased with decreasing temperature above the AF Neél temperature (T(N) = 78.4 K). This is a surprising result given that the AF's uniaxial anisotropy axis was in the plane of the sample, but is consistent with prior experimental and theoretical work. The resonance linewidth had a strong dependence on the direction of the external magnetic field with respect to in-plane FeF(2) crystallographic directions, consistent with interface magnon scattering due to defect-induced demagnetizing fields. Below T(N), the exchange bias field H(E) measured via FMR for the Ni sample was in good agreement with H(E) determined from magnetization measurements if the perpendicular out-of-plane anisotropy was taken into account. A low field resonance line normally observed at H ≈ 0, associated with domain formation during magnetization in ferromagnets, coincided with the exchange bias field for T < T(N), indicating domain formation with the in-plane FM magnetization perpendicular to the AF easy axis. Thus, biquadratic FM-AF coupling is important at temperatures below and above T(N).     

Large variations in the magnetic ordering behavior of EuCu(2)As(2) with the application of external pressure and magnetic field

The influence of external pressure on the electrical transport and magnetic properties of EuCu(2)As(2), crystallizing in a ThCr(2)Si(2)-type structure, is reported. The system is known to be an antiferromagnet below T(N) ≈ 15 K in the absence of external magnetic fields. We find that there is a gradual reduction of T(N) with the application of a magnetic field with an extrapolated value of the critical field of around 18 kOe which can drive T(N) to zero. Electrical resistivity under pressure (<11 GPa) reveals that the magnetic ordering temperature is pushed up dramatically to higher temperatures which is quite interesting if compared with the behavior in isostructural FeAs-based systems containing Eu. Above 7 GPa, the pressure-induced state appears to be ferromagnetic. The results thus reveal interesting changes in the magnetic ordering behavior of this compound with increasing pressure and magnetic fields.     

Symmetry-breaking lattice distortion in Sr₃Ru₂O₇

The electronic nematic phase of Sr?Ru?O? is investigated by high-resolution in-plane thermal expansion measurements in magnetic fields close to 8 T applied at various angles Θ off the c axis. At Θ < 10° we observe a very small (10??) lattice distortion which breaks the fourfold in-plane symmetry, resulting in nematic domains with interchanged a and b axis. At Θ ? 10° the domains are almost fully aligned and thermal expansion indicates an area-preserving lattice distortion of order 2 × 10?? which is likely related to orbital ordering. Since the system is located in the immediate vicinity of a metamagnetic quantum critical end point, the results represent the first observation of a structural relaxation driven by quantum criticality.     

Successive orbital ordering transitions in NaVO2

Physical property measurements on samples of triangular-lattice NaVO2 reveal two successive orbital ordering transitions. At 300 K, the structure is rhombohedral. At 98 K, the system undergoes a second-order transition to a monoclinic phase in which the in-plane V-V distances separate into four short and two long bonds, corresponding to orbital ordering of one electron per V3+. Below 93 K, there is a first-order transition to a second monoclinic phase with four long and two short V-V bonds, consistent with orbital ordering of two electrons per V3+. Long range magnetic ordering of 0.98(2)mu_(B) per V3+ (3d(2)) sets in at the 93 K structural transition. The orbital ordering relieves the geometric frustration and leads to a magnetically ordered ground state.     

Superconductivity at 22 K in Co-doped BaFe2As2 crystals

Here we report bulk superconductivity in BaFe1.8Co0.2As2 single crystals below Tc=22 K, as demonstrated by resistivity, magnetic susceptibility, and specific heat data. Hall data indicate that the dominant carriers are electrons, as expected from simple chemical reasoning. This is the first example of superconductivity induced by electron doping in this family of materials. In contrast with cuprates, the BaFe2As2 system appears to tolerate considerable disorder in the FeAs planes. First principles calculations for BaFe1.8Co0.2As2 indicate the interband scattering due to Co is weak.     

Strain induced changes in the magnetic phase diagram of metamagnetic heteroepitaxial EuSe/PbSe(1-x)Tex multilayers

Multilayers of strained metamagnetic EuSe intercalated with nonmagnetic PbSe1-xTex were grown by molecular beam epitaxy under conditions optimized by electron diffraction. From detailed structural and magnetic characterization using anomalous synchrotron x-ray diffraction and magnetization measurements, the phase transition temperatures and the magnetic phase diagrams of strained EuSe as a function of the in-plane lattice constant are determined. In this way, it is demonstrated that the magnetic properties of the samples can be significantly changed by applying biaxial strain on EuSe in superlattice structures.     

Orbital ordering and magnetic field effect in MnV2O4

We studied the structural properties of an orbital-spin-coupled spinel oxide, MnV2O4, mainly by single-crystal x-ray diffraction measurement. It was found that a structural phase transition from cubic to tetragonal and ferrimagnetic ordering occur at the same temperature (Ts,TN=57 K). The structural phase transition was induced also by magnetic field above Ts. In addition, magnetic-field-induced alignment of tetragonal domains results in large magnetostriction below Ts. We also found that the structural phase transition is caused by the antiferro-type ordering of the V t2g orbitals.     

Confinement of chiral magnetic modulations in the precursor region of FeGe

We report on magnetic susceptibility and specific heat measurements of the cubic helimagnet FeGe in external magnetic fields and temperatures near the onset of long-range magnetic order at TC = 278.2(3) K. Pronounced anomalies in the field-dependent χac(H) data as well as in the corresponding imaginary part χ'ac(H) reveal a precursor region around TC in the magnetic phase diagram. The occurrence of a maximum at T0 = 279.6 K in the zero-field specific heat data indicates a second-order transition into a magnetically ordered state. A shoulder evolves above this maximum as a magnetic field is applied. The field dependence of both features coincides with crossover lines from the field-polarized to the paramagnetic state deduced from χac(T) at constant magnetic fields. The experimental findings are analyzed within the standard Dzyaloshinskii theory for cubic helimagnets. The remarkable multiplicity of modulated precursor states and the complexity of the magnetic phase diagram near the magnetic ordering are explained by the change of the character of solitonic inter-core interactions and the onset of specific confined chiral modulations in this area.     

Review of nuclear magnetic resonance studies on iron-based superconductors

The newly discovered iron-based superconductors have triggered renewed enormous research interest in the condensed matter physics community. Nuclear magnetic resonance （NMR） is a low-energy local probe for studying strongly correlated electrons, and particularly important for high-Tc superconductors. In this paper, we review NMR studies on the structural transition, antiferromagnetic order, spin fluctuations, and superconducting properties of several iron-based high-Tc superconductors, including LaFeAsOl_xFx, LaFeAsOl_x, BaFe2As2, Bal_xKxFe2As2, Cao.23Nao.67Fe2As2, BaFe2（Asl_xPx）2, Ba（Fel_xRux）2As2, Ba（Fel_xCox）2As2, Lil＋xFeAs, LiFel_xCoxAs, NaFeAs, NaFel_xCoxAs, KyFe2_xSe2, and （T1,Rb）yFe2_xSe2.     

Continuous paranematic-to-nematic ordering transitions of liquid crystals in tubular silica nanochannels

The optical birefringence of rodlike nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 microm length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.     

Observation of a second order magnetic phase transition in CsFeS2

Mössbauer experiments performed on CsFeS₂ at temperatures between 4.2 K and 300 K show that the orthorhombic high temperature phase undergoes a second order magnetic phase transition near 69 K, when the previously reported first order magnetic and structural transition to a triclinic modification near 75 K is suppressed by lattice defects or internal stresses. The saturation values of the hyperfine fields are 19.1 T for the triclinic and 15.5 and 14.1 T for the orthorhombic phase.     

Magnetic anomalies in CeIn(2)

The magnetic behavior of binary compound CeIn(2) has been reported to be unusual in the sense that this compound appears to exhibit a first-order ferromagnetic transition at a rather high temperature of (T(C)=)22 K, which is not so common for Ce systems. In order to throw more light on the magnetic behavior of this compound, we have carried out detailed magnetization, and electrical resistivity studies as a function of temperature, magnetic field and external pressure, in addition to heat-capacity measurements. The plots of H/M versus M(2) at low fields are interestingly characterized by negative slopes, not only near T(C), but also at lower temperatures, a source of which could be attributed to magnetic-field-induced transitions at much lower temperatures. The sign of magnetoresistance tends to change from positive to negative with increasing temperature, as though there is a gradual change in the magnetic character. Finally, the magnetic ordering temperature increases with increasing pressure (until 20 kbar), as though this compound lies at the left-hand side of the peak in Doniach’s magnetic phase diagram.     

Phase Transition and Band Structure Tuned by Strains in Al_(1/2)Ga_(1/2)N Alloy of Complex Structure

Phase transition and band structure tuned by uniaxial and biaxial strains are systematically investigated based on the density-functional theory for ordered Al_(1/2)Ga_(1/2)N alloys of complex structures.Although the structural transformations to graphite-like from wurtzite are energetically favorable for both types of strain,the phase transitions are different in nature:the second-order transition induced by uniaxial strain is jointly driven by the mechanical and dynamical instabilities and the first-order transition by biaxial strain only by the mechanical instability.The wurtzite phase always shows the direct band gap,while the band gap of the graphite-like phase is always indirect.Furthermore,the band gaps of the wurtzite phase can be reduced by both types of strain,while that of the graphite-like phase is enhanced by uniaxial strain and is suppressed by biaxial strain.     

Comment on magnetism and superconductivity in rutheno cuprates: RuSr2GdCu2O8 and RuSr2Gd1.5Ce0.5Cu2O10

Both RuSr₂GdCu₂O_8-δ (Ru-1212) and RuSr₂Gd_1.5Ce_0.5Cu₂O_10-δ (Ru-1222) exhibits magnetism and superconductivity, as seen by magnetization vs. temperature behavior measured in 5 Oe field. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization data show branching at around 140 K and 100 K with a cusp at 135 K and 80 K and a diamagnetic transition around 20 K and 30 K in the ZFC part, for Ru-1212 and Ru-1222, respectively. The isothermal magnetization possesses a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behavior of the samples in applied fields of 0, 3 and 7 T confirmed superconductivity, with a different type of broadening of the superconductivity transition under magnetic fields for Ru-1212 from that known for conventional high-T _c superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature, MR displays a positive peak at low fields and becomes negative at higher fields for Ru-1212. For Ru-1222, MR remains negative both above and below the ordering temperature. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for the Ru-1212 sample shows two types of superstructure: one has a weak spot at the centre of the a–b rectangle, and the other only along the b direction. Interestingly, Ru-1222 shows only clean a–b and a–c planes, without any superstructures.