Guocai Liu

Determination of the entropy changes in the compounds with a first-order magnetic transition

Entropy changes in the compounds of La1−xPrxFe11.5Si1.5 (x=0.3 and 0.4) have been experimentally studied. A tower-shaped entropy change of the height of ∼27J∕kgK is obtained based on the analyses of heat capacity, while the Maxwell relation predicts an extra entropy peak of the height of ∼99J∕kgK, slightly varying with Pr content. A careful study indicates that the Maxwell relation cannot be used in the vicinity of the Curie temperature because of the coexistence of paramagnetic and ferromagnetic phases, and the huge entropy peak is a spurious result. Similar conclusions are applicable to MnAs and Mn1−xFexAs, for which huge entropy changes have been reported. Appropriate methods for the determination of entropy change of the compound with phase separation are discussed based on the magnetic data.

Topological superfluid transition induced by a periodically driven optical lattice

We propose a scenario to create a topological superfluid in a periodically driven two-dimensional square optical lattice. We study the phase diagram of a spin-orbit coupled s-wave pairing superfluid in a periodically driven two-dimensional square optical lattice. We find that a phase transition from a trivial superfluid to a topological superfluid occurs when the potentials of the optical lattices are periodically changed. The topological phase is called the Floquet topological superfluid and can host Majorana fermions.

A comparison study of the entropy changes in materials with and without short-range magnetic order

The effects of short-range magnetic order (SRMO) on magnetic entropy changes have been studied for the compounds Sm0.55Sr0.45MnO3, Eu0.55Sr0.45MnO3 and the rare-earth metal Gd. Ferromagnetic clusters composed of ~8 Mn ions are found to be formed in the paramagnetic phase of the manganites based on the analysis of susceptibility. As a result, the entropy change for a field variation of 0?5?T is ~6?J?kg?1?K?1 for Sm0.55Sr0.45MnO3 and ~7?J?kg?1?K?1 for Eu0.55Sr0.45MnO3, well below the values expected without considering the effect of SRMO. (The maximum entropy change will be ~56?J?kg?1?K?1 for both compounds.) Assuming the presence of sizable magnetic clusters, the field-induced entropy change, not only its peak value but also its temperature dependence, can be well reproduced by the mean field theory. The average cluster size is found to be m?8, in agreement with the result obtained from a Curie?Weiss analysis of the susceptibility. The presence of SRMO reduces the entropy change by ~80%. In contrast, the mean field theory gives a good description of the entropy change in Gd, for which no significant SRMO is observed. The present work reveals the important role of SRMO in modifying the magnetocaloric properties of the magnetic materials.

Magnetic, thermal and transport properties of phase-separated La0.27Nd0.4Ca0.33MnO3

The magnetic, resistive and thermal properties of the phase-separated compound La0.27Nd0.4Ca0.33MnO3 have been experimentally studied. The sample is found to experience a charge/orbital ordering transition at ∼175 K and an antiferromagnetic (AFM) transition at ∼156 K without a magnetic field. A magnetic field stabilizes the ferromagnetic (FM) order and the field-induced FM phase coexists with the AFM phase under a field below 2 T in the temperature range below TC ≈ 110 K. However, the magnetic entropy change accompanying the AFM–FM transition is negligibly small when TC TN. A general relation between resistivity and magnetization, ρ = A0T exp[(e − 800m 2 )/T ] (m = normalized magnetization), is established for the paramagnetic phase, which is also applicable to other compounds with different characters, such as the La0.67Ca0.33MnO3 film and the La0.474Bi0.193Ca0.33Mn0.994Cr0.006O3 and Eu0.55Sr0.45MnO3 ceramics. In the FM state, the resistivity is quite sensitive to the change of spin alignment and exhibits an exponential decrease with magnetization ρ = ρ0 exp(−22.7m). Field-induced phase separation is believed to be responsible for the distinct properties of La0.27Nd0.4Ca0.33MnO3. (Some figures in this article are in colour only in the electronic version)

Chiral f-wave topological superfluid in triangular optical lattices

We demonstrate that an exotically chiral f-wave topological superfluid can be induced in cold-fermionic-atom triangular optical lattices through a laser-field-generated effective non-Abelian gauge field, controllable Zeeman fields, and s-wave Feshbach resonance. We find that the chiral f-wave topological superfluid is characterized by three gapless Majorana edge states located on the boundary of the system. More interestingly, these Majorana edge states degenerate into one Majorana fermion bound to each vortex in the superfluid. Our proposal enlarges the topological superfluid family and specifies a unique experimentally controllable system to study Majorana fermion physics.

A universal relation between resistivity and magnetization in paramagnetic state of manganites

A systematic study on the resistivity-magnetization relation above the Curie temperature (TC) has been performed for the manganites with different properties, such as the epitaxial La0.67Ca0.33MnO3 film on a SrTiO3 substrate, which has the weakest magnetic correlation above TC, the La0.27Nd0.4Ca0.33MnO3 polycrystalline, which exhibits a charge ordering in the paramagnetic state, and the ceramics Eu0.55Sr0.45MnO3 and La0.474Bi0.193Ca0.33Mn0.994Cr0.006O3, which show strong short-range magnetic orders above TC. A quantitative, yet universal, relation of the form ρ=ATexp[(e-αm2)∕T] is observed in all of the manganites, with an essentially constant coefficient of α≈800±60K, where m is the normalized magnetization and e is the activation energy for polarons. The underlying physics for this relation has been analyzed based on the double exchange model.

Quantum Hall effects in a non-Abelian honeycomb lattice

We study the tunable quantum Hall effects in a non-Abelian honeycomb optical lattice which is a multi-Dirac-point system. We find that the quantum Hall effects present different features with the change in relative strengths of several perturbations. Namely, the quantum spin Hall effect can be induced by gauge-field-dressed next-nearest-neighbor hopping, which, together with a Zeeman field, can induce the quantum anomalous Hall effect characterized by different Chern numbers. Furthermore, we find that the edge states of the multi-Dirac-point system represent very different features for different boundary geometries, in contrast with the generic two-Dirac-point system. Our study extends the borders of the field of quantum Hall effects in a honeycomb optical lattice with multivalley degrees of freedom.

Hall effect in La{sub 0.7}Ce{sub 0.3}MnO{sub 3+{delta}} films with variable oxygen content

We have recently argued that manganites do not possess stripes of charge order, implying that the electron-lattice coupling is weak [Loudon et al., Phys. Rev. Lett. 94, 097202 (2005)]. Here we independently argue the same conclusion based on transmission electron microscopy measurements of a nanopatterned epitaxial film of La{sub 0.5}Ca{sub 0.5}MnO{sub 3}. In strain relaxed regions, the superlattice period is modified by 2% to 3% with respect to the parent lattice, suggesting that the two are not strongly tied.Minimalist theories of complex systems are broadly of two kinds: mean field and axiomatic. So far, all theories of complex properties absent from simple systems and intrinsic to glasses are axiomatic. Stretched Exponential Relaxation (SER) is the prototypical complex temporal property of glasses, discovered by Kohlrausch 150 years ago, and now observed almost universally in microscopically homogeneous, complex nonequilibrium materials, including luminescent electronic Coulomb glasses. A critical comparison of alternative axiomatic theories with both numerical simulations and experiments strongly favors channeled dynamical trap models over static percolative or energy landscape models. The topics discussed cover those reported since the authors review article in 1996, with an emphasis on parallels between channel bifurcation in electronic and molecular relaxation.The local atomic structure of the Ag induced Si(111)-({radical}(3)x{radical}(3)) surface has been investigated using photoelectron diffraction (PED) at 10 and 300 K. Two surface components, whose intensities varied by changing the photon energy as a consequence of diffraction effects, were observed in the Si 2p core-level spectra at both temperatures. The good agreement between the experimental PED patterns of the Si 2p surface components and the simulated PED patterns indicates that the atomic structure of this surface follows the inequivalent triangle model. Further, since the PED patterns obtained at 10 and 300 K resemble each other closely, we conclude that the local atomic structure of the Ag/Si(111)-({radical}(3)x{radical}(3)) surface is the same at the two temperatures, and thus that the origin of the transition reported in the literature is an order-disorder transition.Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La{sub 2}CuO{sub 4} single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined to drive a continuous reorientation of the copper spins from the orthorhombic b axis to the c axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for nonstoichiometric La{sub 2}CuO{sub 4} is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work.Positron annihilation spectroscopy was applied to study relaxed P-doped n-type and undoped Si{sub 1-x}Ge{sub x} layers with x up to 0.30. The as-grown SiGe layers were found to be defect free and annihilation parameters in a random SiGe alloy could be represented as superpositions of annihilations in bulk Si and Ge. A 2 MeV proton irradiation with a 1.6x10{sup 15} cm{sup -2} fluence was used to produce saturated positron trapping in monovacancy related defects in the n-type layers. The defects were identified as V-P pairs, the E center. The distribution of Si and Ge atoms surrounding the E center was the same as in the host lattice. The process leading to the formation of V-P pairs therefore does not seem to have a significant preference for either Si or Ge atoms. In undoped Si{sub 1-x}Ge{sub x} we find that a similar irradiation produces a low concentration of divacancies or larger vacancy defects and found no evidence of monovacancies surrounded by several Ge atoms.Structural properties of the spin chain and ladder compound Sr{sub 14}Cu{sub 24}O{sub 41} have been studied using high energy x-ray diffraction. Strong incommensurate modulation reflections are observed due to the lattice mismatch of the chain and ladder structure, respectively. While modulation reflections of low orders display only a weak temperature dependence, higher orders dramatically increase in intensity when cooling the sample to 10 K. All observed modulation reflections are indexed within a super space group symmetry and no structural phase transition could be identified between 10 K and room temperature. We argue that these modulation reflections are not caused by a fivefold periodicity of the chain lattice, as claimed by Fukuda et al., Phys. Rev. B 66, 012104 (2002), but that holes localize in the potential given by the lattice modulation, which in turn gives rise to a further deformation of the lattice.We report neutron diffraction experiments on the light-induced metastable state SI in single crystals of Na{sub 2}[Fe(CN){sub 5}NO]{center_dot}2D{sub 2}O. It is shown that the metastable state SI corresponds to a linkage isomer of the NO group, the so-called isonitrosyl configuration where the NO ligand is oxygen bound to the central iron atom.The impact of group-III vacancy diffusion, generated during dielectric cap induced intermixing, on the energy state transition and the inhomogeneity reduction in the InGaAs/GaAs quantum-dot structure is investigated. We use a three-dimensional quantum-dot diffusion model and photoluminescence data to determine the thermal and the interdiffusion properties of the quantum dot. The band gap energy variation related to the dot uniformity is found to be dominantly affected by the height fluctuation. A group-III vacancies migration energy H{sub m} for InGaAs quantum dots of 1.7 eV was deduced. This result is similar to the value obtained from the bulk and GaAs/AlGaAs quantum-well materials confirming the role of SiO{sub 2} capping enhanced group-III vacancy induced interdiffusion in the InGaAs quantum dots.We report vibrating wire viscometer experiments in the concentrated and dilute phase of saturated {sup 3}He-{sup 4}He mixtures showing that the slip length may become orders of magnitude larger than the mean free path due to specular scattering of the {sup 3}He quasiparticles with a {sup 4}He coating adsorbed at the surface of the wire. Since the liquid does not almost stick to the surface, the boundary conditions for fluid flow are unusual and not accounted for by the current theory for slip [H. Hoejgaard Jensen et al., J. Low Temp. Phys. 41, 473 (1980)]. The experimental results are in excellent agreement with a recent theory for slip [R. Bowley and J. Owers-Bradley, J. Low Temp. Phys. 136, 15 (2004)] which accounts for the effect of the cylindrical geometry and for velocity slip in directions normal as well as tangential to the surface of the wire. We find that our viscosity measurements in the dilute phase resulting from the data analysis based on the recent slip theory are in better agreement with the Fermi liquid theory than previous experimental results.Magnetization measurements prove that the magnetic properties of large-angle ({theta}>30 deg. ) bismuth bicrystals with a crystallite interface (CI) of twisting types essentially differ from well-known results on single-crystalline specimens. Two superconducting phases with T{sub c}{approx}8.4 K and {approx}4.3 K were observed at the CI of bicrystals while ordinary rhombohedral Bi is not a superconductor. We conclude that these phases have to do with the central part and the adjacent layers of the CI of bicrystals.The local structure around Ag ions in silver borate glasses g-Ag{sub 2}O{center_dot}nB{sub 2}O{sub 3} (n=2,4) was studied by x-ray absorption spectroscopy at the Ag K edge for temperatures from 77 to 450 K. Extended x-ray absorption fine structure (EXAFS) analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribution functions (RDFs) around Ag ions were reconstructed using a method based on the direct inversion of the EXAFS expression. The RDFs consist of about eight atoms (oxygens and borons), exhibit a relatively weak temperature dependence, and indicate the presence of strong static disorder. Two main components can be identified in RDFs, located at about 2.3-2.4 A and 2.5-3.4 A, respectively. The chemical types of atoms contributing to the RDF were determined via a simulation of configurationally averaged x-ray absorption near-edge structure (XANES) and EXAFS signals. The immediate neighborhood of Ag contains mostly oxygens while borons dominate at larger distances. The combination of EXAFS and XANES techniques allowed us to determine a more complete structural model than would be possible by relying solely on either EXAFS or XANES alone.Hall effects of the La{sub 0.7}Ce{sub 0.3}MnO{sub 3+{delta}} film, which is believed an electron-doped manganite, have been experimentally studied, and a positive normal Hall coefficient is observed below the Curie temperature when the oxygen content of the film varies in a wide range. These observations may be attributed to the presence of excessive oxygen and composition distribution in the film, which may occur companying tetravalent ion doping. Removing excessive oxygen drives the system into the electron-doping state, however, the resistivity increases monotonically with oxygen loss, and the metal-to-semiconductor transition typical for a hole-doped manganite disappears. These results suggest the determinative role of hole doping for the resistive and magnetic behaviors in La{sub 0.7}Ce{sub 0.3}MnO{sub 3+{delta}}.We studied the influence of the disorder introduced in polycrystalline MgB{sub 2} samples by neutron irradiation. To circumvent self-shielding effects due to the strong interaction between thermal neutrons and {sup 10}B we employed isotopically enriched {sup 11}B which contains 40 times less {sup 10}B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low {sup 10}B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1-2x10{sup 18} cm{sup -2}. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behavior has been carried out to identify what scattering and pinning mechanisms come into play. Finally, the correlation between some characteristic lengths and the transition widths is analyzed.The structure of Na{sub 0.5}CoO{sub 2}, the low-temperature insulator that separates the antiferromagnetic and normal metals in the Na{sub x}CoO{sub 2} phase diagram, is studied by high-resolution powder neutron diffraction at temperatures between 10 and 300 K. Profile analysis confirms that it has an orthorhombic symmetry structure, space group Pnmm, consisting of layers of edge-sharing CoO{sub 6} octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The oxygen content is found to be stoichiometric within 1%, indicating that the Na concentration accurately determines the electronic doping. The Na ordering creates two distinct Co sites, in parallel chains running along one crystallographic direction. The differences in their Co-O bond distances and the derived bond valence sums, reflections of the degree of charge ordering in this phase, are very small.The temperature dependence of the local structure of V{sub 2}O{sub 3} in the vicinity of the metal-to-insulator transition (MIT) has been investigated using hard x-ray absorption spectroscopy. It is shown that the vanadium pair distance along the hexagonal c axis changes abruptly at the MIT as expected. However, a continuous increase of the tilt of these pairs sets in already at higher temperatures and reaches its maximum value at the onset of the electronic and magnetic transition. These findings confirm recent theoretical results which claim that electron-lattice coupling is important for the MIT in V{sub 2}O{sub 3}. Our results suggest that the distortion of the symmetry of the basal plane plays a decisive role for the MIT and orbital degrees of freedom drive the MIT via changes in hybridization.We present here ab initio determinations of the nuclear-quadrupole moment Q of hyperfine-probe-nuclear states of three different In isotopes: the 5{sup +} 192 keV excited state of {sup 114}In (probe for nuclear quadrupole alignment spectroscopy), the 9/2{sup +} ground state of {sup 115}In (nuclear magnetic and nuclear quadrupole resonance probe), and the 3/2{sup +} 659 keV excited state of {sup 117}In (perturbed angular correlations probe). These nuclear-quadrupole moments were determined by comparing experimental nuclear-quadrupole frequencies to the electric field gradient tensor calculated with high accuracy at In sites in metallic indium within the density functional theory. These ab initio calculations were performed with the full-potential linearized augmented plane wave method. The results obtained for the quadrupole moments of {sup 114}In [Q({sup 114}In)=-0.14(1) b] are in clear discrepancy with those reported in the literature [Q({sup 114}In)=+0.16(6) b and +0.739(12) b]. For {sup 115}In and {sup 117}In our results are in excellent agreement with the literature and in the last case Q({sup 117}In) is determined with more precision. In the case of Q({sup 117}In), its sign cannot be determined because standard {gamma}-{gamma} perturbed angular correlations experiments are not sensitive to the sign of the nuclear-quadrupole frequency.An original epitaxial system consisting of two ferrimagnetic insulator layers (CoFe{sub 2}O{sub 4} and Fe{sub 3}O{sub 4}) separated by a nonmagnetic metallic layer (Au) has been grown. The transport properties in the current in plane geometry indicate that the conduction of the CoFe{sub 2}O{sub 4}/Au/Fe{sub 3}O{sub 4} trilayer takes place within the thin metallic layer. The giant magnetoresistance (GMR) observed (2.6% at 10 K) is associated to the switching from a parallel to an antiparallel configuration of the magnetization of the two ferrite layers and corresponds to the spin dependence of electron reflection at the interfaces with a large contribution of specular reflections. The increase of the GMR (5% at 10 K) in the symmetrical interface CoFe{sub 2}O{sub 4}/Fe{sub 3}O{sub 4}/Au/Fe{sub 3}O{sub 4} system and the effect of the interface roughness on the GMR confirm the presence of this spin-dependent specular reflection.The effect of externally applied pressure on the magnetic behavior of Cu{sub 2}Te{sub 2}O{sub 5}(Br{sub x}Cl{sub 1-x}){sub 2} with x=0, 0.73, and 1, is investigated by a combination of magnetic susceptibility, neutron diffraction, and neutron inelastic scattering measurements. The magnetic transition temperatures of the x=0 and 0.73 compositions are observed to increase linearly with increasing pressure at a rate of 0.23(2) and 0.04(1) K/kbar, respectively. However, the bromide shows contrasting behavior with a large suppression of the transition temperature under pressure, at a rate of -0.95(9) K/kbar. In neutron inelastic scattering measurements of Cu{sub 2}Te{sub 2}O{sub 5}Br{sub 2} under pressure only a small change to the ambient pressure magnetic excitations were observed. A peak in the density of states was seen to shift from {approx}5 meV in ambient pressure to {approx}6 meV under an applied pressure of 11.3 kbar, which was associated with an increase in the overall magnetic coupling strength.A KH{sub 2}PO{sub 4} (KDP) crystal, irradiated by a 1 MeV hydrogen ion beam to a dose of 10{sup 15} ions/cm{sup 2}, was studied by means of x-ray diffraction (XRD), {sup 1}H nuclear magnetic resonance (NMR), and dielectric constant measurements. The XRD pattern for the a-cut KDP crystal revealed a decrease in the lattice constant along the a axis after the proton irradiation. According to the {sup 1}H NMR spin-lattice relaxation rate measurements, the proton irradiation gave rise to reduction in the activation energy in the paraelectric phase, from 0.42 to 0.28 eV, in agreement with the temperature dependent second moment measurements indicating the proton motion is more activated after the proton irradiation. Besides, analysis of the temperature-dependent dielectric constants using a mean-field approximation revealed a change in the hydrogen bond induced by the proton irradiation.