arXiv subjects

T. A. Lograsso

Publications and source records attributed to T. A. Lograsso.

47 records · Page 3Linked to original sources

arXiv2007

^{17}O and ^{51}V NMR for the zigzag spin-1 chain compound CaV2O4

$^{51}$V NMR studies on CaV2O4 single crystals and $^{17}$O NMR studies on $^{17}$O-enriched powder samples are reported. The temperature dependences of the $^{17}$O NMR line width and nuclear spin-lattice relaxation rate give strong evidence for a long-range antiferromagnetic transition at Tn = 78 K in the powder. Magnetic susceptibility measurements show that Tn = 69 K in the crystals. A zero-field $^{51}$V NMR signal was observed at low temperatures (f $\approx$ 237 MHz at 4.2 K) in the crystals. The field swept spectra with the field in different directions suggest the presence of two antiferromagnetic substructures. Each substructure is collinear, with the easy axes of the two substructures separated by an angle of 19(1) degree, and with their average direction pointing approximately along the b-axis of the crystal structure. The two spin substructures contain equal number of spins. The temperature dependence of the ordered moment, measured up to 45 K, shows the presence of an energy gap Eg in the antiferromagnetic spin wave excitation spectrum. Antiferromagnetic spin wave theory suggests that Eg lies between 64 and 98 K.

cond-mat.str-el↗

arXiv2007

Low temperature heat capacity of Fe_{1-x}Ga_{x} alloys with large magneostriction

The low temperature heat capacity C_{p} of Fe_{1-x}Ga_{x} alloys with large magnetostriction has been investigated. The data were analyzed in the standard way using electron ($γT$) and phonon ($βT^{3}$) contributions. The Debye temperature $Θ_{D}$ decreases approximately linearly with increasing Ga concentration, consistent with previous resonant ultrasound measurements and measured phonon dispersion curves. Calculations of $Θ_{D}$ from lattice dynamical models and from measured elastic constants C_{11}, C_{12} and C_{44} are in agreement with the measured data. The linear coefficient of electronic specific heat $γ$ remains relatively constant as the Ga concentration increases, despite the fact that the magnetoelastic coupling increases. Band structure calculations show that this is due to the compensation of majority and minority spin states at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2007

Addendum: "On the nature of the phase transition in the itinerant helimagnet MnSi", arXiv:cond-mat/0702460v1 [cond-mat.str-el]

New high resolution data for heat capacity, heat capacity under applied magnetic fields and resistivity of high quality single crystal of MnSi are reported. Striking mirror symmetry between temperature derivative of resistivity and thermal expansion coefficient of MnSi is displayed. Close similarity between variation of the heat capacity and the temperature derivative of resistivity through the phase transition is observed. It is shown that the heat capacity and thermal expansion coefficient of the helical phase are not influenced by moderate magnetic field.

cond-mat.str-el↗

arXiv2007

Anomalous phonon behavior in the high temperature shape memory alloy: TiPd:Cr

Ti50 Pd50-xCrx is a high temperature shape memory alloy with a martensitic transformation temperature strongly dependent on the Cr composition. Prior to the transformation a premartensitic phase is present with an incommensurate modulated cubic lattice with wave vector of q0=(0.22, 0.22, 0). The temperature dependence of the diffuse scattering in the cubic phase is measured as a function temperature for x=6.5, 8.5, and 10 at. %. The lattice dynamics has been studied and reveals anomalous temperature and q-dependence of the [110]-TA2 transverse phonon branch. The phonon linewidth is broad over the entire Brillouin zone and increases with decreasing temperature, contrary to the behavior expected for anharmonicity. No anomaly is observed at q0. The results are compared with first principles calculation of the phonon structure.

cond-mat.mtrl-sci↗

arXiv2007

On the nature of the phase transition in the itinerant helimagnet MnSi

A careful study of thermodynamic and transport properties of a high quality single crystal of MnSi at ambient pressure suggests that its transition to a helical magnetic state near 29 K is weakly first order. The heat capacity, temperature derivatives of resistivity, thermal expansion and magnetic susceptibility exhibit a specific structure around the phase transition point, interpreted as a combination of first and second order features. Conclusions drawn from these experiments question prevailing views on the phase diagram of MnSi and propose that the phase transition under study becomes second order at high pressure and low temperature.

cond-mat.str-el↗

arXiv2006

Temperature and magnetic field dependences of the elastic constants of Ni-Mn-Al magnetic Heusler alloys

We report on measurements of the adiabatic second order elastic constants of the off-stoichiometric Ni$_{54}$Mn$_{23}$Al$_{23}$ single crystalline Heusler alloy. The variation in the temperature dependence of the elastic constants has been investigated across the magnetic transition and over a broad temperature range. Anomalies in the temperature behaviour of the elastic constants have been found in the vicinity of the magnetic phase transition. Measurements under applied magnetic field, both isothermal and variable temperature, show that the value of the elastic constants depends on magnetic order, thus giving evidence for magnetoelastic coupling in this alloy system.

cond-mat.mtrl-sci↗

arXiv2006

Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys

Inelastic and elastic neutron scattering have been used to study a single crystal of the Ni$_{54}$Mn$_{23}$Al$_{23}$ Heusler alloy over a broad temperature range. The paper reports the first experimental determination of the low-lying phonon dispersion curves for this alloy system. We find that the frequencies of the TA$_2$ modes are relatively low. This branch exhibits an anomaly (dip) at a wave number $ξ_{0} ={1/3}\approx 0.33$, which softens with decreasing temperature. Associated with this anomalous dip at $ξ_{0}$, an elastic central peak scattering is also present. We have also observed satellites due to the magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2003

Formation of a quasiperiodic copper thin film

We have synthesised a thin film of copper with a quasi-periodic structure by the adsorption of copper atoms on the five-fold surface of the icosahedral quasicrystal Al-Pd-Mn at room temperature. The quasi-periodicity of the thin film is manifested in low energy electronic diffraction (LEED) measurements and in the existence of Fibonacci relationships between rows of copper atoms imaged using scanning tunneling microscopy (STM). These findings demonstrate the feasibility of single-element quasi-periodic thin film formation using quasicrystals as templates.

cond-mat.mtrl-sci↗

arXiv2002

Bulk Termination of the Quasicrystalline Five-Fold Surface of Al_{70}Pd_{21}Mn_{9}

The structure of the Al_{70}Pd_{21}Mn_{9} surface has been investigated using high resolution scanning tunnelling microscopy (STM). From two large five-fold terraces on the surface in a short decorated Fibonacci sequence, atomically resolved surface images have been obtained. One of these terraces carries a rare local configuration in a form of a ring. The location of the corresponding sequence of terminations in the bulk model M of icosahedral i-AlPdMn based on the three-dimensional tiling T*(2F) of an F-phase has been estimated using this ring configuration and the requirement from the LEED work of Gierer et al. that the average atomic density of the terminations is 0.136 atoms per A^2. A termination contains two atomic plane layers separated by a vertical distance of 0.48 A. The position of the bulk terminations is fixed within the layers of Bergman polytopes in the model M: they are 4.08 A in the direction of the bulk from a surface of the most dense Bergman layers. From the coding windows of the top planes in terminations in M we conclude that a Penrose (P1) tiling is possible on almost all five-fold terraces. The shortest edge of the tiling P1, is either 4.8 A or 7.8 A. The experimentally derived tiling of the surface with the ring configuration has an edge-length of 8.0 +- 0.3 A and hence matches the minimal edge-length expected from the model.

cond-mat.mtrl-sci↗

arXiv2001

Formation of a stable deacagonal quasicrystalline Al-Pd-Mn surface layer

We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn quasicrystal overlayer on the 5-fold symmetric surface of an icosahedral Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by x-ray photoelectron diffraction, low-energy electron diffraction and electron backscatter diffraction. This overlayer is also characterized by a reduced density of states near the Fermi edge as expected for quasicrystals. This is the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn is obtained, studied and compared to its icosahedral counterpart.

cond-mat.dis-nn↗

arXiv2001

Tiling of the five-fold surface of Al(70)Pd(21)Mn(9)

The nature of the five-fold surface of Al(70)Pd(21)Mn(9) has been investigated using scanning tunneling microscopy. From high resolution images of the terraces, a tiling of the surface has been constructed using pentagonal prototiles. This tiling matches the bulk model of Boudard et. al. (J. Phys.: Cond. Matter 4, 10149, (1992)), which allows us to elucidate the atomic nature of the surface. Furthermore, it is consistent with a Penrose tiling T^*((P1)r) obtained from the geometric model based on the three-dimensional tiling T^*(2F). The results provide direct confirmation that the five-fold surface of i-Al-Pd-Mn is a termination of the bulk structure.

cond-mat.mtrl-sci↗