arXiv subjects

M. K. Haas

Publications and source records attributed to M. K. Haas.

13 recordsLinked to original sources

arXiv2016

Sample independence of magnetoelastic excitations in the rare earth pyrochlore Tb2Ti2O7

Recent experimental results have emphasized two aspects of Tb2Ti2O7 which have not been taken into account in previous attempts to construct theories of Tb2Ti2O7: the role of small levels of structural disorder, which appears to control the formation of a long-range ordered state of as yet unknown nature; and the importance of strong coupling between spin and lattice degrees of freedom, which results in the hybridization of crystal field excitons and transverse acoustic phonons. In this work we examine the juncture of these two phenomena and show that samples with strongly contrasting behavior vis-a-vis the structural disorder (i.e. with and without the transition to the ordered state), develop identical magnetoelastic coupling. We also show that the comparison between single crystal and powder samples is more complicated than previously thought - the correlation between lattice parameter (as a measure of superstoichiometric Tb$^{3+}$) and the existence of a specific heat peak, as observed in powder samples, does not hold for single crystals.

cond-mat.str-el↗

arXiv2014

Magnetoelastic excitations in the pyrochlore spin liquid Tb$_2$Ti$_2$O$_7$

At low temperatures, Tb$_2$Ti$_2$O$_7$ enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb$_2$Ti$_2$O$_7$ is aptly named a "magnetoelastic spin liquid" and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb$_2$Ti$_2$O$_7$ can now be regarded as a Coulomb phase with propagating bosonic spin excitations.

cond-mat.str-el↗

arXiv2002

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.

cond-mat↗

arXiv2002

Structure and Superconductivity in Zr-Stabilized, Nonstoichiometric Molybdenum Diboride

The structure and physical properties of the Zr-stabilized, nonstoichiometric molybdenum diboride superconductor are reported. Good quality material of the diboride structure type can only be obtained by partial substitution of Zr for Mo, and the quenching of melts. The phase is best made with boron in excess of the ideal 2:1 boron to metal ratio. Powder neutron diffraction measurements show that the non-stoichiometry is accommodated by atom deficiency in the metal layers. The diboride structure type exists for (Mo.96Zr.04)xB2 for x between 0.85 and 1.0. Electron diffraction shows that the stoichiometric material, x=1, has a significant number of stacking faults. Tc increases from 5.9 to 8.2K with the introduction of metal vacancies. Resistivity measurements indicate that (Mo.96Zr.04).88B2 is a bad metal, and specific heat measurements show that gamma= 4.4 mJ/mol K2, and that deltaC/gammaTc = 1.19. Preliminary boron isotope effect measurements indicate an exponent 0.11(5). Analysis of the data in terms of the electronic structure is reported, allowing an estimate of the electron-phonon coupling constant, lamda = 0.1-0.3, making these weak-coupling superconductors. Preliminary characterizations of the superconductivity in the related phases NbxB2 and (Mo.96X.04).85B2 for X=Ti, and Hf are reported.

cond-mat.supr-con↗

arXiv2002

Robust paramagnetism in Bi2-xMxRu2O7 (M=Mn,Fe,Co,Ni,Cu) pyrochlore

We report physical property characterization of Bi2-xMxRu2O7 pyrochlores, including magnetic suseptibility, resistivity, and Seebeck coefficients. The solid solution exists up to x=0.5 for (M=Cu,Ni,Co) and up to x=0.1 for (M=Fe,Mn). None of the doped materials exhibit ferromagnetism or any localized ruthenium moment behavior. Instead we find the Ru-O and Bi-O sublattices to be essentially independent, with any magnetism resulting from the unpaired transition metal dopant spins. Cobalt substitution for bismuth results in localized Co{2+}, and low temperature spin-glass transitions in several cases. Nickel moments on the pyrochlore lattice display properties intermediate to localized and itinerant. Finally, copper doping results in only an enhancement of the Pauli metallic density of states.

cond-mat.supr-con↗

arXiv2001

The complex superstructure in Mg1-xAlxB2 at x~0.5

Electron diffraction and high resolution microscopy have been performed on Mg1-xAlxB2 with x~0.5. This composition displays a superstructure with a repeat period of exactly 2c along the c axis and about 10 nm in the a-b plane. The superstructure results in ring-shaped superreflections in the diffraction pattern. Irradiation by a strong electron beam results in a loss of the superstructure and a decrease of about 1% in the c lattice parameter. In-situ heating and cooling on the other hand showed that the superstructure is stable from 100 K to 700 K. Possible origins for the superstructure are proposed.

cond-mat↗

arXiv2001

Temperature dependence of the structural parameters of the non-oxide perovskite superconductor MgCNi3

We report the structural parameters of superconducting MgCxNi3 (x=0.96, TC=7.3 K) as a function of temperature, from 2 K to 295 K, determined by neutron powder diffraction profile refinement. The compound has the perovskite structure over the whole temperature range, with symmetry Pm3m and a=3.81221(5) A at 295 K: no structural or long range magnetic ordering transitions were observed. The lattice parameter a and the Debye-Waller factors for the individual atoms decrease smoothly with decreasing temperature. There are no unusual changes of the structural parameters near TC.

cond-mat.supr-con↗

arXiv2001

The suppression of superconductivity in MgCNi3 by Ni-site doping

The effects of partial substitution of Cu and Co for Ni in the intermetallic perovskite superconductor MgCNi3 are reported. Calculation of the expected electronic density of states suggests that electron (Cu) and hole (Co) doping should have different effects. For MgCNi3-xCux, solubility of Cu is limited to approximately 3% (x = 0.1), and Tc decreases systematically from 7K to 6K. For MgCNi3-xCox, solubility of Co is much more extensive, but bulk superconductivity disappears for Co doping of 1% (x = 0.03). No signature of long range magnetic ordering is observed in the magnetic susceptibility of the Co doped material.

cond-mat.supr-con↗

arXiv2001

Giant anharmonicity and non-linear electron-phonon coupling in MgB$_{2}$; 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 $σ$ 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$_{2}$

cond-mat.mtrl-sci↗

arXiv2001

Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. With twice the critical temperature of Nb_3Sn and four times that of Nb-Ti alloy, MgB_2 has the potential to reach much higher fields and current densities than either of these technological superconductors. A vital prerequisite, strongly linked current flow, has already been demonstrated even at this early stage. One possible drawback is the observation that the field at which superconductivity is destroyed is modest. Further, the field which limits the range of practical applications, the irreversibility field H*(T), is ~7 T at liquid helium temperature (4.2 K), significantly lower than ~10 T for Nb-Ti and ~20 T for Nb_3Sn. Here we show that MgB_2 thin films can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding H*(4.2 K) above 14 T. In addition, very high critical current densities at 4.2 K, 1 MA/cm_2 at 1 T and 10_5 A/cm_2 at 10 T, are possible. These data demonstrate that MgB_2 has credible potential for high-field superconducting applications.

cond-mat.supr-con↗

arXiv2001

Superconductivity in the non-oxide Perovskite MgCNi3

The oxide perovskites are a large family of materials with many important physical properties. Of particular interest has been the fact that this structure type provides an excellent structural framework for the existence of superconductivity. The high Tc copper oxides are the most famous examples of superconducting perovskites, but there are many others [1]. Intermetallic compounds have been the source of many superconducting materials in the past, but they have been eclipsed in recent years by the perovskite oxides. The recent discovery of superconductivity in MgB2 [2] suggests that intermetallic compounds with simple structure types are worth serious reconsideration as sources of new superconducting materials. Here we report the observation of superconductivity at 8 K in the perovskite structure intermetallic compound MgCNi3, linking what appear at first sight to be mutually exclusive classes of superconducting materials. The observation of superconductivity in MgCNi3 indicates that MgB2 will not be the only one of its kind within the chemical paradigm that it suggests for new superconducting materials.

cond-mat.supr-con↗

arXiv2001

Loss of superconductivity and structural transition in Mg1-xAlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. Therefore the recent announcement of superconductivity at 39 K in the simple binary ceramic compound MgB2 is surprising. This compound, available from common chemical suppliers, and used as a starting material for chemical metathesis reactions, has been known and structurally characterized since the mid 1950's. Here we show that the addition of electrons to MgB2 through partial substitution of Al for Mg results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near 10% Al content. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

cond-mat.supr-con↗

arXiv2001

Electrical Transport Properties of Single Crystal Sr3Ru2O7: The Possible Existence of an Antiferromagnetic Instability at Low Temperatures

We report the results of Hall coefficient R_H and magnetoresistance (MR) measurements on single crystalline samples of Sr3Ru2O7 grown by the floating zone method. R_H was found to be positive over the entire temperature range studied (0.3 - 300K). Its temperature (T) dependence follows closely that of the magnetic susceptibility, including a maximum at a characteristic temperature T=17 K. We show that R_H can be decomposed into normal and anomalous parts as in the case of skew scattering in heavy-fermion compounds and ferromagnetic metals. This, together with the observation that the longitudinal MR is greater than the transverse MR at the same magnetic field and temperature, suggests that magnetic fluctuations dominate the electrical transport properties in Sr3Ru2O7. We found a crossover in the sign of the MR at T*, from positive to negative as the temperature increased, for both the transverse and the longitudinal configurations. In addition, a non-monotonic behavior in the field dependence of the MR was found at low temperatures. These observations suggest that the magnetic correlations in Sr3Ru2O7 at ambient pressure undergo a qualitative change as the temperature is lowered. Above T*, they are dominated by ferromagnetic instability. However, below T*, the system crosses over to a different behavior, controlled possibly by a canted antiferromagnetic instability.

cond-mat.str-el↗