Author
Vivien Zapf
Other affiliations: University of California, San Diego, California Institute of Technology
Bio: Vivien Zapf is an academic researcher from Los Alamos National Laboratory. The author has contributed to research in topics: Magnetization & Magnetic field. The author has an hindex of 32, co-authored 168 publications receiving 4307 citations. Previous affiliations of Vivien Zapf include University of California, San Diego & California Institute of Technology.
Papers published on a yearly basis
Papers
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TL;DR: In this article, it was shown that superconductivity has been observed in the heavy fermion quasiparticles with an effective mass of approximately 50 µm/e.
Abstract: Superconductivity has been observed in ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ at ${T}_{C}=1.85 \mathrm{K}$ and appears to involve heavy fermion quasiparticles with an effective mass ${m}^{*}\ensuremath{\sim}50 {m}_{e}$ as inferred from the jump in the specific heat at ${T}_{C},$ the upper critical field near ${T}_{C},$ and the normal state electronic specific heat. Thermodynamic and transport measurements suggest that the heavy fermion state has a quadrupolar origin, although a magnetic origin cannot be completely ruled out.
589 citations
TL;DR: In this article, a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations was performed, and the authors found evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, in contrast with the currently assumed trigonal three-layer stacking periodicity.
Abstract: The layered honeycomb magnet $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled ${j}_{\mathrm{eff}}=\frac{1}{2}\phantom{\rule{4.pt}{0ex}}{\mathrm{Ru}}^{3+}$ magnetic moments. Here, we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently assumed trigonal three-layer stacking periodicity. We report electronic band-structure calculations for the monoclinic structure, which find support for the applicability of the ${j}_{\mathrm{eff}}=\frac{1}{2}$ picture once spin-orbit coupling and electron correlations are included. Of the three nearest-neighbor Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the $b$ axis nonequivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the previously reported spin gap in powder inelastic neutron scattering measurements, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction, show a single magnetic transition upon cooling below ${T}_{\mathrm{N}}\ensuremath{\approx}13$ K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.
432 citations
249 citations
TL;DR: The results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.
Abstract: It has recently been suggested that the organic compound ${\mathrm{NiCl}}_{2}\mathrm{\text{\ensuremath{-}}}4\mathrm{SC}({\mathrm{NH}}_{2}{)}_{2}$ (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni $S=1$ spins exhibit three-dimensional $XY$ antiferromagnetism above a critical field ${H}_{c1}\ensuremath{\sim}2\text{ }\text{ }\mathrm{T}$. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law $H\ensuremath{-}{H}_{c1}\ensuremath{\propto}{T}_{c}^{\ensuremath{\alpha}}$ near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of $\ensuremath{\alpha}=1.5$.
164 citations
TL;DR: The data are consistent with an inverse-DM mechanism for multiferroic behavior, as the electric polarization exhibits an unusual memory effect, as it remembers the direction of the previous two magnetic-field pulses applied.
Abstract: We present the first example of magnetic ordering-induced multiferroic behavior in a metal–organic framework magnet. This compound is [CH3NH3][Co(HCOO)3] with a perovskite-like structure. The A-site [CH3NH3]+ cation strongly distorts the framework, allowing anisotropic magnetic and electric behavior and coupling between them to occur. This material is a spin canted antiferromagnet below 15.9 K with a weak ferromagnetic component attributable to Dzyaloshinskii–Moriya (DM) interactions and experiences a discontinuous hysteretic magnetic-field-induced switching along [010] and a more continuous hysteresis along [101]. Coupling between the magnetic and electric order is resolved when the field is applied along this [101]: a spin rearrangement occurs at a critical magnetic field in the ac plane that induces a change in the electric polarization along [101] and [10-1]. The electric polarization exhibits an unusual memory effect, as it remembers the direction of the previous two magnetic-field pulses applied. Th...
163 citations
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Journal Article•
28,685 citations
01 Sep 1955
TL;DR: In this paper, the authors restrict their attention to the ferrites and a few other closely related materials, which are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present.
Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.
2,659 citations