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We describe a minimal model, based on a spin only Hamiltonian with a single energy scale for itinerant electron metamagnetism. Within this model the metamagnetic critical field is directly proportional to the temperature where a peak in the linear susceptibility occurs which in turn is related in a simple manner to the temperature where the nonlinear susceptibilities also peak. The spin dependent thermodynamic properties are derived in a straightforward manner and bear a striking resemblance to observations in such strongly correlated systems as heavy fermion materials. We also consider extensions of the model by including effects such as a mean field to encompass observed deviations from a minimal metamagnetic behavior.

We report here low temperature magnetization isotherms for the single molecule magnet, $(UO_2-L)_3$. By analyzing the low temperature magnetization in terms of $M= X_1*B + X_3*B^3$ we extract the linear susceptibility $X_1$ and the leading order nonlinear susceptibility $X_3$. We find that $X_1$ exhibits a peak at a temperature of $T_1=10.4 K$ with $Chi_3$ also exhibiting a peak but at a reduced temperature $T3 = 5 K$. At the lowest temperatures the isotherms exhibit a critical field $B_c = 11.5 T$ marked by a clear point of inflection. A minimal Hamiltonian employing S=1 (pseudo) spins with only a single energy scale (successfully used to model the behavior of bulk f-electron metamagnets) is shown to provide a good description of the observed linear scaling between $T_1, T_3$ and $B_c$. We further show that a Heisenberg Hamiltonian previously employed by Carretta et al. (2013 J. Phys.Cond. Matt. 25 486001) to model this single molecule magnet gives formulas for the angle averaged susceptibilities (in the Ising limit) very similar to those of the minimal model.

For a decade and a half, CeCoIn$_5$ and related alloys have served as a rich playground to explore the interplay between magnetism and unconventional superconductivity. Despite this extended study, the presence/absence of metamagnetism (MM) in this ternary system remains as an unresolved issue. Here we show that the linear and non-linear magnetic response in CeMIn$_5$ (M = Rh, Ir and Co) can be understood within the context of the recently proposed single energy scale (SES) model of MM. New measurements of the third-order susceptibility, $\chi_3$, in CeCoIn$_5$ are presented and together with the known systematics of the linear susceptibility in all three compounds, are shown to be consistent with the SES model. Predictions are made for the MM critical field in CeCoIn$_5$ and the fifth-order susceptibility, $\chi_5$.

We report longitudinal ultrasound velocity measurements for magnetic fields up to 33 T applied parallel to the a-axis of the heavy electron compound UPt$_{3}$. A characteristic dip in the sound velocity at the metamagnetic critical field, $H_{c}=20$ T, reported in earlier work is reproduced and shown to be independent of temperature at very low temperatures. We show that the single energy scale model (B.S. Shivaram et al., Phys. Rev. B89, 241107(R), 2014) captures the observed key features of the field dependence in the sound velocity shift, $\delta v_{s}$. The shift $\delta v_{s}$ at $H_{c}$ is found to be inversely dependent on temperature above 3\thinspace K and assumes a fixed value at low T. This saturation in $\delta v_{s}$ below 3 K is accounted for by level broadening of the Uranium spin states.

We present numerical simulations of a theory of resonant processes in a frozen gas of excited atoms interacting via dipole-dipole potentials that vary as $r^{-3}$, where $r$ is the interatomic separation. The simulations calculate time-dependent averages of transition amplitudes and transition probabilities for a single atom in a given state interacting resonantly with a uniformly distributed random gas of atoms in a different state. The averages are over spatial configurations of the gas atoms, which are held fixed while the resonant interaction creates a Frenkel exciton that can travel from atom to atom. We check that the simulations reproduce previously known exact results when the exciton is not allowed to propagate [Phys. Rev. A 59, 4358 (1999)]. Further, we develop an approximation for the average transition amplitude that compares well with the numerical results for a wide range of values of the system parameters.

We present the theory for Multiple Energy X-ray Holography (MEXH), using a multipole expansion for the scattered field. We find that light polarization plays a crucial role in the reconstruction of the image, and we suggest how to use it in order to eliminate aberration effects. The method we propose is alternative to the SWIFT method (scattered-wave--included Fourier transform), but has the advantage that no theoretical calculations are required to redefine the hologram., Comment: Revtex 17 pages, 4epsi files, 5figures

Laser experiments with optically excited frozen gases entail the excitation of polarization waves. In a continuum approximation the waves are dispersionless, but their frequency depends on the direction of the propagation vector. An outline is given of the theory of transient phenomena that involve the excitation of these waves by a resonant dipole-dipole transfer process., Comment: 12 pages including 1 figure

We present a theory of resonant processes in a frozen gas of atoms interacting via dipole-dipole potentials that vary as $r^{-3}$, where $r$ is the interatomic separation. We supply an exact result for a single atom in a given state interacting resonantly with a random gas of atoms in a different state. The time development of the transition process is calculated both on- and off-resonance, and the linewidth with respect to detuning is obtained as a function of time $t$. We introduce a random spin Hamiltonian to model a dense system of resonators and show how it reduces to the previous model in the limit of a sparse system. We derive approximate equations for the average effective spin, and we use them to model the behavior seen in the experiments of Anderson et al. and Lowell et al. The approach to equilibrium is found to be proportional to $\exp (-\sqrt{\gamma_{eq}t}$), where the constant $\gamma _{eq}$ is explicitly related to the system's parameters., Comment: 30 pages, 6 figures