Non-Fermi-liquid behavior in Ce(Ru1‑xFex)2Ge2 : Cause and effect

We present inelastic neutron scattering measurements on the intermetallic compounds Ce(Ru1−xFex)2Ge2 (x=0.65, 0.76, and 0.87). These compounds represent samples in a magnetically ordered phase, at a quantum critical point, and in the heavy-fermion phase, respectively. We show that at high temperatures the three compositions have the identical response of a local moment system. However, at low temperatures the spin fluctuations in the critical composition are given by non-Fermi-liquid dynamics, while the spin fluctuations in the heavy-fermion system show a simple exponential decay in time. In both compositions, the lifetime of the fluctuations is determined solely by the distance to the quantum critical point. We discuss the implications of these observations regarding the possible origins of non-Fermi-liquid behavior in this system.Work at the University of Missouri was supported by Missouri University research board Grant No. RB-03-081

The ground state of a quantum critical system

The competition between the tendency of magnetic moments to order at low temperatures, and the tendency of conduction electrons to shield these moments, can result in a phase transition that takes place at zero Kelvin, the quantum critical point (QCP). So far, the ground state of these types of systems has remained unresolved. We present neutron scattering experiments that show that the ground state of a sample representative of a class of QCP-systems is determined by the residual interactions between the conduction electrons, resulting in a state with incommensurate intermediate-range order. However, long-range order is thwarted by quantum fluctuations that locally destroy magnetic moments, leaving the system with too few moments to achieve long-range order

A comparison of the hidden order transition in URu2_2Si2_2 to the λ\lambda-transition in 4^4He

The low-temperature states of ambient URu$_2$Si$_2$ and superfluid $^4$He are both characterized by momentum-dependent energy gaps between the ground and excited states. This behavior weakly persists even above the transitions temperatures but becomes over-damped because of the number of excitations present at elevated temperature. We show that akin to the normal fluid to superfluid transition in $^4$He, the hidden-order (HO) transition in URu$_2$Si$_2$ can be understood by a change of the gapped transitions to elementary excitations (EE) of the unknown ordered state. These underdamped EEs reflect the basic character and order parameters of the different phase transitions. This view accounts for the full amount of entropy released in these transitions, the jumps in the resistivity and thermal conductivity directly below the transition, as well as the reduction of the Fermi surface. We argue that the behavior in the HO phase is that of a gas of weakly interacting excitations from charge density wave or crystal field states in a similar manner to that of the phonon-roton excitations of the superfluid $^4$He phase. We discuss the influence of applying pressure and magnetic fields within this scenario and the role of the small moment antiferromagnetic clustering in the hidden order phase.Comment: 14 pages, 12 figures, to be submitted to PR

The dynamics of superfluid 4He

We present neutron scattering results for the dynamic response by superfluid and normal-fluid 4He and the results of a simple perturbation-theory analysis which allows us to describe all aspects of the observed behavior by considering only density fluctuations. We show that the three key features that are characteristic of the dynamic reponse of superfluid 4He, as well as their dramatic variations with temperature, can be attributed predominantly to the Bose statistics obeyed by all the 4He atoms. It appears that the presence of the Bose condensate exerts at most a minor influence on the dynamic response.Comment: 12 pages, 4 figures. Material closely related to W. Montfrooij and E.C. Svensson, Journal of Low Temperature Physics, Vol. 121, p. 293-302 (2000

Teaching superfluidity at the introductory level

Standard introductory modern physics textbooks do not exactly dwell on superfluidity in 4He. Typically, Bose-Einstein condensation (BEC) is mentioned in the context of an ideal Bose gas, followed by the statement that BEC happens in 4He and that the ground state of 4He exhibits many interesting properties such as having zero viscosity. Not only does this approach not explain in any way why 4He becomes a superfluid, it denies students the opportunity to learn about the far reaching consequences of energy gaps as they develop in both superfluids and superconductors. We revisit superfluid 4He by starting with Feynman's explanation of superfluidity based on Bose statistics as opposed to BEC, and we present exercises for the students that allow them to arrive at a very accurate estimate of the superfluid transition temperature and of the energy gap separating the ground state from the first excited state. This paper represents a self-contained account of superfluidity, which can be covered in one or two lessons in class.Comment: This paper was written to compensate for the lack of any useful treatment of superfluidity in standard (introduction) modern physics textbooks. The paper contains some interesting estimates that might be of interest to people involved in superfluid researc

Flux growth of LixM2O4 spinel for use in testing the Quantum Critical Point (QCP) [abstract]

Abstract only availableFaculty Mentor: Keary Schoen, Research ReactorLixM2O4 is a crystal known as a spinel that has the ability to have lithium removed without affecting the structure. By varying the amount of lithium in the substance, the chemical pressure inside the crystal can be changed. This can change the ground state, which is instrumental in testing the quantum critical point (QCP). QCP is the lowest temperature point at which there a change in existence in a material, which typically occurs near absolute zero. Also, the trivalent variable element (M), can be changed to alter the nature near the QCP. These changes are observable using neutron diffraction. However, all of the current LixM2O4 spinels have existed in minute particle sizes, which is unusable in neutron diffraction. To create sufficiently sized crystals, a flux growth method is used. This process uses salts at very high temperatures (~1000K), which dissociate and form the spinel and byproducts (2 LiCl + 4 M(NO3)2 = 2 LiM2O4 + 8 NO2 + Cl2) By varying the time and intensity of the heating process along with the length of cooling, the production of large crystals has been optimized. The introduction of a previous crystal will induce further growth, and is known as seeding. Best results have been formed using manganese, with seeding. Further optimization in crystal size has been shown by increasing the duration of elevated temperature and by slowing the rate of cooling. Other forms of crystal using chromium and cerium have proven extremely difficult to create in significant quantities or size. Using neutron powder diffraction and x-ray powder diffraction, the structure of the created spinel has been confirmed to be that of the desired LixM2O4 crystal