Magneto-elastic coupling and competing entropy changes in substituted CoMnSi metamagnets

We use neutron diffraction, magnetometry and low temperature heat capacity to probe giant magneto-elastic coupling in CoMnSi-based antiferromagnets and to establish the origin of the entropy change that occurs at the metamagnetic transition in such compounds. We find a large difference between the electronic density of states of the antiferromagnetic and high magnetisation states. The magnetic field-induced entropy change is composed of this contribution and a significant counteracting lattice component, deduced from the presence of negative magnetostriction. In calculating the electronic entropy change, we note the importance of using an accurate model of the electronic density of states, which here varies rapidly close to the Fermi energy.Comment: 11 pages, 9 figures. Figures 4 and 6 were updated in v2 of this preprint. In v3, figures 1 and 2 have been updated, while Table II and the abstract have been extended. In v4, Table I has updated with relevant neutron diffraction dat

Phase diagram and magnetocaloric effect of CoMnGe_{1-x}Sn_{x} alloys

We propose the phase diagram of a new pseudo-ternary compound, CoMnGe_{1-x}Sn_{x}, in the range x less than or equal to 0.1. Our phase diagram is a result of magnetic and calometric measurements. We demonstrate the appearance of a hysteretic magnetostructural phase transition in the range x=0.04 to x=0.055, similar to that observed in CoMnGe under hydrostatic pressure. From magnetisation measurements, we show that the isothermal entropy change associated with the magnetostructural transition can be as high as 4.5 J/(K kg) in a field of 1 Tesla. However, the large thermal hysteresis in this transition (~20 K) will limit its straightforward use in a magnetocaloric device.Comment: 12 pages, 5 figure

Negative magnetocaloric effect from highly sensitive metamagnetism in CoMnSi_{1-x}Ge_{x}

We report a novel negative magnetocaloric effect in CoMnSi_{1-x}Ge_{x} arising from a metamagnetic magnetoelastic transition. The effect is of relevance to magnetic refrigeration over a wide range of temperature, including room temperature. In addition we report a very high shift in the metamagnetic transition temperature with applied magnetic field. This is driven by competition between antiferromagnetic and ferromagnetic order which can be readily tuned by applied pressure and compositional changes.Comment: 5 pages, 5 figures, REVTeX, submitted to Physical Revie

Transition to Mach reflexion of shock waves in steady and pseudosteady flow with and without relaxation

Experiments were conducted in the free-piston shock tube and shock tunnel with dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon to measure the transition condition in pseudosteady and steady flow. The transition condition in the steady flow, in which the wall was eliminated by symmetry, agrees with the calculated von Neumann condition. In the real gases this calculation assumed thermo-dynamic equilibrium after the reflected shock. In the pseudosteady flow of reflexion from a wedge the measured transition angle lies on the Mach-reflexion side of the calculated detachment condition by an amount which may be explained in terms of the displacement effect of the boundary layer on the wedge surface. A single criterion based on the availability of a length scale at the reflexion point explains the difference between the pseudosteady and steady flow transition condition and predicts a hysteresis effect in the transition angle when the shock angle is varied during steady flow. No significant effects on the transition condition due to finite relaxation length could be detected. However, new experiments in which interesting relaxation effects should be evident are suggested

Electrical stimulation treatment for facial palsy after revision pleomorphic adenoma surgery.

Surgery for pleomorphic adenoma recurrence presents a significant risk of facial nerve damage that can result in facial weakness effecting patients' ability to communicate, mental health and self-image. We report two case studies that had marked facial weakness after resection of recurrent pleomorphic adenoma and their progress with electrical stimulation. Subjects received electrical stimulation twice daily for 24 weeks during which photographs of expressions, facial measurements and Sunnybrook scores were recorded. Both subjects recovered good facial function demonstrating Sunnybrook scores of 54 and 64 that improved to 88 and 96, respectively. Neither subjects demonstrated adverse effects of treatment. We conclude that electrical stimulation is a safe treatment and may improve facial palsy in patients after resection of recurrent pleomorphic adenoma. Larger studies would be difficult to pursue due to the low incidence of cases

Impurity and strain effects on the magnetotransport of La1.85Sr0.15Cu(1-y)Zn(y)O4 films

The influence of zinc doping and strain related effects on the normal state transport properties(the resistivity, the Hall angle and the orbital magneto- resistance(OMR) is studied in a series of La1.85Sr0.15Cu(1-y)Zn(y)O4 films with values of y between 0 and 0.12 and various degrees of strain induced by the mismatch between the films and the substrate. The zinc doping affects only the constant term in the temperature dependence of cotangent theta but the strain affects both the slope and the constant term, while their ratio remains constant.OMR is decreased by zinc doping but is unaffected by strain. The ratio delta rho/(rho*tan^2 theta) is T-independent but decreases with impurity doping. These results put strong constraints on theories of the normal state of high- temperature superconductors

Quantum and Topological Criticalities of Lifshitz Transition in Two-Dimensional Correlated Electron Systems

We study electron correlation effects on quantum criticalities of Lifshitz transitions at zero temperature, using the mean-field theory based on a preexisting symmetry-broken order, in two-dimensional systems. In the presence of interactions, Lifshitz transitions may become discontinuous in contrast to the continuous transition in the original proposal by Lifshitz for noninteracting systems. We focus on the quantum criticality at the endpoint of discontinuous Lifshitz transitions, which we call the marginal quantum critical point. It shows remarkable criticalities arising from its nature as a topological transition. At the point, for the canonical ensemble, the susceptibility of the order parameter chi is found to diverge as ln 1/|delta Delta| when the ``neck'' of the Fermi surface collapses at the van Hove singularity. More remarkably, it diverges as 1/|delta Delta| when the electron/hole pocket of the Fermi surface vanishes. Here delta Delta is the amplitude of the mean field measured from the Lifshitz critical point. On the other hand, for the grand canonical ensemble, the discontinuous transitions appear as the electronic phase separation and the endpoint of the phase separation is the marginal quantum critical point. Especially, when a pocket of the Fermi surface vanishes, the uniform charge compressibility kappa diverges as 1/|delta n|, instead of chi, where delta n is the electron density measured from the critical point. Accordingly, Lifshitz transition induces large fluctuations represented by diverging chi and/or kappa. Such fluctuations must be involved in physics of competing orders and influence diversity of strong correlation effects.Comment: 16 pages, 15 figures, to appear in Jounal of the Physical Society of Japa

Split transition in ferromagnetic superconductors

The split superconducting transition of up-spin and down-spin electrons on the background of ferromagnetism is studied within the framework of a recent model that describes the coexistence of ferromagnetism and superconductivity induced by magnetic fluctuations. It is shown that one generically expects the two transitions to be close to one another. This conclusion is discussed in relation to experimental results on URhGe. It is also shown that the magnetic Goldstone modes acquire an interesting structure in the superconducting phase, which can be used as an experimental tool to probe the origin of the superconductivity.Comment: REVTeX4, 15 pp, 7 eps fig

Observational Limit on Gravitational Waves from Binary Neutron Stars in the Galaxy

Using optimal matched filtering, we search 25 hours of data from the LIGO 40-meter prototype laser interferometric gravitational-wave detector for gravitational-wave chirps emitted by coalescing binary systems within our Galaxy. This is the first test of this filtering technique on real interferometric data. An upper limit on the rate R of neutron star binary inspirals in our Galaxy is obtained: with 90% confidence, R< 0.5/hour. Similar experiments with LIGO interferometers will provide constraints on the population of tight binary neutron star systems in the Universe.Comment: RevTeX, minor revisions, exactly as published in PRL 83 (1999) p1498, 4 pages, 2 figures include

Heat-capacity anomalies at TscT_{sc} and T∗T^{*} in the ferromagnetic superconductor UGe2_2

The heat-capacity and magnetization measurements under high pressure have been carried out in a ferromagnetic superconductor UGe$_2$. Both measurements were done using a same pressure cell in order to obtain both data for one pressure. Contrary to the heat capacity at ambient pressure, an anomaly is found in the heat capacity at the characteristic temperature $T^{*}$ where the magnetization shows an anomalous enhancement under high pressure where the superconductivity appears. This suggests that a thermodynamic phase transition takes place at $T^{*}$ at least under high pressure slightly below $P_{c}^{*}$ where $T^{*}$ becomes zero. The heat-capacity anomaly associated with the superconducting transition is also investigated, where a clear peak of $C/T$ is observed in a narrow pressure region ($\Delta P \sim 0.1$ GPa) around $P_{c}^{*}$ contrary to the previous results of the resistivity measurement. Present results suggest the importance of the thermodynamic critical point $P_{c}^{*}$ for the appearance of the superconductivity.Comment: 4 pages, 4 figures, to appear in Phys. Rev. B, Rapid Communication