Top-loading Small-sample Calorimeters for Measurements as a Function of Magnetic Field Angle

In quasi-low-dimensional systems, the existence of a particular physical state and the temperature and magnetic-field-dependence of its phase boundary often strongly depends on magnetic field orientation. To investigate magnetic field orientation dependent phase transitions in these materials, we have developed rotatable miniature and sub-miniature sample-in-vacuum calorimeters that operate in dc magnetic fields up to 18 and 45 tesla. The calorimeters cover the temperature range from below 0.1 K to above 10 K; they are able rotate a full 360 degrees relative to the applied magnetic field while remaining at base temperature. Samples are typically ontheorderof1mginmassandupto2mm2 x0.5mminvolume

Calorimetric Measurements of Magnetic-Field-Induced Inhomogeneous Superconductivity Above The Paramagnetic Limit

We report the first magneto-caloric and calorimetric observations of a magnetic-field-induced phase transition within a superconducting state to the long-sought exotic "FFLO" superconducting state first predicted over 50 years ago. Through the combination of bulk thermodynamic calorimetric and magnetocaloric measurements in the organic superconductor $\kappa$ - (BEDT-TTF)$_2$Cu(NCS)$_2$, as a function of temperature, magnetic field strength, and magnetic field orientation, we establish for the first time that this field-induced first-order phase transition at the paramagnetic limit $H_p$ for traditional superconductivity is to a higher entropy superconducting phase uniquely characteristic of the FFLO state. We also establish that this high-field superconducting state displays the bulk paramagnetic ordering of spin domains required of the FFLO state. These results rule out the alternate possibility of spin-density wave (SDW) ordering in the high field superconducting phase. The phase diagram determined from our measurements --- including the observation of a phase transition into the FFLO phase at $H_p$ --- is in good agreement with recent NMR results and our own earlier tunnel-diode magnetic penetration depth experiments, but is in disagreement with the only previous calorimetric report.Comment: 5 pages, 5 figure

Calorimetric Measurements of Magnetic-Field-Induced Inhomogeneous Superconductivity Above The Paramagnetic Limit

We report the first magnetocaloric and calorimetric observations of a magnetic-field-induced phase transition within a superconducting state to the long-sought exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state, first predicted over 50 years ago. Through the combination of bulk thermodynamic calorimetric and magnetocaloric measurements in the organic superconductor κ−(BEDT−TTF)2Cu(NCS)2 as a function of temperature, magnetic field strength, and magnetic field orientation, we establish for the first time that this field-induced first-order phase transition at the paramagnetic limit Hp is a transition to a higher-entropy superconducting phase, uniquely characteristic of the FFLO state. We also establish that this high-field superconducting state displays the bulk paramagnetic ordering of spin domains required of the FFLO state. These results rule out the alternate possibility of spin-density wave ordering in the high-field superconducting phase. The phase diagram determined from our measurements—including the observation of a phase transition into the FFLO phase at Hp—is in good agreement with recent NMR results and our own earlier tunnel-diode magnetic penetration depth experiments but is in disagreement with the only previous calorimetric report

Responding to the U.S. Research Community\u27s Liquid Helium Crisis

An Action Plan to Preserve U.S. Innovation. This report lays out the issues facing researchers who use liquid helium and the negative impact on U.S. innovation. The report then proposes five key steps that will have a transformative effect on the ability to maintain the ready availability of helium and ensure the vibrancy of the U.S. low-temperature research capability. These recommendations focus on: conservation of helium use; a mechanism to pay for the capital investment required for helium recycling; a mechanism to ensure an appropriate price is paid by researchers for helium; and a methodology which allows researchers to best explore the options available to them. A SCIENCE POLICY REPORT ISSUED BY: American Physical Society, Materials Research Society, American Chemical Society Representing more than 200,000 scientists, engineers, and innovator worldwide. This report was overseen by the APS Panel on Public Affairs (POPA). POPA routinely produces reports on timely topics being debated in government so as to inform the debate with the perspectives of physicists working in the relevant issue areas

Evolution of magnetic field induced ordering in the layered quantum Heisenberg triangular-lattice antiferromagnet Ba\u3csub\u3e3\u3c/sub\u3e CoSb\u3csub\u3e2\u3c/sub\u3e O\u3csub\u3e9\u3c/sub\u3e

Quantum fluctuations in the effective spin- 1/2 layered triangular-lattice quantum Heisenberg antiferromagnet Ba3CoSb2O9 lift the classical degeneracy of the antiferromagnetic ground state in magnetic field, producing a series of novel spin structures for magnetic fields applied within the crystallographic ab plane, including a celebrated collinear “up-up-down” spin ordering with magnetization equal to 1/3 of the saturation magnetization over an extended field range. Theoretically unresolved, however, are the effects of interlayer antiferromagnetic coupling and transverse magnetic fields on the ground states of this system. Additional magnetic field induced phase transitions are theoretically expected and in some cases have been experimentally observed, but details regarding their number, location, and physical character appear inconsistent with the predictions of existing models. Conversely, an absence of experimental measurements as a function of magnetic-field orientation has left other key predictions of these models untested. To address these issues, we have used specific heat, neutron diffraction, thermal conductivity, and magnetic torque measurements to map out the phase diagram as a function of magnetic field intensity and orientation relative to the crystallographic ab plane. For H||ab, we have discovered an additional magnetic field induced phase transition at low temperature and an unexpected tetracritical point in the high-field phase diagram, which coupled with the apparent second-order nature of the phase transitions eliminates several theoretically proposed spin structures for the high-field phases. Our calorimetric measurements as a function of magnetic field orientation are in general agreement with theory for field-orientation angles close to plane parallel (H||a) but diverge at angles near plane perpendicular; a predicted convergence of two phase boundaries at finite angle and a corresponding change in the order of the field induced phase transition are not observed experimentally. Our results emphasize the role of interlayer coupling in selecting and stabilizing field induced phases, provide guidance on the nature of the magnetic order in each phase, and reveal the need for new physics to account for the nature of magnetic ordering in this archetypal two-dimensional spin- 1/2 triangular-lattice quantum Heisenberg antiferromagnet

Calorimetric Determination of the Angular Dependent Phase Diagram of an S = 1/2 Heisenberg Triangular-lattice Antiferromagnet

An antiferromagnetic system on a 2-D triangular lattice leads to geometric topological frustration. This ideal system has been the subject of theoretical investigations. One experimental realization of this system is the compound Cs2CuCl4. Various magnetization, heat capacity, neutron scattering and NMR studies have identified several magnetic transitions when the magnetic field is applied along one of the three principal axes. The current work investigates the evolution of these phases at intermediate angles as the crystal is rotated relative to the magnetic field. These phases were investigated using a novel rotating calorimeter allowing complete coverage of the experimental parameter space. New magnetic phases only existing at intermediate angles have been found

Magnetic-field-induced Heisenberg to XY Crossover in a Quasi-2D Quantum Antiferromagnet

The magnetic-field-dependent ordering temperature of the quasi-2D quantum Heisenberg antiferromagnet (QHAF) Cu(pz)2(ClO4)2 was determined by calorimetric measurement in applied dc fields up to 33 tesla. The magnetic phase diagram shows a round maximum at 5.95 K and 17.5 T (at ≈ 1/3 of its saturation field), a 40 percent enhancement of the ordering temperature above the zero field value of 4.25 K. The enhancement and reentrance are consistent with predictions of a field-induced Heisenberg to XY crossover behavior for an ideal 2D QHAF system

Cascade of Magnetic-Field-Induced Quantum Phase Transitions in a Spin- 12 Triangular-Lattice Antiferromagnet

We report magnetocaloric and magnetic-torque evidence that in Cs2CuBr4—a geometrically frustrated Heisenberg S 1⁄4 12 triangular-lattice antiferromagnet—quantum fluctuations stabilize a series of spin states at simple increasing fractions of the saturation magnetization Ms. Only the first of these states—at M 1⁄4 13 Ms—has been theoretically predicted. We discuss how the higher fraction quantum states might arise and propose model spin arrangements. We argue that the first-order nature of the transitions into those states is due to strong lowering of the energies by quantum fluctuations, with implications for the general character of quantum phase transitions in geometrically frustrated systems