Superfluidity, liquid flow without friction, is familiar in helium. The first evidence for "supersolidity", its analogue in quantum solids, came from recent torsional oscillator (TO) measurements 1,2 involving 4 He. At temperatures below 200 mK, TO frequencies increased, suggesting that some of the solid decoupled from the oscillator. This behavior has been replicated by several groups 3,4,5,6,7 , but solid 4 He does not respond to pressure differences 8 and persistent currents and other signatures of superflow have not been seen. Both experiments and theory 9,10,11,12,13,14 indicate that defects are involved. These should also affect the solid's mechanical behavior and so we have measured the shear modulus of solid 4 He at low frequencies and strains. We observe large increases below 200 mK, with the same dependence on measurement amplitude, 3 He impurity concentration and annealing as the decoupling seen in TO experiments. This unusual elastic behavior is explained in terms of a dislocation network which is pinned by 3 He at the lowest temperatures but becomes mobile above 100 mK. The frequency changes in TO experiments appear to be related to the motion of these dislocations, perhaps by disrupting a possible supersolid state.Although the amount of helium which decouples in different TOs varies widely, the measurements have many common features. Decoupling occurs below about 200 mK, with a gradual onset accompanied by a dissipation peak at lower temperatures. It decreases at large oscillation amplitudes, which is interpreted in terms of a superflow critical velocity (v c ≈ 10 µm/s). The magnitude of the decoupling is frequency
In the quest for superconductors with higher transition temperatures ( ), one emerging motif is that electronic interactions favourable for superconductivity can be enhanced by fluctuations of a broken-symmetry phase. Recent experiments have suggested the existence of the requisite broken symmetry phase in the highcuprates, but the impact of such a phase on the ground-state electronic interactions has remained unclear. We use magnetic fields exceeding 90 tesla to access the underlying metallic state of the cuprate YBa 2 Cu 3 O 6+δ over a wide range of doping, and observe magnetic quantum oscillations that reveal a strong enhancement of the quasiparticle effective mass toward optimal doping. This mass enhancement results from increasing electronic interactions approaching optimal doping, and suggests a quantum-critical point at a hole doping of ≈ . .In several classes of unconventional superconductors, such as the heavy fermions, organics, and iron pnictides, superconductivity has been linked to a quantum critical point (QCP). At a QCP, the system undergoes a phase transition and a change in symmetry at zero temperature; the associated quantum fluctuations enhance interactions, which can give rise to (or enhance) superconductivity [1, 2]. As the QCP is approached, these fluctuations produce stronger and stronger electronic correlations, resulting in an experimentally-observable enhancement of the electron effective mass [1, 3, 4, 5]. It is widely believed that spin fluctuations in the vicinity of an antiferromagnetic QCP are important for superconductivity in many heavy-fermion, organic, and pnictide superconductors [6, 2], leading to the ubiquitous phenomenon of a superconducting dome surrounding a QCP. The role of quantum-criticality in cuprate high-temperature superconductors is more controversial [7]: do the collapsing experimental energy scales [8], enhanced superconducting properties (see Fig. 1), and evidence for a change in ground-state symmetry near optimal doping [9, 10, 11, 12, 13, 14, 15, 16] support the existence of strong fluctuations that are relevant to superconductivity [17, 18, 19, 2]? Alternative explanations for the phenomenology of the cuprate phase diagram focus on the physics of a lightly doped Mott insulator [7, 20], rather than a metal with competing broken-symmetry phases. Several investigations, both theoretical and experimental, suggest that competing order is present in the cuprates, and is associated with the charge (rather than spin) degree of freedom (such as charge density wave order, orbital current order, or nematicity, see Fig. 1) [12, 15, 16,17, 18, 21, 22, 23, 24, 25, 26, 27, 28]. What has been missing is direct, low-temperature evidence that the disappearance of competing order near optimal doping, and the associated change in ground-state symmetry, is accompanied by enhanced electronic interactions in the ground state.A powerful technique for measuring low-temperature Fermi surface properties is the magnetic quantum-oscillation phenomenon, which directly accesses quasi...
Torsional oscillator experiments on solid 4He show frequency changes which suggest mass decoupling, but the onset is broad and is accompanied by a dissipation peak. We have measured the elastic shear modulus over a broad frequency range, from 0.5 Hz to 8 kHz, and observe similar behavior-stiffening and a dissipation peak. These features are associated with a dynamical crossover in a thermally activated relaxation process in a disordered system rather than a true phase transition. If there is a transition to a dc response, e.g., to a supersolid state, it must occur below 55 mK.
Measurements of quantum oscillations in the cuprate superconductors afford a new opportunity to assess the extent to which the electronic properties of these materials yield to a description rooted in Fermi liquid theory. However, such an analysis is hampered by the small number of oscillatory periods observed. Here we employ a genetic algorithm to globally model the field, angular, and temperature dependence of the quantum oscillations observed in the resistivity of Y Ba2Cu3O6.59. This approach successfully fits an entire data set to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key feature of the data is the first identification of the effect of Zeeman splitting, which separates spin-up and spin-down contributions, indicating that the quasiparticles in the cuprates behave as nearly free spins, constraining the source of the Fermi surface reconstruction to something other than a conventional spin density wave with moments parallel to the CuO2 planes.A magnetic field H applied to a metal puts electrons into discrete Landau levels whose spacing increases linearly with the field. As a consequence, the physical properties of a metal oscillate with a characteristic 1/H periodicity due to successive Landau levels passing through the Fermi energy which separates filled from empty states. [7] in spite of their greater cation disorder. Comparisons between different compounds and dopings suggest a drastic reorganization of the electronic structure, from a large hole-like Fermi surface in overdoped compounds[6] to small pockets in underdoped compounds. Recently, measurements in the YBa 2 Cu 3 O 6+x (YBCO) system have been exploring the underdoped side in greater detail, to determine the extent to which the measurements can be modeled by conventional Fermi liquid treatments, despite the strong electronic correlations in the underdoped region. In these studies, additional oscillation frequencies and thus a more complicated Fermi surface have been reported [3,8,9]. We concentrate here on the observation that the original low frequency oscillation is actually comprised of multiple, closely spaced frequencies [8]. In this situation, angle dependent measurements clarify whether the multiple frequencies arise from a warped quasi-2D Fermi cylinder. Below, we present such a set of measurements together with an analysis method that uncovers for the first time the effects of Zeeman splitting.In Fig. 1 we present oscillations of the longitudinal resistivity (Shubnikov-de Haas effect) in the direction perpendicular to the CuO 2 planes (ĉ-axis), and with the magnetic field also aligned along theĉ-axis. Although it is theĉ-axis resistivity being measured, the oscillations are probing the cyclotron motion of electrons within the CuO 2 planes. At 1.5 K, the onset of non-zero resistivity shows the vortex lattice begins to depin or melt at 24 Tesla. This transition occurs at lower fields with increasing temperature, giving a trade-off between available field range and oscillation amplitude. The derivative of thes...
The recent torsional oscillator results of Kim and Chan suggest a supersolid phase transition in solid 4 He. We have used a piezoelectrically driven diaphragm to study the flow of solid helium through an array of capillaries. Our measurements showed no indication of low temperature flow, placing stringent restrictions on supersolid flow in response to a pressure difference. The average flow speed at low temperatures was less than 1.2x10 −14 m/s, corresponding to a supersolid velocity at least 7 orders of magnitude smaller than the critical velocities inferred from the torsional oscillator measurements.PACS numbers: 67.40. Hf, 67.80.Mg, 67.90.+z Recent experiments by Kim and Chan [1,2] showed that solid helium decouples from a torsional oscillator at temperatures below about 0.2 K. In liquid 4 He, such decoupling reflects the non-classical rotational inertia (NCRI) associated with superfluidity and these experiments suggest that 4 He also exhibits "supersolidity". The possibility of supersolidity in helium has been discussed for many years [3,4,5] but previous experimental searches [6,7,8] were unsuccessful. Following Kim and Chan's experiments, a number of papers have discussed the possible microscopic origins of supersolidity [9,10,11,12,13,14,15,16] and the properties that such a state might exhibit [12,17]. However, there is not yet a consensus on whether supersolidity can occur in a defect-free crystal and further experiments are needed to establish whether solid helium displays any of the other unusual properties associated with superfluidity. We recently[18] used a capacitive method to look for pressure-driven flow of solid helium confined in the pores of vycor glass, but saw no evidence of superflow at temperatures down to 30 mK, nor has supersolidity been seen in recent ultrasonic experiments in vycor [19]. In this Letter, we report measurements of DC and low frequency AC flow of solid 4 He through an array of glass capillaries. Near the melting temperature, applying a pressure difference caused solid helium to flow through the capillaries, but the rate decreased with temperature; below about 1 K no flow was detected. Our experiments extended to 35 mK, well into the temperature range where Kim and Chan observed NCRI, and used isotopically pure 4 He. Our results place stringent limits on possible pressureinduced supersolid flow.The essential results of the torsional oscillator measurements were similar for 4 He confined in the nanometer pores of vycor glass[1] and for bulk 4 He[2]. Each showed a gradual transition at T c ≈ 0.2 K with about 1% of the helium (the "supersolid fraction" ρ s /ρ) decoupling at the lowest temperatures and amplitudes. The decoupling was smaller at large oscillation amplitudes, suggesting a supersolid critical velocity v c ∼ 10 µm/s in both systems. The similarities support the interpretation that NCRI is an intrinsic property of solid helium rather than, for example, occurring in liquid layer at pore surfaces. The measurements in vycor revealed a remarkable sensitivity to 3 He...
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