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In this article, we review the experimental studies in pursuit of supersolidity and on recent studies of mass flow through solid 4He samples. [ABSTRACT FROM AUTHOR]

The resonant period drop observed at low temperatures in torsional oscillators containing solid helium had been interpreted as a signature of a supersolid. However, it was found that the shear modulus increase found in solid helium at the same low temperature could also decrease the resonant period of the torsional oscillator. We report the results of a study in two different torsional oscillators that were specially designed to minimize the shear modulus effect and maximize any possible supersolid response. We were able to place an upper limit on the nonclassical rotational inertia or supersolid fraction of 4 × 10-6. Moreover, we have repeated an earlier experiment on the hep 3He solid, which shows similar low-temperature stiffening to that in hep 4He. We found that the small drop of the resonant period measured in the hep 3He sample is comparable in size to that found in the hep 4He samples. These results strongly suggest that the resonant period drop reported in most torsional oscillator studies in the last decade is primarily a consequence of the shear modulus stiffening effect. [ABSTRACT FROM AUTHOR]

In quasi-one-dimensional nanowires, superconductivity is destroyed by phase slip events. Phase slips can be caused by thermal activation over a free energy barrier (TAPS) or quantum tunneling through the barrier (QPS). Quantum phase slip is an example of macroscopic quantum tunneling. Here, we report the observation of QPS experimentally separated from interference of TAPS in aluminum nanowires. This separation between the low- temperature QPS and the high-temperature TAPS regions is made possible by a phase slip free superconducting region stabilized by the dissipative environment viz, the normal electrodes. Individual QPSs are detected by means of a single-shot voltage measurement protocol, in which they appear as stochastic switching events from the superconducting to the normal state. [ABSTRACT FROM AUTHOR]

In 2004, Kim and Chan carried out torsional oscillator measurements of solid helium confined in porous Vycor glass and found an abrupt drop in the resonant period below 200 mK. The period drop was interpreted as probable experimental evidence of nonclassical rotational inertia. This experiment sparked considerable activities in the studies of superfluidity in solid helium. More recent ultrasound and torsional oscillator studies, however, found evidence that shear modulus stiffening is responsible for at least a fraction of the period drop found in bulk solid helium samples. The experimental configuration of Kim and Chan makes it unavoidable to have a small amount of bulk solid inside the torsion cell containing the Vycor disk. We report here the results of a new helium in Vycor experiment with a design that is completely free from any bulk solid shear modulus stiffening effect. We found no measurable period drop that can be attributed to nonclassical rotational inertia. [ABSTRACT FROM AUTHOR]

Pays tribute to Daniel Richard Frankl, an emeritus professor of physics at the Pennsylvania State University, who died on June 1, 2003 in State College, Pennsylvania.

We report heat capacity measurements of solid 4He and 3He-4He solid mixture samples grown in silica aerogel. In addition to the Debye T3 term, the heat capacity of solid 4He samples includes T linear and T2 terms which are attributed to the amorphous and two-dimensional helium layers confined near the silica strands of aerogel. Beside these regular polynomial terms, a broad heat capacity peak is found between 0.1 and 0.35 K. The peak probably has its origin in the vibration of the dislocation lines pinned by the silica strands. Compared to 4He samples, the heat capacity of solid 3He-4He mixture samples shows evidence of phase separation below a characteristic temperature Tc that increases with 3He concentration (X3). Below this temperature the heat capacity shows X31/2 dependence suggesting that the 3He atoms are phase separated from being uniformly distributed in the pore space into two-dimensional patches enfolding the silica strands that are themselves coated with a strongly bound amorphous 4He layer. [ABSTRACT FROM AUTHOR]

A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana modes. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these modes in a millimeter-size QAH-niobium hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. Here, we studied similar hybrid devices with a well-controlled and transparent interface between the superconductor and the QAH insulator. When the devices are in the QAH state with well-aligned magnetization, the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid devices and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions in samples with a highly transparent interface. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the influence of direct imaging of electron transfer on the superconducting transition temperature (Tc) of monolayer iron selenide (FeSe) films on SrTiO3) (STO). It discusses the direct electron transfer from STO to the FeSe layer using low temperature electron energy-loss spectroscopy (EELS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).

The quantum anomalous Hall (QAH) effect is predicted to possess, at a zero magnetic field, chiral edge channels that conduct a spin polarized current without dissipation. While edge channels have been observed in previous experimental studies of the QAH effect, their dissipationless nature at a zero magnetic field has not been convincingly demonstrated. By a comprehensive experimental study of the gate and temperature dependences of local and nonlocal magnetoresistance, we unambiguously establish the dissipationless edge transport. By studying the onset of dissipation, we also identify the origin of dissipative channels and clarify the surprising observation that the critical temperature of the QAH effect is 2 orders of magnitude smaller than the Curie temperature of ferromagnetism. [ABSTRACT FROM AUTHOR]

The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two-dimensional electron layer under a strong magnetic field. The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon to that of the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism. Here, we report the experimental observation of the QAH state in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down to 0.00013 ± 0.00007h/e2 (~3.35 ± 1.76 Ω), Hall conductance reaching 0.9998 ± 0.0006e2/h and the Hall angle becoming as high as 89.993° ± 0.004° at T = 25 mK. A further advantage of this system comes from the fact that it is a hard ferromagnet with a large coercive field (Hc > 1.0 T) and a relative high Curie temperature. This realization of a robust QAH state in hard ferromagnetic topological insulators (FMTIs) is a major step towards dissipationless electronic applications in the absence of external fields. [ABSTRACT FROM AUTHOR]

We demonstrate by high resolution low temperature electron energy loss spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order in the vanadium- (V-)doped topological insulator Sb2Te3 has the nature of van Vleck-type ferromagnetism. The positions and the relative amplitudes of two core-level peaks (L3 and L2) of the V EELS spectrum show unambiguous change when the sample is cooled from room temperature to T = 10 K. Magnetotransport and comparison of the measured and simulated EELS spectra confirm that these changes originate from the onset of FM order. Crystal field analysis indicates that in V-doped Sb2Te3, partially filled core states contribute to the FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner. [ABSTRACT FROM AUTHOR]

The physical origin and the nature of superconductivity in nanostructured Bi remains puzzling. Here, we report transport measurements of individual cylindrical single-crystal Bi nanowires, 20 and 32 nm in diameter. In contrast to nonsuperconducting Bi nanoribbons with two flat surfaces, cylindrical Bi nanowires show superconductivity below 1.3 K. However, their superconducting critical magnetic fields decrease with their diameter, which is the opposite of the expected behavior for thin superconducting wires. Quasiperiodic oscillations of magnetoresistance were observed in perpendicular fields but were not seen in the parallel orientation. These results can be understood by a model of surface superconductivity with an enhanced surface-to-bulk volume in small diameter wires, where the superconductivity originates from the strained surface states of the nanowires due to the surface curvature-induced stress. [ABSTRACT FROM AUTHOR]

Recent studies show that the electronic structures of graphene can be modified by strain, and it was predicted that strain in graphene can induce peaks in the local density of states (LDOS), mimicking Landau levels (LLs) generated in the presence of a large magnetic field. Here we report the scanning tunneling spectroscopy (STS) observation of nine strain-induced peaks in LDOS at 77 K along a graphene ridge created when the graphene layer was cleaved from a sample of highly oriented pyrolytic graphite (HOPG). The energies of these peaks follow the progression of LLs of massless "Dirac fermions" (DFs) in a magnetic field of 230 T The results presented here suggest a possible route to realize zero-field quantum Hall-like effects at 77 K. [ABSTRACT FROM AUTHOR]

We report the observation of the superconducting proximity effect in nanoribbons of a candidate topological insulator (Bi2Se3), which is interfaced with superconducting (tungsten) contacts. We observe a supercurrent and multiple Andreev reflections for channel lengths that are much longer than the inelastic and diffusive thermal lengths deduced from normal-state transport. This suggests that the proximity effect couples preferentially to a ballistic surface transport channel, even in the presence of a coexisting diffusive bulk channel. When a magnetic field is applied perpendicular to the plane of the nanoribbon, we observe magnetoresistance oscillations that are periodic in magnetic field. Quantitative comparison with a model of vortex blockade relates the occurrence of these oscillations to the formation of Pearl vortices in the region of proximity-induced superconductivity. [ABSTRACT FROM AUTHOR]

The antiproximity effect, where the superconductivity in superconducting nanowires is suppressed or weakened when the wires are contacted by bulk superconducting electrodes, first revealed in arrays of Zn nanowires by tuning the electrodes from the superconducting to the normal state by means of an external magnetic field, has been confirmed in single-crystal aluminum nanowires. The critical current at zero magnetic field of an individual aluminum nanowire contacted by superconducting electrodes was found to be significantly smaller than that with normal electrodes, showing that the effect is not a consequence of the magnetic field. [ABSTRACT FROM AUTHOR]

The first clear evidence for supersolidity in helium came from experiments in which the resonant period of a torsional oscillator decreased below ∼0.2 K, indicating that some of the solid 4He decoupled from the oscillation. More recently, shear-modulus measurements on solid 4He revealed an unexpected increase with the same dependence on temperature and 3He impurities. The similarities raised the possibility that the period drop in torsion experiments is simply due to the stiffening of the solid. Here, we report the same measurements on solid 3He, a Fermi solid instead of a Bose solid. The anomalous modulus increase found in hexagonal close-packed (hcp) 4He, is also found in hcp 3He. However, in the case of 3He, the shear modulus increase is not accompanied by a corresponding period change of the torsional oscillator. We conclude that elastic stiffening alone does not produce the changes in the torsional-oscillator period and that decoupling occurs only in a stiffened Bose solid. [ABSTRACT FROM AUTHOR]

The "magnetoelectric effect" arises from the coupling between magnetic and electric properties in materials. The Z2 invariant of topological insulators (TIs) leads to a quantized version of this phenomenon, known as the topological magnetoelectric (TME) effect. This effect can be realized in a new topological phase called an "axion insulator" whose surface states are all gapped but the interior still obeys time reversal symmetry. We demonstrate such a phase using electrical transport measurements in a quantum anomalous Hall (QAH) sandwich heterostructure, in which two compositionally different magnetic TI layers are separated by an undoped TI layer. Magnetic force microscopy images of the same sample reveal sequential magnetization reversals of the top and bottom layers at different coercive fields, a consequence of the weak interlayer exchange coupling due to the spacer. When the magnetization is antiparallel, both the Hall resistance and Hall conductance show zero plateaus, accompanied by a large longitudinal resistance and vanishing longitudinal conductance, indicating the realization of an axion insulator state. Our findings thus show evidence for a phase of matter distinct from the established QAH state and provide a promising platform for the realization of the TME effect. [ABSTRACT FROM AUTHOR]

The possible realization of dissipationless chiral edge current in a topological insulator/magnetic insulator heterostructure is based on the condition that the magnetic proximity exchange coupling at the interface is dominated by the Dirac surface states of the topological insulator. Here we report a polarized neutron reflectometry observation of Dirac-electron-mediated magnetic proximity effect in a bulk-insulating topological insulator (Bi0.2Sb0.8)2Te3/magnetic insulator EuS heterostructure. We are able to maximize the proximity-induced magnetism by applying an electrical back gate to tune the Fermi level of topological insulator to be close to the Dirac point. A phenomenological model based on diamagnetic screening is developed to explain the suppressed proximity-induced magnetism at high carrier density. Our work paves the way to utilize the magnetic proximity effect at the topological insulator/magnetic insulator heterointerface for low-power spintronic applications. [ABSTRACT FROM AUTHOR]

We report the observation of superfluidlike mass flow through coin-shaped 8 μm thick solid 4He samples sandwiched between superfluid leads. Mass flow is found from the melting pressure to at least 30 bar with a concomitant decrease in the onset temperature from 1 to 0.25 K. The mass-flow rate is found to be sample dependent and can be enhanced by thermal annealing. The flow rate decreases with temperature and decays nearly exponentially with the pressure of the samples. The dissipation associated with the mass flow decreases with temperature and becomes superfluidlike near 0.1 K. In contrast to earlier studies on centimeter-thick samples, we do not see a sharp cutoff in the mass-flow rate at low temperature. [ABSTRACT FROM AUTHOR]

Fundamental insight into the nature of the quantum phase transition from a superconductor to an insulator in two dimensions, or from one plateau to the next or to an insulator in the quantum Hall effect, has been revealed through the study of its scaling behavior. Here, we report on the experimental observation of a quantum phase transition from a quantum-anomalous-Hall insulator to an Anderson insulator in a magnetic topological insulator by tuning the chemical potential. Our experiment demonstrates the existence of scaling behavior from which we extract the critical exponent for this quantum phase transition. We expect that our work will motivate much further investigation of many properties of quantum phase transition in this new context. [ABSTRACT FROM AUTHOR]

Three-dimensional topological insulators are characterized by Dirac-like conducting surface states, the existence of which has been confirmed in relatively clean metallic samples by angle-resolved photoemission spectroscopy, as well as by anomalous Aharonov-Bohm oscillations in the magnetoresistance of nanoribbons. However, a fundamental aspect of these surface states, namely, their robustness to time-reversal-invariant disorder, has remained relatively untested. In this work, we have synthesized thin nanotubes of Bi2Te3 with extremely insulating bulk at low temperatures due to disorder. Nonetheless, the magnetoresistance exhibits quantum oscillations as a function of the magnetic field along the axis of the nanotubes, with a period determined by the cross-sectional area of the outer surface. Detailed numerical simulations based on a recursive Green function method support that the resistance oscillations are arising from the topological surface states which have substantially longer localization length than that of other nontopological states. This observation demonstrates coherent transport at the surface even for highly disordered samples, thus providing a direct confirmation of the inherently topological character of surface states. The result also demonstrates a viable route for revealing the properties of topological states by suppressing the bulk conduction using disorder. [ABSTRACT FROM AUTHOR]