204 research outputs found
Absence of supersolidity in solid helium in porous Vycor glass
In 2004, Kim and Chan (KC) carried out torsional oscillator (TO) 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 (NCRI). This
experiment sparked considerable activities in the studies of superfluidity in
solid helium. More recent ultrasound and TO 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 KC makes it unavoidable to have a small amount of bulk solid inside the
torsion cell containing the Vycor disc. 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 measureable period drop that
can be attributed to NCRI
Observation of Landau level-like quantizations at 77 K along a strained-induced graphene ridge
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 scanning
tunnelling 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
Unconventional Planar Hall Effect in Exchange-Coupled Topological Insulator-Ferromagnetic Insulator Heterostructures
The Dirac electrons occupying the surface states (SSs) of topological
insulators (TIs) have been predicted to exhibit many exciting magneto-transport
phenomena. Here we report on the first experimental observation of an
unconventional planar Hall effect (PHE) and an electrically gate-tunable
hysteretic planar magnetoresistance (PMR) in EuS/TI heterostructures, in which
EuS is a ferromagnetic insulator (FMI) with an in-plane magnetization. In such
exchange-coupled FMI/TI heterostructures, we find a significant (suppressed)
PHE when the in-plane magnetic field is parallel (perpendicular) to the
electric current. This behavior differs from previous observations of the PHE
in ferromagnets and semiconductors. Furthermore, as the thickness of the 3D TI
films is reduced into the 2D limit, in which the Dirac SSs develop a
hybridization gap, we find a suppression of the PHE around the charge neutral
point indicating the vital role of Dirac SSs in this phenomenon. To explain our
findings, we outline a symmetry argument that excludes linear-Hall mechanisms
and suggest two possible non-linear Hall mechanisms that can account for all
the essential qualitative features in our observations.Comment: 17 pages, 4 figures, accepted by Phys. Rev.
Zero-field dissipationless chiral edge transport and the nature of dissipation in the quantum anomalous Hall state
The quantum anomalous Hall (QAH) effect is predicted to possess, at zero
magnetic field, chiral edge channels that conduct 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 two orders of magnitude smaller than the Curie
temperature of ferromagnetism.Comment: main text+supporting materials. This is the accepted version for PRL.
Comments are welcom
- …