Topological states of matter are characterized by topological invariants, which are physical quantities whose values are quantized and do not depend on the details of the system (such as its shape, size and impurities). Of these quantities, the easiest to probe is the electrical Hall conductance, and fractional values (in units of e/h, where e is the electronic charge and h is the Planck constant) of this quantity attest to topologically ordered states, which carry quasiparticles with fractional charge and anyonic statistics. Another topological invariant is the thermal Hall conductance, which is harder to measure. For the quantized thermal Hall conductance, a fractional value in units of κ (κ = πk/(3h), where k is the Boltzmann constant) proves that the state of matter is non-Abelian. Such non-Abelian states lead to ground-state degeneracy and perform topological unitary transformations when braided, which can be useful for topological quantum computation. Here we report measurements of the thermal Hall conductance of several quantum Hall states in the first excited Landau level and find that the thermal Hall conductance of the 5/2 state is compatible with a half-integer value of 2.5κ, demonstrating its non-Abelian nature.

The quantum of thermal conductance of ballistic (collisionless) one-dimensional channels is a unique fundamental constant. Although the quantization of the electrical conductance of one-dimensional ballistic conductors has long been experimentally established, demonstrating the quantization of thermal conductance has been challenging as it necessitated an accurate measurement of very small temperature increase. It has been accomplished for weakly interacting systems of phonons, photons and electronic Fermi liquids; however, it should theoretically also hold in strongly interacting systems, such as those in which the fractional quantum Hall effect is observed. This effect describes the fractionalization of electrons into anyons and chargeless quasiparticles, which in some cases can be Majorana fermions. Because the bulk is incompressible in the fractional quantum Hall regime, it is not expected to contribute substantially to the thermal conductance, which is instead determined by chiral, one-dimensional edge modes. The thermal conductance thus reflects the topological properties of the fractional quantum Hall electronic system, to which measurements of the electrical conductance give no access. Here we report measurements of thermal conductance in particle-like (Laughlin-Jain series) states and the more complex (and less studied) hole-like states in a high-mobility two-dimensional electron gas in GaAs-AlGaAs heterostructures. Hole-like states, which have fractional Landau-level fillings of 1/2 to 1, support downstream charged modes as well as upstream neutral modes, and are expected to have a thermal conductance that is determined by the net chirality of all of their downstream and upstream edge modes. Our results establish the universality of the quantization of thermal conductance for fractionally charged and neutral modes. Measurements of anyonic heat flow provide access to information that is not easily accessible from measurements of conductance.

Electrons living in a two-dimensional world under a strong magnetic field -the socalled fractional quantum Hall effect (FQHE) -often manifest themselves as fractionally charged quasiparticles (anyons). Moreover, being under special conditions they are expected to be immune to the environment, thus may serve as building blocks for future quantum computers. Interference of such anyons is the very first step towards understanding their anyonic statistics. However, the complex edge-modes structure of the fractional quantum Hall states, combined with upstream neutral modes, have been suspected to prevent an observation of the much sought after interference of anyons. Here, we report of finding a direct correlation between the appearance of neutral modes and the gradual disappearance of interference in a Mach-Zehnder interferometer (MZI), as the bulk filling factor is lowered towards Landau filling =1; followed by a complete interference quench at =1. Specifically, the interference was found to start diminishing at ~1.5 with a growing upstream neutral mode, which was detected by a born upstream shot noise in the input quantum point contact (QPC) to the MZI. Moreover, at the same time a =1/3 conductance plateau, carrying shot-noise, appeared in the transmission of the QPC -persisting until bulk filling =1/2. We identified this conductance plateau to result from edge reconstruction, which leads to an upstream neutral mode. Here, we also show that even the particle-like quasiparticles are accompanied by upstream neutral modes, therefore suppressing interference in the FQHE regime.

Slow intrinsic fluctuations of resistance, also known as the flicker noise or 1/f-noise, in the surface transport of strong topological insulators (TIs) is a poorly understood phenomenon. Here, we have systematically explored the 1/f-noise in field-effect transistors (FET) of mechanically exfoliated Bi1.6Sb0.4Te2Se TI films when transport occurs predominantly via the surface states. We find that the slow kinetics of the charge disorder within the bulk of the TI induces mobility fluctuations at the surface, providing a new source of intrinsic 1/f-noise that is unique to bulk TI systems. At small channel thickness, the noise magnitude can be extremely small, corresponding to the phenomenological Hooge parameter γH as low as ≈10(-4), but it increases rapidly when channel thickness exceeds ∼1 μm. From the temperature (T)-dependence of noise, which displayed sharp peaks at characteristic values of T, we identified generation-recombination processes from interband transitions within the TI bulk as the dominant source of the mobility fluctuations in surface transport. Our experiment not only establishes an intrinsic microscopic origin of noise in TI surface channels, but also reveals a unique spectroscopic information on the impurity bands that can be useful in bulk TI systems in general.

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In species whose evolutionary history has provided natural tolerance to dehydration and freezing, metabolic depression is often a pre-requisite for survival. We tested the hypothesis that preconditioning of mammalian cells with 5-aminoimidazole-4-carboxamide-1-b-D-ribofuranoside (AICAR) to achieve metabolic depression will promote greater survivorship during cryopreservation. AICAR is used extensively to stimulate AMP-activated protein kinase (AMPK), which can result in downregulation of biosynthetic processes. We showed that the metabolic interconversion of AICAR was cell-type dependent. Accumulation of 5-aminoimidazole-4-carboxamide-1b-D-ribofuranosyl-5′-monophosphate (ZMP), as well as other metabolites that possess multiple phosphates (i.e., ZDP, ZTP), varied approximately 3.5-fold across the cell lines tested. AICAR treatment also significantly influenced the concentrations of cellular adenylates (ATP, ADP, AMP). Depression of cell metabolism and proliferation with AICAR treatment differed among cell lines. Proliferation for a given cell line was negatively correlated with the fold-increase achieved in the 'effective adenylate ratio' ([AMP]+[ZMP])/[ATP]) after AICAR treatment. Metabolic preconditioning with AICAR promoted a significant increase in viability post-freezing in J774.A1 macrophages, HepG2/C3A cells and primary hepatocytes but not in NIH/ 3T3 fibroblasts or OMK cells. The effect of AICAR on viability after freezing was positively correlated (r 2 = 0.94) with the fold-increase in the 'effective adenylate ratio'. Thus for each cell line, the greater the depression of metabolism and proliferation due to preconditioning with AICAR, the greater was the survivorship post-freezing.

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*Implementing topological insulators as elementary units in quantum technologies requires a comprehensive understanding of the dephasing mechanisms governing the surface carriers in these materials, which impose a practical limit to the applicability of these materials in such technologies requiring phase coherent transport. To investigate this, we have performed magneto-resistance (MR) and conductance fluctuations (CF) measurements in both exfoliated and molecular beam epitaxy grown samples. The phase breaking length (l φ ) obtained from MR shows a saturation below sample dependent characteristic temperatures, consistent with that obtained from CF measurements. We have systematically eliminated several factors that may lead to such behavior of l φ in the context of TIs, such as finite size effect, thermalization, spin-orbit coupling length, spin-flip scattering, and surface-bulk coupling. Our work indicates the need to identify an alternative source of dephasing that dominates at low T in topological insulators, causing saturation in the phase breaking length and time./h) B (T) -60 0 60 120 6 8 10 6 8 10 V G (V) R EXF (K:) R MBE (K:) 10 …m (b) (a) Figure 1. Quantum transport in topological insulator FETs. (a) Typical R -VG for exfoliated TI (R EXF ) TBN11 and epitaxially grown TI (R M BE ) M10 at 20mK. Inset: optical micrograph of a typical exfoliated TI FET (b) Weak-antilocalisation behavior observed in different samples at T = 300 mK. The solid black lines are fits to the data using Eq. 1.Topological insulators (TIs) [1-4] are a new class of materials characterized by the presence of gapless and linearly dispersing metallic surface states present in the bulk band gap due to non-trivial topology of the bulk band structure. The surface carriers are prohibited from back-scattering against non-magnetic impurities and exhibit a plethora of fundamentally important effects such as spin-momentum locking, hosting Majorana fermions in the presence of a superconductor, topological magnetoelectric effect, and quantum anomalous Hall effect [1,5]. The topological protection of these surface states makes these materials a strong contender for the building blocks of qubits, which require long phase coherence length (l φ ) * isaurav@iisc.ac.in; SI and SB contributed equally for error tolerant quantum computation. Hence, it is critical to understand the mechanisms responsible for dephasing or decoherence, which is equivalent to loss of information, in the surface states of TIs. The most common dephasing mechanism in TIs at low temperature (T ) has been known to be electron-electron interaction [6][7][8][9][10], and the coupling of the surface states to localized charged puddles in the bulk [11]. Li et al. have demonstrated that electron-phonon interaction is also required to explain the dependence of l φ on T [12]. Although theoretically, all these mechanisms lead to a diverging l φ with decreasing T [6,7,13,14], experimentally, the increase of l φ with reducing T is often followed by its saturation for T ≤ 2 − 5 K [11,12,15,16...