24 research outputs found
Kosterlitz-Thouless scaling at many-body localization phase transitions
We propose a scaling theory for the many-body localization (MBL) phase
transition in one dimension, building on the idea that it proceeds via a
'quantum avalanche'. We argue that the critical properties can be captured at a
coarse-grained level by a Kosterlitz-Thouless (KT) renormalization group (RG)
flow. On phenomenological grounds, we identify the scaling variables as the
density of thermal regions and the lengthscale that controls the decay of
typical matrix elements. Within this KT picture, the MBL phase is a line of
fixed points that terminates at the delocalization transition. We discuss two
possible scenarios distinguished by the distribution of rare, fractal thermal
inclusions within the MBL phase. In the first scenario, these regions have a
stretched exponential distribution in the MBL phase. In the second scenario,
the near-critical MBL phase hosts rare thermal regions that are power-law
distributed in size. This points to the existence of a second transition within
the MBL phase, at which these power-laws change to the stretched exponential
form expected at strong disorder. We numerically simulate two different
phenomenological RGs previously proposed to describe the MBL transition. Both
RGs display a universal power-law length distribution of thermal regions at the
transition with a critical exponent , and continuously varying
exponents in the MBL phase consistent with the KT picture.Comment: 17 pages, 10 figures; v3. minor changes, as published; v2. added
section and appendix with new numerical simulations, expanded discussio
Order and Disorder in AKLT Antiferromagnets in Three Dimensions
The models constructed by Affleck, Kennedy, Lieb, and Tasaki describe a
family of quantum antiferromagnets on arbitrary lattices, where the local spin
S is an integer multiple M of half the lattice coordination number. The equal
time quantum correlations in their ground states may be computed as finite
temperature correlations of a classical O(3) model on the same lattice, where
the temperature is given by T=1/M. In dimensions d=1 and d=2 this mapping
implies that all AKLT states are quantum disordered. We consider AKLT states in
d=3 where the nature of the AKLT states is now a question of detail depending
upon the choice of lattice and spin; for sufficiently large S some form of Neel
order is almost inevitable. On the unfrustrated cubic lattice, we find that all
AKLT states are ordered while for the unfrustrated diamond lattice the minimal
S=2 state is disordered while all other states are ordered. On the frustrated
pyrochlore lattice, we find (conservatively) that several states starting with
the minimal S=3 state are disordered. The disordered AKLT models we report here
are a significant addition to the catalog of magnetic Hamiltonians in d=3 with
ground states known to lack order on account of strong quantum fluctuations.Comment: 7 pages, 2 figure
Coulomb-driven band unflattening suppresses K-phonon pairing in moire graphene
It is a matter of current debate whether the gate-tunable superconductivity in twisted bilayer graphene is phonon-mediated or arises from electron-electron interactions. The recent observation of the strong coupling of electrons to so-called K-phonon modes in angle-resolved photoemission spectroscopy experiments has resuscitated early proposals that K-phonons drive superconductivity. We show that the bandwidth-enhancing effect of interactions drastically weakens both the intrinsic susceptibility towards pairing as well as the screening of Coulomb repulsion that is essential for the phonon attraction to dominate at low temperature. This rules out purely K-phonon-mediated superconductivity with the observed transition temperature of ∼1 K. We conclude that the unflattening of bands by Coulomb interactions challenges any purely phonon-driven pairing mechanism, and must be addressed by a successful theory of superconductivity in moiré graphen
Spin skyrmion gaps as signatures of strong-coupling insulators in magic-angle twisted bilayer graphene
The flat electronic bands in magic-angle twisted bilayer graphene (MATBG)
host a variety of correlated insulating ground states, many of which are
predicted to support charged excitations with topologically non-trivial spin
and/or valley skyrmion textures. However, it has remained challenging to
experimentally address their ground state order and excitations, both because
some of the proposed states do not couple directly to experimental probes, and
because they are highly sensitive to spatial inhomogeneities in real samples.
Here, using a scanning single-electron transistor, we observe thermodynamic
gaps at even integer moir\'e filling factors at low magnetic fields. We find
evidence of a field-tuned crossover from charged spin skyrmions to bare
particle-like excitations, suggesting that the underlying ground state belongs
to the manifold of strong-coupling insulators. From the spatial dependence of
these states and the chemical potential variation within the flat bands, we
infer a link between the stability of the correlated ground states and local
twist angle and strain. Our work advances the microscopic understanding of the
correlated insulators in MATBG and their unconventional excitations.Comment: Supplementary information available at
https://www.nature.com/articles/s41467-023-42275-
Interacting multi-channel topological boundary modes in a quantum Hall valley system
Symmetry and topology play key roles in the identification of phases of
matter and their properties. Both concepts are central to understanding quantum
Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously
broken spin or pseudospin symmetry whose wavefunctions also have topological
properties. Domain walls between distinct broken symmetry QHFM phases are
predicted to host gapless one-dimensional (1D) modes that emerge due to a
topological change of the underlying electronic wavefunctions at such
interfaces. Although a variety of QHFMs have been identified in different
materials, probing interacting electronic modes at these domain walls has not
yet been accomplished. Here we use a scanning tunneling microscope (STM) to
directly visualize the spontaneous formation of boundary modes, within a
sign-changing topological gap, at domain walls between different
valley-polarized quantum Hall phases on the surface of bismuth. By changing the
valley occupation and the corresponding number of modes at the domain wall, we
can realize different regimes where the valley-polarized channels are either
metallic or develop a spectroscopic gap. This behavior is a consequence of
Coulomb interactions constrained by the symmetry-breaking valley flavor, which
determines whether electrons in the topological modes can backscatter, making
these channels a unique class of interacting Luttinger liquids
Nations within a nation: variations in epidemiological transition across the states of India, 1990–2016 in the Global Burden of Disease Study
18% of the world's population lives in India, and many states of India have populations similar to those of large countries. Action to effectively improve population health in India requires availability of reliable and comprehensive state-level estimates of disease burden and risk factors over time. Such comprehensive estimates have not been available so far for all major diseases and risk factors. Thus, we aimed to estimate the disease burden and risk factors in every state of India as part of the Global Burden of Disease (GBD) Study 2016
Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo
Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level