8 research outputs found
Transport and criticality in topological systems and spin models
No full textThis thesis presents work done on transport in topological insulators and graphene-based systems, and quantum criticality in one- and two-dimensional spin
models. In particular we study the following: transport on surfaces of
three-dimensional topological insulators in the presence of time-independent and
time-dependent barriers, Majorana modes in a one-dimensional topological insulator in proximity with a -wave superconductor, the phase diagram of the
Hubbard model on a triangular lattice periodically driven by an in-plane electric
field, quantum criticality of a Ising model with three-spin interactions and
a transverse field, the origin of spin-orbit coupling in a graphene-WSe heterostructure, and a prediction of edge states in trilayer graphene.
In the first chapter, we give a brief introduction to the concepts relevant to the
rest of the thesis such as topological insulators, superconductivity, Floquet theory for studying periodically driven Hamiltonians, graphene and the spin-orbit coupling terms, quantum phase transitions, and the transverse field Ising model.
In the second chapter, we consider a thin-film topological insulator (TI) in which the top and the bottom surfaces are separated by a small distance. The hybridisation
between the states on the top and bottom surfaces of this system is characterized by a coupling strength . We study the various features of transport when a potential or
magnetic barrier is applied on one of the surfaces. We find that the conductance of this system oscillates with the barrier strength with the period of oscillations
varying with the coupling strength . This gives us an indirect way of estimating the extent of hybridisation in such thin films by looking at the
conductance. The period of these oscillations changes from to as increases from zero to a value close to the energy of the incident electrons. Next we
study the effects of a magnetic barrier, and we find that the conductance reaches a
non-zero and -dependent value as the barrier strength
is increased. This is in sharp contrast to the behavior of the
conductance of a single TI surface where it approaches zero with increasing magnetic
barrier strength. We also find oscillations in the case of a magnetic barrier for
large barrier widths. The period of these oscillations depends on .
In the third chapter, we consider a similar magnetic barrier whose strength is periodically driven in time. We explore the behaviour of the conductance as a function of the driving parameters. Such a barrier can be realised by shining linearly
polarised light over a region of width on the surface of a TI.
We find that the conductance of
this system exhibits a number of interesting features like prominent peaks and dips as the parameters of the system are varied. This also paves the way to have an optical
(electromagnetic) control over the electrical current in such junctions where we can go from a high-conductance regime to a low-conductance regime or vice versa by tuning the
amplitude and frequency of the light. We also see that this system can act as a frequency detector or an optically controlled switch as a function of the incident
energy of the electron.
In the fourth chapter, we consider a model of a TI which is now constricted to a narrow and long strip running along the direction. We study what happens to the Majorana
modes when such a system is placed in proximity to an -wave superconductor. This model hosts a spin-dependent chirality and only has a right-moving spin-up and a left-moving
spin-down branch. We find that this leads to a number of unusual features, such as only one zero energy Majorana mode at each end of a finite system, a single Andreev bound
state at a Josephson junction instead of two states, and multiple Shapiro steps for particular frequencies of an AC driving.
In the fifth chapter, we study a Hubbard model on a triangular lattice at half-filling in the limit of large interaction. At half-filling, this is known to
describe a Heisenberg spin Hamiltonian with equal nearest-neighbour couplings. We study the effects of driving this system periodically with an in-plane electric field. Taking
the driving to be the perturbation, we find, using Floquet perturbation theory, that the effective Hamiltonian up to third order has two-spin Heisenberg couplings with different
magnitudes in the three different directions of the triangular lattice. We also get a three-spin interaction chiral term in the third order with its sign being opposite on
up- and down-pointing triangles.
We study the ground state phase diagram as a function of the three couplings
using exact diagonalization. We find that driving leads to new phases in the system apart from the spiral phase. We have three collinear ordered phases, one
coplanar ordered phase, and three disordered (spin-liquid) phases. These phases are distinguished by looking at the peaks of the static spin structure function in
the Brillouin zone, the ground state fidelity susceptibility, the minimum value of the correlation function in real space, and the crossings of the
energies of the ground state and first excited state.
In the sixth chapter, we consider a one-dimensional Ising model with a three-spin interaction with a transverse field of magnitude . We find that this model has
duality and a second-order phase transition at the self-dual point . We find from finite-size scaling that the correlation length exponent is
close to in this model. Having a dynamical critical exponent and a central charge , we find that the model displays weak universality and lies somewhere in
the middle of the Ashkin-Teller line of models, with the two extreme
limits of the line being the transverse field Ising and four-state Potts models.
Unlike the transverse Ising model,
our model is non-integrable, with the level spacing statistics being governed
by the Wigner-Dyson Gaussian orthogonal ensemble. We also find
that this model has a subset of zero energy states which are
rather special as they are independent of the value of
and have very low entanglement entropy compared to the states in the neighbourhood of the energy eigenvalues. These states are quantum many-body scars and they
violate the eigenstate thermalisation hypothesis (ETH).
Chapters and describe works done in collaboration with some experimental groups. In Chapter , we study the system of graphene-WSe heterostructure
where we have a strong proximity-induced spin-orbit coupling. The quantum Shubnikov-de Haas (SdH) oscillations observed experimentally show a beating implying
the presence of two closely spaced frequencies. The energy dispersion thus extracted is then studied theoretically using an effective Hamiltonian with all possible spin-orbit
couplings present. The Fermi velocity of the sample is about times that of pristine graphene. The data fitting and perturbation calculations show that the
spin-splitting
energy of nearly meV comes dominantly from the valley-Zeeman and Rashba
spin-orbit couplings in the system. In chapter , we study a system of trilayer graphene under the influence of a perpendicular electric field. The non-local
and local resistance measurements done in this system show a scaling relation given by with for a range of values of the
displacement field. The
value of is seen to be close to 1 up to temperatures around which the bulk gap closes in the system. This strongly suggests that the transport is dominated in this
sample by edge modes. We study a theoretical model for trilayer graphene with displacement fields consistent with the experiments, and show that in this regime the
valley Chern number is non-zero with a large value of for a given valley and a given spin. We also show that the system host zig-zag edge
modes for the displacement fields of interest, although they are not protected from backscattering. A simple resistor circuit model that mimics the inter-valley
scattering through dissipation then explains the linear relation between the non-local and local resistances.
At the end, we summarise our results and discuss possible future
studies in these areas of research
Transport in a thin topological insulator with potential and magnetic barriers
No full textWe study transport across either a potential or a magnetic barrier which is placed on the top surface of a three-dimensional thin topological insulator (TI). For such thin TIs, the top and bottom surfaces interact via a coupling lambda which influences the transport properties of junctions constructed out of them. We find that for junctions hosting a potential barrier, the differential conductance oscillates with the barrier strength. The period of these oscillations doubles as the coupling lambda changes from small values to a value close to the energy of the incident electrons. In contrast, for junctions with a magnetic barrier, the conductance approaches a nonzero constant as the barrier strength is increased. This feature is in contrast to the case of transport across a single TI surface where the conductance approaches zero as the strength of a magnetic barrier is increased. We also study the spin currents for these two kinds of barriers; in both cases, the spin current is found to have opposite signs on the top and bottom surfaces. Thus this system can be used to split applied charge currents to spin currents with opposite spin orientations which can be collected by applying opposite spin-polarized leads to the two surfaces. We show that several of these features of transport across finite width barriers can be understood analytically by studying the delta-function barrier limit. We discuss experiments which may test our theory
One-dimensional spin-orbit coupled Dirac system with extended s-wave superconductivity:Majorana modes and Josephson effects
No full textMotivated by the spin-momentum locking of electrons at the boundaries of certain topological insulators, we study a one-dimensional system of spin-orbit coupled massless Dirac electrons with s-wave superconducting pairing. As a result of the spin-orbit coupling, our model has only two kinds of linearly dispersing modes, and we take these to be right-moving spin-up and left-moving spin-down. Both lattice and continuum models are studied. In the lattice model, we find that a single Majorana zero energy mode appears at each end of a finite system provided that the s-wave pairing has an extended form, with the nearest-neighbor pairing being larger than the on-site pairing. We confirm this both numerically and analytically by calculating the winding number. We find that the continuum model also has zero energy end modes. Next we study a lattice version of a model with both Schrodinger and Dirac-like terms and find that the model hosts a topological transition between topologically trivial and non-trivial phases depending on the relative strength of the Schrodinger and Dirac terms. We then study a continuum system consisting of two s-wave superconductors with different phases of the pairing, with a delta-function potential barrier lying at the junction of the two superconductors. Remarkably, we find that the system has a single Andreev bound state (ABS) which is localized at the junction. When the pairing phase difference crosses a multiple of 2 pi, an ABS touches the top of the superconducting gap and disappears, and a different state appears from the bottom of the gap. We also study the AC Josephson effect in such a junction with a voltage bias that has both a constant V-0 and a term which oscillates with a frequency omega. We find that, in contrast to standard Josephson junctions, Shapiro plateaus appear when the Josephson frequency omega(J) = 2eV(0)/PLANCK CONSTANT OVER TWO PI is a rational fraction of omega. We discuss experiments which can realize such junctions.</p
Weak universality, quantum many-body scars and anomalous infinite-temperature autocorrelations in a one-dimensional spin model with duality
No full textWe study a one-dimensional spin- model with three-spin interactions and
a transverse magnetic field . The model has a symmetry, and
a duality between and . The self-dual point at is a quantum
critical point with a continuous phase transition. We compute the critical
exponents , , and , and the central charge
numerically using exact diagonalization (ED) for systems with periodic boundary
conditions. We find that both and are equal to , implying that the
critical point is governed by a conformal field theory. The values obtained for
, , and from ED suggest that the model exhibits
Ashkin-Teller criticality with an effective coupling that is intermediate
between the four-state Potts model and two decoupled transverse field Ising
models. An analysis on larger systems but with open boundaries using
density-matrix renormalization group calculations, however, shows that the
self-dual point may be in the same universality class as the four-state Potts
model. An energy level spacing analysis shows that the model is not integrable.
For a system with periodic boundary conditions, there are an exponentially
large number of exact mid-spectrum zero-energy eigenstates. A subset of these
eigenstates have wave functions which are independent of and have unusual
entanglement structure, suggesting that they are quantum many-body scars. The
number of such states scales at least linearly with system size. Finally, we
study the infinite-temperature autocorrelation functions close to one end of an
open system. We find that some of the autocorrelators relax anomalously in
time, with pronounced oscillations and very small decay rates if or
. If is close to the critical point, the autocorrelators decay
quickly to zero except for an autocorrelator at the end site.Comment: 23 pages, 20 figures; corrected some typo
The SIB Swiss Institute of Bioinformatics' resources: focus on curated databases
No full textThe SIB Swiss Institute of Bioinformatics (www.isb-sib.ch) provides world-class bioinformatics databases, software tools, services and training to the international life science community in academia and industry. These solutions allow life scientists to turn the exponentially growing amount of data into knowledge. Here, we provide an overview of SIB's resources and competence areas, with a strong focus on curated databases and SIB's most popular and widely used resources. In particular, SIB's Bioinformatics resource portal ExPASy features over 150 resources, including UniProtKB/Swiss-Prot, ENZYME, PROSITE, neXtProt, STRING, UniCarbKB, SugarBindDB, SwissRegulon, EPD, arrayMap, Bgee, SWISS-MODEL Repository, OMA, OrthoDB and other databases, which are briefly described in this article
Characterising acute and chronic care needs: insights from the Global Burden of Disease Study 2019
No full textChronic care manages long-term, progressive conditions, while acute care addresses short-term conditions. Chronic conditions increasingly strain health systems, which are often unprepared for these demands. This study examines the burden of conditions requiring acute versus chronic care, including sequelae. Conditions and sequelae from the Global Burden of Diseases Study 2019 were classified into acute or chronic care categories. Data were analysed by age, sex, and socio-demographic index, presenting total numbers and contributions to burden metrics such as Disability-Adjusted Life Years (DALYs), Years Lived with Disability (YLD), and Years of Life Lost (YLL). Approximately 68% of DALYs were attributed to chronic care, while 27% were due to acute care. Chronic care needs increased with age, representing 86% of YLDs and 71% of YLLs, and accounting for 93% of YLDs from sequelae. These findings highlight that chronic care needs far exceed acute care needs globally, necessitating health systems to adapt accordingly.
© 2025. The Author(s)
Tracking development assistance for health and for COVID-19: a review of development assistance, government, out-of-pocket, and other private spending on health for 204 countries and territories, 1990-2050
No full textBackground The rapid spread of COVID-19 renewed the focus on how health systems across the globe are financed, especially during public health emergencies. Development assistance is an important source of health financing in many low-income countries, yet little is known about how much of this funding was disbursed for COVID-19. We aimed to put development assistance for health for COVID-19 in the context of broader trends in global health financing, and to estimate total health spending from 1995 to 2050 and development assistance for COVID-19 in 2020. Methods We estimated domestic health spending and development assistance for health to generate total health-sector spending estimates for 204 countries and territories. We leveraged data from the WHO Global Health Expenditure Database to produce estimates of domestic health spending. To generate estimates for development assistance for health, we relied on project-level disbursement data from the major international development agencies' online databases and annual financial statements and reports for information on income sources. To adjust our estimates for 2020 to include disbursements related to COVID-19, we extracted project data on commitments and disbursements from a broader set of databases (because not all of the data sources used to estimate the historical series extend to 2020), including the UN Office of Humanitarian Assistance Financial Tracking Service and the International Aid Transparency Initiative. We reported all the historic and future spending estimates in inflation-adjusted 2020 US per capita, purchasing-power parity-adjusted US8. 8 trillion (95% uncertainty interval UI] 8.7-8.8) or 40.4 billion (0.5%, 95% UI 0.5-0.5) was development assistance for health provided to low-income and middle-income countries, which made up 24.6% (UI 24.0-25.1) of total spending in low-income countries. We estimate that 13.7 billion was targeted toward the COVID-19 health response. 1.4 billion was repurposed from existing health projects. 2.4 billion (17.9%) was for supply chain and logistics. Only 1519 (1448-1591) per person in 2050, although spending across countries is expected to remain varied. Interpretation Global health spending is expected to continue to grow, but remain unequally distributed between countries. We estimate that development organisations substantially increased the amount of development assistance for health provided in 2020. Continued efforts are needed to raise sufficient resources to mitigate the pandemic for the most vulnerable, and to help curtail the pandemic for all. Copyright (C) 2021 The Author(s). Published by Elsevier Ltd
Anemia prevalence in women of reproductive age in low- and middle-income countries between 2000 and 2018
No full textAnemia is a globally widespread condition in women and is associated with reduced economic productivity and increased mortality worldwide. Here we map annual 2000–2018 geospatial estimates of anemia prevalence in women of reproductive age (15–49 years) across 82 low- and middle-income countries (LMICs), stratify anemia by severity and aggregate results to policy-relevant administrative and national levels. Additionally, we provide subnational disparity analyses to provide a comprehensive overview of anemia prevalence inequalities within these countries and predict progress toward the World Health Organization’s Global Nutrition Target (WHO GNT) to reduce anemia by half by 2030. Our results demonstrate widespread moderate improvements in overall anemia prevalence but identify only three LMICs with a high probability of achieving the WHO GNT by 2030 at a national scale, and no LMIC is expected to achieve the target in all their subnational administrative units. Our maps show where large within-country disparities occur, as well as areas likely to fall short of the WHO GNT, offering precision public health tools so that adequate resource allocation and subsequent interventions can be targeted to the most vulnerable populations
