| 1. |
- Banerjee, Saikat, 1989-
(författare)
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Interacting Dirac Matter
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The discovery of graphene in 2004 has led to a surge of activities focused on the theoretical and experimental studies of materials hosting linearly dispersive quasiparticles during the last decade. Rapid expansion in the list of materials having similar properties to graphene has led to the emergence of a new class of materials known as the Dirac materials. The low energy quasiparticles in this class of materials are described by a Dirac-like equation in contrast to the Schrödinger equation which governs the low energy dynamics in any conventional materials such as metals. The Dirac fermions, as we call these low-energy quasiparticles, in a wide range of materials ranging from the d-wave superconductors, graphene to the surface states of topological insulators share the common property. The particles move around as if they have lost their mass. This feature results in a completely new set of physical effects consisting of various transport and thermodynamic quantities, that are absent in conventional metals.This thesis is devoted to studying the properties of bosonic analogs of the commonly known Dirac materials where the quasiparticle are fermionic. In chapter one, we discuss the microscopic origin of the Dirac equation in several fermionic and bosonic systems. We observe identical features of the Dirac materials with quasiparticles of either statistics when the interparticle interaction is absent. Dirac materials with both types of quasiparticles possess the nodal excitations that are described by an effective Dirac-like equation. The possible physical effects due to the linear dispersions in fermionic and bosonic Dirac materials are also outlined.In chapter two, we propose a system of superconducting grains arranged in honeycomb lattice as a realization for Bosonic Dirac Materials (BDM). The underlying microscopic dynamics, which give rise to the emergence of Dirac structure in the spectrum of the collective phase oscillations, is discussed in detail. Similarities and differences of BDM systems to the conventional Dirac materials with fermionic quasiparticles are also mentioned. Chapter three is dedicated to the detailed analysis of the interaction effects on the stability and renormalization of the conical Dirac band structure. We find that the type of interaction dictates the possible fate of renormalized Dirac cone in both fermionic and bosonic Dirac materials. We study interaction effects in four different individual systems : (a) Dirac fermions in graphene interacting via Coulomb interactions, (b) Dirac fermions subjected to an onsite Hubbard repulsion, (c) Coulomb repulsion in charged Cooper pairs in honeycomb lattice and (d) Dirac magnons interacting via Heisenberg exchange interaction. The possibility of interaction induced gap opening at the Dirac nodal point described is also discussed in these cases.Chapter four mainly concerns the study of a related topic of the synthetic gauge fields. We discuss the possibility of Landau quantization in neutral particles. Possible experimental evidence in toroidal cold atomic traps is also mentioned. A connection to Landau levels in case of magnons is also described. We finally conclude our thesis in chapter five and discuss the possible future directions that can be taken as an extension for our works in interacting Dirac materials.
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| 2. |
- Bouman, Daniël, et al.
(författare)
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Triplet-blockaded Josephson supercurrent in double quantum dots
- 2020
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 102:22
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Tidskriftsartikel (refereegranskat)abstract
- Serial double quantum dots created in semiconductor nanostructures provide a versatile platform for investigating two-electron spin quantum states, which can be tuned by electrostatic gating and an external magnetic field. In this Rapid Communication, we directly measure the supercurrent reversal between adjacent charge states of an InAs nanowire double quantum dot with superconducting leads, in good agreement with theoretical models. In the even charge parity sector, we observe a supercurrent blockade with increasing magnetic field, corresponding to the spin singlet to triplet transition. Our results demonstrate a direct spin to supercurrent conversion, the superconducting equivalent of the Pauli spin blockade. This effect can be exploited in hybrid quantum architectures coupling the quantum states of spin systems and superconducting circuits.
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| 3. |
- Kirsanskas, Gediminas, et al.
(författare)
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Yu-Shiba-Rusinov states in phase-biased superconductor-quantum dot-superconductor junctions
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 92:23
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Tidskriftsartikel (refereegranskat)abstract
- We study the effects of a phase difference on Yu-Shiba-Rusinov (YSR) states in a spinful Coulomb-blockaded quantum dot contacted by a superconducting loop. In the limit where charging energy is larger than the superconducting gap, we determine the subgap excitation spectrum, the corresponding supercurrent, and the differential conductance as measured by a normal-metal tunnel probe. In absence of a phase difference only one linear combination of the superconductor lead electrons couples to the spin, which gives a single YSR state. With finite phase difference, however, it is effectively a two-channel scattering problem and therefore an additional state emerges from the gap edge. The energy of the phase-dependent YSR states depend on the gate voltage and one state can cross zero energy twice inside the valley with odd occupancy. These crossings are shifted by the phase difference towards the charge degeneracy points, corresponding to larger exchange couplings. Moreover, the zero-energy crossings give rise to resonant peaks in the differential conductance with magnitude 4e(2)/h. Finally, we demonstrate that the quantum fluctuations of the dot spin do not alter qualitatively any of the results.
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| 4. |
- Korsholm, Kasper, et al.
(författare)
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Left Atrial Appendage Occlusion versus Novel Oral Anticoagulation for Stroke Prevention in Atrial Fibrillation: Rationale and Design of the Multicenter Randomized Occlusion-AF Trial.
- 2021
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Ingår i: American heart journal. - : Elsevier BV. - 1097-6744 .- 0002-8703. ; 243, s. 28-38
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Tidskriftsartikel (refereegranskat)abstract
- The prevalence of atrial fibrillation (AF) is increasing globally, which is a major clinical and public health concern due to the five-fold increased risk of stroke. Oral anticoagulation with novel oral anticoagulants (NOACs) is the current primary option for stroke prevention in patients with AF, although it increases the risk of major bleeding. Patients with prior ischemic cerebrovascular events are at particularly high risk of both recurrent ischemic events and major bleeding. Left atrial appendage occlusion (LAAO) provides an alternative option for stroke prevention in high-risk patients, however, with currently limited evidence. Thus, randomized trials comparing LAAO to NOACs are needed.The Occlusion-AF trial is designed to assess whether LAAO is non-inferior to NOAC therapy for reduction of the combined endpoint of stroke, systemic embolism, major bleeding (Bleeding Academic Research Consortium ≥ 3) and all-cause mortality in patients with AF and a recent ischemic stroke or transient ischemic attack (TIA).Investigator-initiated multicenter, multinational, randomized open-label non-inferiority trial with blinded outcome evaluation (PROBE design). Patients with documented AF, and an ischemic stroke or TIA within 6 months will be eligible for enrollment. Major exclusion criteria are modified Rankin Scale > 3 at enrollment, glomerular filtration rate < 15 ml/min, and life-expectancy less than 2 years. A total of 750 patients will be randomized 1:1 to receive either a NOAC or LAAO using the Amplatzer Amulet (Abbott, MN, USA) or Watchman FLX (Boston Scientific, MN, USA) with subsequent life-long aspirin 75 mg daily. Follow-up will be based on in-office and telephone follow-up in combination with long-term follow-up (10 years) through national hospital discharge registries in the individual Nordic countries. The primary outcome will be a composite endpoint of stroke, systemic embolism, major bleeding (BARC ≥ 3) and all-cause mortality at 2-year follow-up.The Occlusion-AF trial is designed to compare LAAO to NOAC therapy for secondary stroke prevention in AF patients with a high risk of recurrent thromboembolic events, i.e. with previous ischemic stroke or TIA, and otherwise eligible for anticoagulation. The results are expected to contribute significantly to the understanding of the effects of LAAO compared to the standard contemporary pharmacological treatment in these patients.
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| 5. |
- Krarup, Niels Henrik, et al.
(författare)
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Quality of cardiopulmonary resuscitation in out-of-hospital cardiac arrest is hampered by interruptions in chest compressions-A nationwide prospective feasibility study
- 2011
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Ingår i: Resuscitation. - : Elsevier BV. - 1873-1570 .- 0300-9572. ; 82:3, s. 263-269
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Tidskriftsartikel (refereegranskat)abstract
- Aim of the study: Quality of cardiopulmonary resuscitation (CPR) is a critical determinant of outcome following out-of-hospital cardiac arrest. The aim of our study was to evaluate the quality of CPR provided by emergency medical service providers (Basic Life Support (BLS) capability) and emergency medical service providers assisted by paramedics, nurse anesthetists or physician-manned ambulances (Advanced Life Support (ALS) capability) in a nationwide, unselected cohort of out-of-hospital cardiac arrest cases. Methods: We conducted a prospective, observational study of out-of-hospital cardiac arrest with non-traumatic etiology (>18 years of age) occurring from the 1st to the 31st of January 2009 and treated by the primary Danish emergency medical service operator, covering approximately 85% of the population. One hundred and ninety-one cases were eligible for analysis. Follow-up was up to one year or death. Quality of CPR was evaluated using measurements of transthoracic impedance. Results: The majority of patients were treated by ambulances with ALS capability (54%). Interruptions in CPR related to loading of the patient into the emergency medical service vehicle were substantial, but independent of whether patients were managed by ALS or BLS capable units (222s versus 224s, P=0.76) as were duration of interruptions during rhythm analysis alone (20s versus 22s, P=0.33) and defibrillation (24s versus 26s, P=0.07). Conclusions: Nationwide, routine monitoring of transthoracic impedance is feasible. CPR is hampered by extended interruptions, particularly during loading of the patient into the emergency medical service vehicle, rhythm analysis and defibrillation. (C) 2010 Elsevier Ireland Ltd. All rights reserved.
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| 6. |
- Nielsen-Kudsk, Jens Erik, et al.
(författare)
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Left atrial appendage occlusion versus standard medical care in patients with atrial fibrillation and intracerebral haemorrhage : a propensity score-matched follow-up study
- 2017
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Ingår i: EuroIntervention. - : EUROPA EDITION. - 1774-024X .- 1969-6213. ; 13:3, s. 371-378
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Tidskriftsartikel (refereegranskat)abstract
- Aims: The aim of this study was to investigate the prognosis in patients with atrial fibrillation (AF) and intracerebral haemorrhage (ICH) having a left atrial appendage occlusion (LAAO) versus patients receiving standard medical therapy. Methods and results: A total of 151 patients from the Nordic countries with AF and previous ICH who underwent LAAO using the AMPLATZER Cardiac Plug or the AMPLATZER AMULET were compared to a propensity score-matched group of 151 patients receiving standard medical therapy. The two groups were matched so that their risks for stroke and bleeding were similar (CHA2DS2-VASc and HAS-BLED scores). The standard care patients were identified from the Danish Stroke Registry among 787 patients with AF and ICH. The primary endpoint was a composite of all-cause mortality, ischaemic stroke and major bleeding. Patients with AF and a prior ICH treated with LAAO had a lower risk of the composite outcome as compared to patients treated with standard medical care (events/1,000 years [95% confidence interval]: 53.3 [44.3-64.1] vs. 366.7 [298.2-450.9]; hazard ratio 0.16 [0.07-0.37]). Conclusions: LAAO is suggested to be of major clinical benefit in AF patients having sustained an ICH. These results have to be confirmed in a randomised clinical trial.
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| 7. |
- Ortega-Taberner, Carlos, 1993-, et al.
(författare)
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Anomalous Josephson current through a driven double quantum dot
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:11
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Tidskriftsartikel (refereegranskat)abstract
- Josephson junctions based on quantum dots offer a convenient tunability by means of local gates. Here we analyze a Josephson junction based on a serial double quantum dot in which the two dots are individually gated by phase-shifted microwave tones of equal frequency. We calculate the time-averaged current across the junction and determine how the phase shift between the drives modifies the current-phase relation of the junction. Breaking particle-hole symmetry on the dots is found to give rise to a finite average anomalous Josephson current with phase bias between the superconductors fixed to zero. This microwave gated weak link thus realizes a tunable "Floquet v0 junction" with maximum critical current achieved for driving frequencies slightly off resonance with the subgap excitation energy. We provide numerical results supported by an analytical analysis for infinite superconducting gap and weak interdot coupling. We identify an interaction-driven 0-ir transition of anomalous Josephson current as a function of driving phase difference. Finally, we show that this junction can be tuned so as to provide for complete rectification of the time-averaged Josephson current-phase relation.
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| 8. |
- Pedersen, Kim G. L., et al.
(författare)
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Quantum interference in off-resonant transport through single molecules
- 2014
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 90:12
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Tidskriftsartikel (refereegranskat)abstract
- We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance.
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