Akbari, Alireza | Quantum correlation dynamics of two qubits coupled to gapped quantum-critical environment | Abstract |
Bandopadhyay, Swarnali | Stochastic dynamics and feedback control in small magnetic system | Abstract |
Chatterjee, Shubhayu | Probing excitations in insulators by injecting spin currents | Abstract |
Costi, Theo | Charge Kondo Effect in thermoelectric properties of lead telluride doped with thallium impurities | Abstract |
Golosov, Denis | Stoner Magnetism in an Inversion Layer | Abstract |
Grushin, Adolfo G. | Stability of chern and fractional chern insulators | Abstract |
He, Yin-Chen | Spin liquids and their transition in kagome antiferromagnets | Abstract |
Hoyer, Mareike | Pair breaking due to orbital magnetism in iron-based superconductors | Abstract |
Joshi, Darshan G. | Nonlinear bond-operator theory and 1/d expansion for coupled-dimer magnets | Abstract |
Juricic, Vladimir | Interplay between electronic topology and crystal symmetry: Dislocation-line modes in topological insulators | Abstract |
Karnaukhov, Igor | Exactly solvable 2D topological Kondo lattice model | Abstract |
Kogan, Eugene | RKKY interaction in graphene | Abstract |
Komendova, Lucia | Band hybridization induced odd-frequency pairing in multi-band superconductors | Abstract |
Kopeć, Tadeusz | Bose condensation in systems with p-particle tunneling and multi-body interactions | Abstract |
Lahtinen, Ville | Weyl anomalous graphene on grain boundaries in topological band insulators | Abstract |
Meng, Tobias | Fractional topological phases in three-dimensional coupled-wire systems | Abstract |
Morampudi, Siddhardh | Diagnosing the statistics of excitations from the dynamical structure factor | Abstract |
Nghiem, Hoa | Time Evolution of a Quantum Impurity System in Response to a General Pulse: a td-NRG Study | Abstract |
Rachel, Stephan | Exotic phases in extended Kitaev models | Abstract |
Syzranov, Sergey | Non-Anderson localisation transition in high dimensions | Abstract |
Tchelidze, Tamar | Defect composition in acceptor doped ZnO quantum structures | Abstract |
Van Dyke, John | Local response of a heavy fermion superconductor to point defects | Abstract |
Wakeham, Nicholas | From Kondo behavior to high temperature superconductivity in Sr(Ni1-xFex)2As2 | Abstract |
Yee, Chuck-Hou | Guided design of copper oxysulfide superconductors | Abstract |
Zaleski, Tomasz | Structure factor of ultra-cold bosons in two-dimensional optical lattices | Abstract |
Zamani, Farzaneh | Steady state dynamics in a model system of strongly correlated electrons: Effective temperatures near local quantum criticality | Abstract |
Zhang, Jiang-min | Nonsmooth and level-resolved dynamics illustrated with a periodically driven tight-binding model | Abstract |
Quantum correlation dynamics of two qubits coupled to gapped quantum-critical environment Akbari, Alireza (Pohang University of Science and Technology (POSTECH), APCTP, Pohang, Korea, Republic of) |
The quantum correlation dynamics of two qubits which interact uniformly to a compass spin-chain environment are calculated and discussed. We study the energy gap induction on dy- namical process of the decoherence factor, entanglement and quantum discord in both weak and strong-coupling cases for two-qubit initially prepared in a class of X-structure state. We show that, gapped critical environment could remarkably prevent an enhanced decay of decoherence factor and quantum correlations at the critical point, which is nontrivially different from the ones in a gapless critical environment (Quan, et.al Rev. Lett. 96, 140604 (2006)). The results indicate that quan- tum correlations show very fast decaying at the critical point in the very beginning of time while maximum decaying occurs away from the critical point. In the week coupling regime the transient time at which the quantum correlations decay rapidly to their local minimum at the critical point of the environment, shows a power-law singularity as a function of gap, and also quantum correlations decay exponentially with second power of relaxation time. |
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Stochastic dynamics and feedback control in small magnetic system Bandopadhyay, Swarnali (TIFR Centre for Interdisciplinary Sciences (TCIS), India) |
Many of the information storage and revival devices, including magnetic read heads and random access memory elements utilize manipulation of magnetization in layered magnetic structures. The amount of transport current through magnetic multi-layered systems changes drastically depending on the relative orientation of magnetizations that can be controlled by external magnetic field, resulting in phenomena like giant magnetoresistance, or tunneling magnetoresistance. A spin-polarized current through layered magnetic systems can induce spin transfer torques (STT) initiating magnetization switching or precession. In this talk, I will discuss the role of different current induced torques (STT, field-like-toque) and alignment angle on switching time distribution. Again, the miniaturization of magnetic memory devices make them vulnerable to thermal fluctuations. In this case, current induced STT can be used to suppress the noise, enhancing fidelity of magnetic spin valves. Further, I will discuss entropy production, and related fluctuation theorems in such small single-domain magnetized systems. |
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Probing excitations in insulators by injecting spin currents Chatterjee, Shubhayu (Harvard University, Physics, Cmabridge, USA) |
Observation of fractional excitations in insulating spin-systems has been a long-sought goal in physics. In spite of promising evidence for observation of spin liquids, the exact nature of possible ground states, and in particular, the presence of a spin-gap is still unclear. Most experiments till this point have focused on thermodynamic measurements. We suggest a transport measurement as an alternate window into the nature of excitations of insulating spin systems. We couple a metal with a non-equilibrium spin-accumulation to an equilibrium insulating spin-system [1], and develop a general formalism to compute the spin current. We use this to calculate the current into ordered antiferromagnets as well as spin liquids, and note salient features in the spin conductance. [1] Takei et. al. Phys. Rev. B 90, 094408 (2014) |
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Charge Kondo Effect in thermoelectric properties of lead telluride doped with thallium impurities Costi, Theo (FZ Jülich, Peter Grünberg Institut and Institute for Advanced Simulation, PGI-2 and IAS-3, Jülich, Germany) |
Semiconducting PbTe is one of the most interesting materials for thermoelectric applications. When doped with a small concentration of Tl impurities, acting as acceptors, a number of anomalous properties are found: e.g.,beyond a critical concentration of about 0.3 at.% Tl, the system exhibits superconductivity with remarkably high critical temperatures for such a low carrier system. This, and other anomalous phenomena, prompted the idea that Tl impurities in PbTe act as negative-U centres giving rise to a charge Kondo effect and to superconductivity. We explore the consequences of this model for the normal state properties, and show that it can qualitatively explain the measured thermoelectric power, resistivity, thermopower, carrier concentration and point contact spectra. |
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Stoner Magnetism in an Inversion Layer Golosov, Denis (Bar-Ilan University, Department of Physics, Ramat-Gan, Israel) |
Motivated by recent experimental work on magnetic properties of Si-MOSFETs, we perform a calculation of magnetisation and susceptibility of electrons in an inversion layer, while taking into account the co-ordinate dependence of electron wave function in the direction perpendicular to the plane. It is assumed that the inversion-layer carriers interact via a contact repulsive potential, which is treated at a mean-field level, resulting in a self-consistent change of profile of the wave functions. We find that the results differ significantly from those obtained in the pure 2DEG case (where no provision is made for a quantum motion in the transverse direction). Specifically, the critical value of interaction needed to attain the ferromagnetic (Stoner) instability is decreased, and the Stoner criterion is therefore relaxed. Results are discussed in the context of the available experimental data. |
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Stability of chern and fractional chern insulators Grushin, Adolfo G. (Max Planck Institute for the Physics of Complex Systems (MPIPKS), Max Planck Institute for the Physics of Complex Systems, Condensed Matter, Dresden, Germany) |
Chern insulators (CI) and fractional Chern insulators (FCI) are zero field lattice analogues of the integer and fractional quantum Hall effects respectively. Here we address the important problem of when and how they are induced by interactions, an issue relevant for both cold-atomic and condensed matter experiments. For the former, we revisit the existing disagreement between mean field theory results and exact diagonalization using the infinite density matrix renormalization group (iDMRG) approach. We show that quantum fluctuations prevent the emergence of the Haldane CI state in the honeycomb lattice via short range interactions. For the FCI state we exemplify its full numerical characterization with the help of iDMRG, a method which will allows us to address the character of the Metal-FCI phase transition, a possible benchmark for future experiments. |
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Spin liquids and their transition in kagome antiferromagnets He, Yin-Chen (Max Planck Institute for the Physics of Complex Systems (MPIPKS), Max Planck Institute for the Physics of Complex Systems (MPIPKS), Condensed matter, Dresden, Germany) |
In recent years, growing experimental and theoretical evidence suggests the existence of quantum spin liquid phase in kagome antiferromagnets, however its nature is still controversial. In this talk, I will introduce our study on kagome antiferromagnets with XXZ anisotropy. Numerically (by DMRG), we find that the emergence of the spin-liquid phase is independent of the anisotropy of the XXZ interaction. In particular, the two extreme limits-the easy-axis and the easy-plane—host the same spin-liquid phases as the isotropic Heisenberg model. Both a time-reversal-invariant spin liquid and a chiral spin liquid with spontaneous time-reversal symmetry breaking are obtained. We show that they evolve continuously into each other by tuning the second- and the third-neighbor interactions. Theoretically, we focus on the strong easy axis limit, by performing a duality transformation we obtain an effective model, which is basically a compact U(1)-Higgs lattice gauge model. I will also discuss the possible spin liquid phase of this effective model, which naively is not captured by Anderson's RVB picture for spin liquid. |
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Pair breaking due to orbital magnetism in iron-based superconductors Hoyer, Mareike (Karlsruhe Institute of Technology (KIT), Institute for Theoretical Condensed Matter Physics, Karlsruhe, Germany) |
We consider superconductivity in the presence of impurities in a two-band model suited for the description of iron-based superconductors. We analyze the effect of interband scattering processes on superconductivity, allowing for orbital, i.e., nonspin-magnetic but time-reversal symmetry-breaking impurities. Pair breaking in such systems is described by a nontrivial phase in an interband-scattering matrix element. We find that the transition temperature of conventional superconductors can be suppressed due to interband scattering, whereas unconventional superconductors may be unaffected. We also discuss the stability of density wave phases in the presence of impurities. As an example, we consider impurities associated with imaginary charge density waves that are of interest for iron-based superconductors. |
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Nonlinear bond-operator theory and 1/d expansion for coupled-dimer magnets Joshi, Darshan G. (TU Dresden, Institut für Theoretische Physik, Dresden, Germany) |
For coupled-dimer Heisenberg magnets, a paradigm of magnetic quantum phase transition, we develop a systematic expansion in 1/d, where d is spatial dimension, using bond operators. We apply this technique to a model of dimers on a hyper-cubic lattice, a generalization of the square-lattice bilayer Heisenberg model to arbitrary d. We calculate physical observables at zero temperature in both the quantum paramagnetic and antiferromagnetic phases and show that the 1/d expansion consistently describes the entire phase diagram including the quantum critical point. In particular, we determine the dispersion and spectral weight distribution of the elementary excitations, including the amplitude (i.e., Higgs) mode of the antiferromagnetic phase. |
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Interplay between electronic topology and crystal symmetry: Dislocation-line modes in topological insulators Juricic, Vladimir (Universiteit Utrecht, Institute for Theoretical Physics, Utrecht, Netherlands) |
Topological insulators are states of quantum matter that, due to the presence of time-reversal symmetry, feature an insulating bulk band gap together with metallic edge or surface states protected by a Z2 topological invariant. I will first discuss the role of crystal symmetries in the physics of these states of quantum matter, and derive the classification of topological band insulators taking into account crystalline symmetries [1]. I will then consider dislocations as probes of different topological states resulting from this classification. As a warm up, I will first discuss the case of two-dimensional topological insulators [2]. The main part of the talk will be devoted to the response of dislocation lines in three-dimensional topological insulators. In particular, I will introduce the K-b-t rule which gives a precise condition for the appearance of the propagating modes in the core of a lattice dislocation oriented along a vector ${bf t}$ and with a Burgers vector b, in a topologically-insulating state characterized by the band-inversion momenta Kinv [3]. I will then consider some concrete examples of topological phases protected by crystalline symmetries and show numerical evidence for the appearance of the dislocation-line modes therein. Finally, I will discuss possible experimentally consequential examples in which the modes are oblivious to the direction of propagation, such as recently proposed topological state in electron-doped BaBiO3 with the cubic (Oh) symmetry. [1] R.-J. Slager, A. Mesaros, V. Juricic, and J. Zaanen, Nature Physics 9, 98 (2013). [2] V. Juricic, A. Mesaros, R.-J. Slager, and J. Zaanen, Phys. Rev. Lett.108, 106403 (2012). [3] R.-J Slager, A. Mesaros, V. Juricic, and J. Zaanen, Phys. Rev. B 90, 241403(R) (2014). |
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Exactly solvable 2D topological Kondo lattice model Karnaukhov, Igor (The National Academy of Sciences of Ukraine, G. V. Kurdyumov Institute for Metal Physics, Department of Theory of Nonideal Crystals, Kyiv , Ukraine) |
A spin-1/2 Kitaev sublattice interacting with a subsystem of spinless fermions is studied on a honeycomb lattice when the fermion band is half-filled. The model Hamiltonian describes a topological Kondo lattice with the Kitaev interaction, it is solved exactly by reduction to free Majorana fermions in a static Z2 gauge field. A yet unsolved problem of a hybridization of fermions and local moments in the Kondo lattice at low temperatures is solved in the framework of the proposed model. The Kondo hybridization gap is opened and the system is fixed in insulator and spin insulator states, due to the spin-fermion nature of the gap. We will show that the hybridization between local moments and itinerant fermions should be understood as hybridization between corresponding Majorana fermions of the spin and charge sectors. The RKKI interaction between local moments is not realized in the model, a system demonstrates a ''quasi-Kondo'' scenario of behavior with realization chiral gapless edge states in topological nontrivial phases. The ground-state phase diagram of the interacting subsystems calculated in the parameter space is rich. |
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RKKY interaction in graphene Kogan, Eugene (Bar-Ilan University, Jack and Pearl Resnick Institute of Advanced Technology, Physics, Ramat-Gan, Israel) |
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities in graphene was theoretically studied quite intensely during last several years[1-9]. One may ask, why the problem, which is in principle so simple (in the lowest order of perturbation theory, as it was treated in all the papers referenced above, the problem is equivalent to calculation of a single bubble diagram), was the subject of so many publication, using different approaches? One of the answers to this question is connected with the fact that a simply written integral is not necessarily a simply calculated integral, and formally identical formulas can give different results. So much so, that in some of the referenced papers one has to deal with divergent integrals, while in the others, only convergent integrals appear. We'll present a review of existing approaches to the calculation of RKKY interaction in graphene and of the obtained results. We'll also briefly mention RKKY interaction in bilayer graphene and in molybdenum disulfide. References [1] V. K. Dugaev, V. I. Litvinov and J. Barnas, Phys. Rev. B 74, 224438 (2006). [2] L. Brey, H. A. Fertig and S. D. Sarma, Phys. Rev. Lett. 99, 116802 (2007). [3] S. Saremi, Phys. Rev. B 76, 184430 (2007). [4] A. M. Black--Schaffer, Phys. Rev. B 81, 205416 (2010). [5] В M. Sherafati and S. Satpathy, Phys. Rev. B 83, 165425 (2011). [6] B. Uchoa, T. G. Rappoport, and A. H. Castro Neto, Phys. Rev. Lett. 106, 016801 (2011). [7] J. E. Bunder and H.-H. Lin, Phys. Rev. B 80, 153414 (2009). [8] E. Kogan, Phys. Rev. B 84, 115119 (2011). [9] E. Kogan, Graphene 2, N 1, 8 (2013). |
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Band hybridization induced odd-frequency pairing in multi-band superconductors Komendova, Lucia (Uppsala University, Department of Physics and Astronomy, Materials Theory, Uppsala, Sweden) |
We investigated how the hybridization (single-particle tunneling) between two superconducting bands induces odd-frequency superconductivity in the system. We found explicit formula for the odd-frequency pairing correlations. We also calculated the density of states, which gets modified from the sum of the two BCS spectra and develops additional hybridization gaps at higher energies. Full article: L. Komendová, A. V. Balatsky, A. M. Black-Schaffer, arXiv:1506.05395 |
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Bose condensation in systems with p-particle tunneling and multi-body interactions Kopeć, Tadeusz (Insitute of Low Temperature ans Structure Research, I, Wroclaw, Poland) |
In this work we have studied a class of Hamiltonians that generalises Bose-Hubbard (BH) model of interacting bosons to include p-particle hopping as well as the multi-body interactions Ul(l≥2). While the case of two body interaction (U2 ≡ U) and single particle tunneling (p=1) is well known, it interesting to examine if the the resulting phase diagram for the general case will have different features from that of the standard BH system. To this end we have applied a mean-filed theory to to locate critical lines, combined with the resolvent and Laplace transform method for the efficient calculation of the statistical sum for the quantum Hamiltonian. We have studied both ground state properties and temperature evolution of phase diagram. Several new features, as an expanding of Mott-insulator lobes for multi-body interaction and the emergence of a new insulating plateaus for p>2, have been demonstrated. These results may be interest for experiments on cold atom systems in optical lattices. |
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Weyl anomalous graphene on grain boundaries in topological band insulators Lahtinen, Ville (FU Berlin, Dahlem Center for Complex Quantum Systems, Physics, Berlin, Germany) |
Authors: Robert-Jan Slager, Vladimir Juricić, Ville Lahtinen and Jan Zaanen Abstract: Semi-metallic materials, such as graphene in two dimensions (2D) and vari- ous Dirac and Weyl semi-metals in three dimensions (3D), are characterized by nodal band structures that give rise to exotic electronic properties. Their stability requires the presence of lattice symmetries or application of external fields, making them lack the inherent topological protection enjoyed by surface states of topological band insulators. Here we bridge this divide by showing that a topologically protected 2D semi-metal exhibiting an odd-integer quantum Hall effect and ballistic conductance the Weyl anomalous graphene can emerge and be experimentally observed in the bulk of a 3D topological insulator. This state emerges on a grain boundary, a ubiquitous extended lattice defect in any crystalline material, thereby providing a novel and experimentally accessible route to topological semi-metals. The underlying mechanism is the hybridization of spinon modes bound to the grain boundary, whose generality suggests that new states of matter can emerge in any topological band insulator where lattice dislocations bind localized topological modes. |
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Fractional topological phases in three-dimensional coupled-wire systems Meng, Tobias (Technische Universität Dresden, Institut für Theoretische Physik, Department of Physics, Dresden, Germany) |
While the understanding of fractionalized topological phases has impressively developed in two dimensions (2D), much less is known in three dimensions (3D). In this poster, I present a 3D system of coupled quantum wires that exhibits fractional topological phases composed of closed loops and open planes of two-dimensional fractional quantum Hall subsystems, and illustrate that the coupled-wire approach provides a new, and powerful tool for the analysis of interacting topological physics in 3D. In the proposed array, the coupled wire approach allows to identify the protected edge states associated with the topologically non-trivial bulk gapped phases. It also shows that these phases are separated by exotic quantum phase transitions corresponding to a rearrangement of fractional quantum Hall edge modes. Also an extended exotic critical phase may exist. Without electron-electron interactions, similar but unfractionalized bulk gapped phases based on coupled integer quantum Hall states exist. They are separated by an extended critical Weyl semimetal phase. |
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Diagnosing the statistics of excitations from the dynamical structure factor Morampudi, Siddhardh (Max Planck Institute for Physics of Complex Systems, Dresden, Germany) |
We show that the statistics of excitations in quantum spin liquids yield characteristic features in the dynamical structure factor (DSF). Quantum spin liquids are exotic phases of matter which fall beyond the traditional paradigm of symmetry breaking. Originally proposed by Anderson with regard to high temperature superconductivity, they are now widely believed to arise in frustrated spin systems such as the antiferromagnetic Heisenberg model on the kagome lattice. Recently, various theoretical methods to characterize spin liquids have been introduced, especially with regard to numerical simulations. In this work, we obtain results connecting the statistics of the excitations to features of the DSF which can be obtained from neutron scattering. More precisely, we show that the statistics can be deduced from the energy dependence of the DSF. We also use numerical exact diagonalization to confirm the results on finite size systems. |
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Time Evolution of a Quantum Impurity System in Response to a General Pulse: a td-NRG Study Nghiem, Hoa (Forschungszentrum Jülich GmbH, Peter Grünberg Institut and Institute for Advanced Simulation, Jülich, Germany) |
To study the time evolution of a quantum impurity system in response to a general pulse, we generalize the time-dependent numerical renormalization group (td-NRG) approach for a single quench to multiple quenches and also generalize the method to arbitrary temperatures. We measure both static and dynamic observables, e.g. occupation number and spectral function. In strongly correlated systems, we quantify the results in the short and long time limits by comparing them to the known thermodynamic values of the initial and final states. In the short time limit, the td-NRG result is numerically exact to the thermodynamic value for general pulses, while, in the long time limit, we obtain the results close to the expected values by varying the pulse slowly enough. |
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Exotic phases in extended Kitaev models Rachel, Stephan (TU Dresden, Institute for Theoretical Physics, Dresden, Germany) |
Heisenberg-Kitaev models have been successfully used to describe the exotic magnetic phases as well as the predicted spin-liquid phases of certain honeycomb-lattice transition metal oxides (TMO). While the original proposal of Jackeli and Khaliullin contained a nearest-neighbor Heisenberg spin exchange as well as the exactly solvable nearest-neighbor Kitaev-term, recently it has been pointed out that second-neighbor Kitaev exchange plays a significant role. Here we propose the K1-K2 model featuring the following properties: (i) it describes the magnetic phases of all the debated honeycomb lattice TMOs and serves as a minimal model, (ii) it contains not only the Kitaev spin liquids but also a plethora of exotic magnetic phases, (iii) its zero-temperature quantum phase diagram is fully distinct from its classical finite-temperature counterpart. |
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Non-Anderson localisation transition in high dimensions Syzranov, Sergey (University of Colorado at Boulder, Physics Department, Boulder, USA) |
It is usually believed that increasing disorder strength in a d>2-dimensional system leads to the Anderson localisation transition with universal properties depending only on the space dimensionality. We demonstrate that systems with a power-law quasiparticle dispersion ξk ∝ kα in dimensions d>2α exhibit another type of a disorder-driven quantum phase transition at the bottom of the band, that lies in a universality class distinct from the Anderson transition. In contrast to the conventional wisdom, it manifests itself in, e.g., the disorder-averaged density of states. For systems in symmetry classes that permit localisation, the striking signature of this transition is a non-analytic behaviour of the mobility edge, that is pinned to the bottom of the band for subcritical disorder and grows for disorder exceeding a critical strength. Focussing on the conductivity and the density of states, we calculate the critical behaviour (exponents and scaling functions), using a renormalisation group, controlled by an ε = 2α-d expansion. We also discuss how this novel transition can be observed in 3D Weyl semimetals and in recently realised 1D chains of ultracold ions with long-range hopping. |
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Defect composition in acceptor doped ZnO quantum structures Tchelidze, Tamar (Ivane Javakhishvili Tbilisi State University, Faculty of Exact and Natural Sciences, Physics, Tbilisi, Georgia) |
We investigated defect composition in ZnO quantum structures - quantum wells (QW), quantum wires (QW) and Quantum dots doped with different impurities. The motivation is that ZnO suffers from the doping asymmetry problem, in that it can be doped n-type rather easily, but highly p-type doping is still problematic. This fact continues to impede the development of ZnO-based light emitters. The dificulties of p-type doping can arise from a variety of causes. One of the main reasons that makes difficult obtaining low ohmic hole conductivity is compensation of dopants by low energy native defects, such as VO or Zni, or background impurities. Compensating processes is strongly affected by electronic structure of system: band gap, ionization energies of donors, acceptors and their compensation centers, formation enthalpy of defects. Our aim was to reveal the way size confinement alters compensation processes, and to find optimal sizes of quantum structures for which it can be suppressed. For this porpose we calculated acceptor and donor ionization energies for ZnO QW, QD, and NW of different size. The space and dielectric confinement is taken into account. These activation energies were used then in the Kroger method of quasi-chemical equations, which gives defect, impurities and charge carriers concentrations vs. temperature and oxygen pressure. The calculation for Ag doped ZnO NW shows that for NW of definite radius p-conductivity becomes achievable. We defined optimal for p-conductivity geometric sizes for ZnO QD and QW doped with Li and Na atoms too. The existence of optimal sizes in ZnO quantum structures is connected to the fact that acceptor ionization energy is less sensitive to space and dielectric confinement, than those for compensating donors. In NW, e.g., the ionization energy of oxygen vacancy remains higher than corresponding bulk value until r=8nm, while ionization energy of acceptor impurity returns to its bulk value for r=3.5 nm. |
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Local response of a heavy fermion superconductor to point defects Van Dyke, John (University of Illinois at Chicago, Physics, Chicago, USA) |
We calculate the differential conductance dI/dV for an STM study of the heavy fermion compound CeCoIn5 in the superconducting state. The results agree with recent experiments and illustrate the multi-gap nature of the system. The local response in the vicinity of point defects is determined, leading to impurity bound states that are also seen experimentally. Finally, a self-consistent calculation reveals the qualitative changes expected in the hybridization, magnetic correlations, and superconducting gap near defects point-like defects. |
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From Kondo behavior to high temperature superconductivity in Sr(Ni1-xFex)2As2 Wakeham, Nicholas (Los Alamos National Laboratory, MPA-CMMS, Los Alamos, USA) |
Sr(Ni1-xFex)2As2 is a conventional superconductor with Tcsim 0.6,K, while Sr(Ni1-xFex)2As2 has an antiferromagnetic groundstate at ambient pressure. It has been shown that in Sr(Ni1-xFex)2As2 there is a dome of superconductivity between x=0.95 and x=0.9, resembling the phase diagram of Ce-based heavy fermion superconductors. However, Sr(Ni1-xFex)2As2 has been little studied for small values of x. In particular, interesting questions in this system are whether there is evidence for single ion or possibly even coherent Kondo physics, how this evolves with Fe concentration, and whether it is relevant to the superconductivity. Here, we present the systematic study of Sr(Ni1-xFex)2As2 through measurements of the specific heat, magnetic susceptibility and resistivity. At x<0.01 we find evidence for single ion Kondo behavior, and at concentrations greater than xsim 0.2 there is a transition into a spin-glass state. Evidence for this spin-glass state disappears before reaching the large concentrations of Fe in which superconductivity is observed. The concentration and energy scales governing these transitions will be discussed. |
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Guided design of copper oxysulfide superconductors Yee, Chuck-Hou (Rutgers University, Physics & Astronomy, Piscataway, USA) |
We describe a framework for designing novel materials, combining modern first-principles electronic structure tools, materials databases, and evolutionary algorithms capable of exploring large configurational spaces. Guided by the chemical principles introduced by Antipov, emph {et. al.}, for the design and synthesis of the Hg-based high-temperature superconductors, we apply our framework to screen 333 proposed compositions to design a new layered copper oxysulfide, Hg(CaS)2CuO2. We evaluate the prospects of superconductivity in this oxysulfide using theories based on charge-transfer energies, orbital distillation and uniaxial strain. |
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Structure factor of ultra-cold bosons in two-dimensional optical lattices Zaleski, Tomasz (Polish Academy of Sciences, Institute of Low Temperature and Structure Research, Department of Solid State Theory, Wroclaw, Poland) |
We study the structure factor of the interacting ultra-cold atoms in a square optical lattice. Using a combined Bogoliubov method and the quantum-rotor approach, the Bose-Hubbard Hamiltonian of strongly interacting bosons is mapped onto the U(1)-phase action. This allows to calculate the momentum and energy dependence of the structure factor in the presence of the Mott insulator and superfluid phases. It is shown that superfluidity manifests itself as a sharp coherence peak resulting from the emergence of the long-range order. On the other hand, correlation effects lead to the appearance of a smearing of the excitation spectra of incoherent particles although the remnants of the Bogoliubov band are still present in the part linked to coherent particles. |
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Nonsmooth and level-resolved dynamics illustrated with a periodically driven tight-binding model Zhang, Jiang-min (Max-Planck-Institut für Physik komplexer Systeme, Condensed matter (Masudul Haque) |
We point out that in the first-order time-dependent perturbation theory, the transition probability may behave nonsmoothly in time and have kinks periodically. Moreover, the detailed temporal evolution can be sensitive to the exact locations of the eigenvalues in the continuum spectrum, in contrast to coarse-graining ideas. Underlying this nonsmooth and level-resolved dynamics is a simple equality about the sinc function sinc x ≡ sin x/x. These physical effects appear in many systems with approximately equally spaced spectra, and are also robust for larger amplitude coupling beyond the domain of perturbation theory. We use a one-dimensional periodically driven tight-binding model to illustrate these effects, both within and outside the perturbative regime. [1] J. M. Zhang and M. Haque, ScienceOpen Research (2014). |
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