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Single Particle Excitation Spectrum of a Proximized Quantum Dot: The Flow Equation Study

100%
Zapalska M., Domański T.
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vol. 126
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issue 4a
A-137-A-140
EN
We study the in-gap states of a quantum dot hybridized with the metallic and superconducting reservoirs applying the continuous unitary transformation to the Anderson-type Hamiltonian. We derive the set of flow equations and analyse the effective single particle excitation spectrum of the correlated quantum dot in presence of the induced electron pairing.
2
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Application of Contractor Renormalization Group to the Heisenberg Zig-Zag and the Hubbard Chain

100%
Cichy K., Tomczak P.
Acta Physica Polonica A
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2007
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vol. 111
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issue 5
773-780
EN
The contractor renormalization group method was devised in 1994 by Morningstar and Weinstein. It was primarily aimed at extracting the physics of lattice quantum field theories (like lattice quantum chromodynamics). However, it is a general method of analyzing Hamiltonian lattice systems, e.g. Ising, Heisenberg or Hubbard models. The aim of this work is to show the application of contractor renormalization group method to one-dimensional quantum systems - the Heisenberg zig-zag model and the Hubbard chain. As a test of the method, the ground state energy of these systems will be calculated.
3
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Order in Quantum Compass and Orbital e_{g} Models

100%
Czarnik P., Dziarmaga J., Oleś A.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
359-361
EN
We investigate thermodynamic phase transitions in the compass model and in e_{g} orbital model on an infinite square lattice by variational tensor network renormalization (VTNR) in imaginary time. The onset of nematic order in the quantum compass model is estimated at 𝓣_{c}/J=0.0606(4). For the e_{g} orbital model one finds: (i) a very accurate estimate of 𝓣_{c}/J=0.3566± 0.0001 and (ii) the critical exponents in the Ising universality class. Remarkably large difference in frustration results in so distinct values of 𝓣_{c}, while entanglement influences the quality of 𝓣_{c} estimation.
4
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Renormalization Group Approach for the Double Exchange Ferromagnets

100%
Zapalska M., Domański T.
Acta Physica Polonica A
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2012
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vol. 122
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issue 6
1099-1101
EN
The discovery of the colossal magnetoresistance (CMR) in the manganese oxides with perovskite structures T_1 - x DMnO_3 (T = La, Pr, Nd; D=Sr, Ca, Ba, Pb) and its potential technological application motivated theoretical and experimental researchers to study the itinerant ferromagnetism. A first theoretical description of this phenomenon in terms of the double-exchange mechanism was given a long time ago by Zener. In this model, the spin orientation of adjacent Mn-moments is associated with kinetic exchange of conduction e_g electrons. Consequently, alignment of the core Mn-spins by an external magnetic field causes higher conductivity. The Mn ions are considered as localized forming a spin of S = 3/2 and they are coupled to the itinerant electrons by a strong ferromagnetic Hund coupling, J_H > 0. We apply the flow equation technique (nonperturbative method, based on continuous canonical transformation) to the double-exchange model for ferromagnetism described by the Kondo type Hamiltonian. We want to eliminate the interaction term responsible for non-conservation of magnon number and to take into account fermion and magnon degrees of freedom. We express the spin operators of Mn ions via the magnon operators (the Holstein-Primakoff transformation) and investigate the magnon excitation spectrum determined by Green's function.
5
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Electromagnetic Response of the BCS Superconductor: Flow Equation Approach

100%
Zapalska M., Domański T.
Acta Physica Polonica A
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2012
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vol. 121
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issue 4
854-857
EN
We address the linear response for superconductors using the non-perturbative technique based on the continuous unitary transformation. We discuss how this method rigorously reproduces the standard BCS solution and point out how such result can be generalized to the case of electron systems with strong pairing fluctuations and in presence of the nonequilibrium phenomena.
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