All (qubit) decoherences: Complete characterization and physical implementation

We investigate decoherence channels that are modelled as a sequence of collisions of a quantum system (e.g., a qubit) with particles (e.g., qubits) of the environment. We show that collisions induce decoherence when a bi-partite interaction between the system qubit and an environment (reservoir) qubit is described by the controlled-U unitary transformation (gate). We characterize decoherence channels and in the case of a qubit we specify the most general decoherence channel and derive a corresponding master equation. Finally, we analyze entanglement that is generated during the process of decoherence between the system and its environment.Comment: 10 pages, 3 figure

Nernst effect in the electron-doped cuprates

We calculate the normal state Nernst signal in the cuprates resulting from a reconstruction of the Fermi surface due to spin density wave order. An order parameter consistent with the reconstruction of the Fermi surface detected in electron-doped materials is shown to sharply enhance the Nernst signal close to optimal doping. Within a semiclassical treatment, the obtained magnitude and position of the enhanced Nernst signal agrees with Nernst measurements in electron-doped cuprates.Comment: 9 pages, 5 figures, revised version as accepted by Phys. Rev. B, changed several citations and reference

Scattering of charge carriers by point defects in bilayer graphene

Theory of scattering of massive chiral fermions in bilayer graphene by radial symmetric potential is developed. It is shown that in the case when the electron wavelength is much larger than the radius of the potential the scattering cross-section is proportional to the electron wavelength. This leads to the mobility independent on the electron concentration. In contrast with the case of single-layer, neutral and charged defects are, in general, equally relevant for the resistivity of the bilayer graphene.Comment: final versio

Collective excitations and low temperature transport properties of bismuth

We examine the influence of collective excitations on the transport properties (resistivity, magneto- optical conductivity) for semimetals, focusing on the case of bismuth. We show, using an RPA approximation, that the properties of the system are drastically affected by the presence of an acoustic plasmon mode, consequence of the presence of two types of carriers (electrons and holes) in this system. We found a crossover temperature T* separating two different regimes of transport. At high temperatures T > T* we show that Baber scattering explains quantitatively the DC resistivity experiments, while at low temperatures T < T* interactions of the carriers with this collective mode lead to a T^5 behavior of the resistivity. We examine other consequences of the presence of this mode, and in particular predict a two plasmon edge feature in the magneto-optical conductivity. We compare our results with the experimental findings on bismuth. We discuss the limitations and extensions of our results beyond the RPA approximation, and examine the case of other semimetals such as graphite or 1T-TiSe_2

Mott transition in the Hubbard model away from particle-hole symmetry

We solve the Dynamical Mean Field Theory equations for the Hubbard model away from the particle-hole symmetric case using the Density Matrix Renormalization Group method. We focus our study on the region of strong interactions and finite doping where two solutions coexist. We obtain precise predictions for the boundaries of the coexistence region. In addition, we demonstrate the capabilities of this precise method by obtaining the frequency dependent optical conductivity spectra.Comment: 4 pages, 4 figures; updated versio

Evidence for biquadratic exchange in the quasi-two-dimensional antiferromagnet FePS3_3

FePS$_3$ is a van der Waals compound with a honeycomb lattice that is a good example of a two-dimensional antiferromagnet with Ising-like anisotropy. Neutron spectroscopy data from FePS3 were previously analysed using a straight-forward Heisenberg Hamiltonian with a single-ion anisotropy. The analysis captured most of the elements of the data, however some significant discrepancies remained. The discrepancies were most obvious at the Brillouin zone boundaries. The data are subsequently reanalysed allowing for unequal exchange between nominally equivalent nearest-neighbours, which resolves the discrepancies. The source of the unequal exchange is attributed to a biquadratic exchange term in the Hamiltonian which most probably arises from a strong magnetolattice coupling. The new parameters show that there are features consistent with Dirac magnon nodal lines along certain Brillouin zone boundaries.Comment: 8 pages, 4 figures. The following article has been accepted by the Journal of Applied Physics. After it is published, it will be found at (https://publishing.aip.org/resources/librarians/products/journals/). The article was submitted as part of a special topic edition (https://publishing.aip.org/publications/journals/special-topics/jap/2d-quantum-materials-magnetism-and-superconductivity/