Valley relaxation in graphene due to charged impurities

Monolayer graphene is an example of materials with multi-valley electronic structure. In such materials, the valley index is being considered as an information carrier. Consequently, relaxation mechanisms leading to loss of valley information are of interest. Here, we calculate the rate of valley relaxation induced by charged impurities in graphene. A special model of graphene is applied, where the $p_z$ orbitals are two-dimensional Gaussian functions, with a spatial extension characterised by an effective Bohr radius $a_\textrm{eB}$. We obtain the valley relaxation rate by solving the Boltzmann equation, for the case of noninteracting electrons, as well as for the case when the impurity potential is screened due to electron-electron interaction. For the latter case, we take into account local-field effects and evaluate the dielectric matrix in the random phase approximation. Our main findings: (i) The valley relaxation rate is proportional to the electronic density of states at the Fermi energy. (ii) Charged impurities located in the close vicinity of the graphene plane, at distance $d \lesssim 0.3\,\textrm{\AA}$, are much more efficient in inducing valley relaxation than those farther away, the effect of the latter being suppressed exponentially with increasing graphene-impurity distance $d$. (iii) Both in the absence and in the presence of electron-electron interaction, the valley relaxation rate shows pronounced dependence on the effective Bohr radius $a_\textrm{eB}$. The trends are different in the two cases: in the absence (presence) of screening, the valley relaxation rate decreases (increases) for increasing effective Bohr radius. This last result highlights that a quantitative calculation of the valley relaxation rate should incorporate electron-electron interactions as well as an accurate knowledge of the electronic wave functions on the atomic length scale.Comment: 15 pages, 8 figure

Basák az iskolában

Az iskolákban megnyilvánuló erőszak, a basáskodás az iskolák világszerte súlyos problémája, mint ahogy – bár nem beszélünk róla – Magyarországon is. Gyökerei az evolúciós örökségként bennünk levő rangsoragresszióban keresendőek, így nem lehet kizárólag a mai társadalom rovására írni, és nem lehet egyszer és mindenkorra megszabadulni tőle. A basáskodás elleni küzdelmet csakis a jelenség tudatosításával és erre a célra kidolgozott speciális, a diákokat-tanárokat-szülőket egyaránt megcélzó (basáskodás elleni) programokkal lehet eredményesen felvenni. A problémának itthon egyelőre nincs gazdája

Control of valley dynamics in silicon quantum dots in the presence of an interface step

Recent experiments on silicon nanostructures have seen breakthroughs toward scalable, long-lived quantum information processing. The valley degree of freedom plays a fundamental role in these devices, and the two lowest-energy electronic states of a silicon quantum dot can form a valley qubit. In this work, we show that a single-atom high step at the silicon/barrier interface induces a strong interaction of the qubit and in-plane electric fields, and analyze the consequences of this enhanced interaction on the dynamics of the qubit. The charge densities of the qubit states are deformed differently by the interface step, allowing non-demolition qubit readout via valley-to-charge conversion. A gate-induced in-plane electric field together with the interface step enables fast control of the valley qubit via electrically driven valley resonance. We calculate single- and two-qubit gate times, as well as relaxation and dephasing times, and present predictions for the parameter range where the gate times can be much shorter than the relaxation time and dephasing is reduced.Comment: 12 pages, 6 figure

Töltéssel rendelkező oldalláncok szerepe retrovirális proteinázok szubsztrát-specificitásában = Role of the charged residues on the substrate specificity of retroviral proteinases

A HIV-1 életciklusában betöltött szerepe miatt terápiás (AIDS) célponttá vált retrovirális proteáz (PR) vizsgálatával betekintést nyerhetünk az enzim inhibitorokkal szembeni rezisztencia kialakulásának molekuláris mechanizmusáról. A rezisztenciában megjelenő HIV-1 mutánsok szubsztrát-specificitási, stabilitási, gátolhatósági és szerkezeti vizsgálatai mellett részletesen összehasonlítottuk a HIV-1, HTLV-1, MLV és BLV proteázok tulajdonságait. Ezen vizsgálatokhoz kidolgoztunk egy nagy teljesítményű fluoreszcens mérési módszert. Vizsgálatainkat kiterjesztettük egy klinikai kipróbálás alatt álló ígéretes HIV-1 proteáz inhibitornak, valamint szubsztrát-alapú peptid analóg inhibitoroknak vad tipusú és mutáns HIV-1 proteázokkal alkotott komplexeinek szerkezetvizsgálatával. A HFV PR különleges tulajdonságaiért felelős aminosavak feltérképezése céljából mutáns HFV proteázokkal pH-optimum és urea-stabilitási vizsgálatokat végeztünk. | As the retroviral protease became a useful therapeutic target of the AIDS due to its essential role in the life cycle of the HIV-1, the molecular mechanism of the raising resitance against HIV-1 protease inhibitors can be revealed by studying of the enzyme. Besides the substrate specificity, stability, inhibitory and structural studies on the mutant forms of HIV-1 protease appearing in the resistance we compared the features of the wild type HTLV-1, MLV and BLV proteases to the HIV-1 protease. We developed a high-throughput fluorescent method for these studies. We extended our studies to structural analysis of the wild type and mutant HIV-1 proteases complexed with a new potent and promising HIV-1 protease inhibitor which is in clinical trial as well as substrate based peptide analog inhibitors. Urea stability and pH-optimum of mutants of HFV PR were measured to map their special features

Dephasing of Majorana qubits due to quasistatic disorder

Quantum bits based on Majorana zero modes are expected to be robust against certain noise types, and hence provide a quantum computing platform that is superior to conventional qubits. This robustness is not complete though: imperfections can still lead to qubit decoherence and hence to information loss. In this work, we theoretically study Majorana-qubit dephasing in a minimal model: in a Kitaev chain with quasistatic disorder. Our approach, based on numerics as well as first-order non-degenerate perturbation theory, provides a conceptually simple physical picture and predicts Gaussian dephasing. We show that, as system parameters are varied, the dephasing rate due to disorder oscillates out-of-phase with respect to the oscillating minigap of the clean system. In our model, first-order dephasing sweet spots are absent, a feature that can be used to characterize the spatial structure of noise in a dephasing experiment. We expect that our results will be utilized for the design and interpretation of future Majorana-qubit experiments.Comment: 15 pages, 5 figure