Ab initio calculation of the shift photocurrent by Wannier interpolation

We describe and implement a first-principles algorithm based on maximally-localized Wannier functions for calculating the shift-current response of piezoelectric crystals in the independent-particle approximation. The proposed algorithm presents several advantages over existing ones, including full gauge invariance, low computational cost, and a correct treatment of the optical matrix elements with nonlocal pseudopotentials. Band-truncation errors are avoided by a careful formulation of $k\cdot p$ perturbation theory within the subspace of wannierized bands. The needed ingredients are the matrix elements of the Hamiltonian and of the position operator in the Wannier basis, which are readily available at the end of the wannierization step. If the off-diagonal matrix elements of the position operator are discarded, our expressions reduce to the ones that have been used in recent tight-binding calculations of the shift current. We find that this `diagonal' approximation can introduce sizeable errors, highlighting the importance of carefully embedding the tight-binding model in real space for an accurate description of the charge transfer that gives rise to the shift current.Comment: 13 pages, 7 figure

Non-degenerate valleys in the half-metallic ferromagnet Fe/WS2_2

We present a first principles investigation of the electronic properties of monolayer WS$_2$ coated with an overlayer of Fe. Our ab initio calculations reveal that the system is a half-metallic ferromagnet with a gap of $\sim 1$ eV for the majority spin channel. Furthermore, the combined effect of time-reversal symmetry breaking due to the magnetic Fe overlayer and the large spin-orbit coupling induced by W gives rise to non-degenerate K and K$'$ valleys. This has a tremendous impact on the excited state properties induced by externally applied circularly polarized light. Our analysis demonstrates that the latter induces a singular hot spot structure of the transition probability around the K and K$'$ valleys for right and left circular polarization, respectively. We trace back the emergence of this remarkable effect to the strong momentum dependent spin-noncollinearity of the valence band involved. As a main consequence, a strong valley-selective magnetic circular dichroism is obtained, making this system a prime candidate for spintronics and photonics applications

Oculto sendero. Elena Fortúnen eleberri autobiografikoa: emakume baten bildungsroman

Eskuartean duzuen lanaren xedea Elena Fortúnen Oculto sendero eleberri autobiografikoaren azterketa bat egitea da bildungsroman literatura azpigeneroan kokatzen dela baieztatzeko, izan ere, azpigeneroari aho batez dagozkion ezaugarriak oso bistakoak dira eleberrian. Hasierako sarreraz eta bukaerako ondorioez gain, lana hiru atal nagusitan dago banatuta: bi atal teorikoak dira eta bestea liburuaren azterketa. Sarreraren ostean, bigarren atalean, bildungsroman literatura azpigeneroaren gerturatze historiko eta teoriko bat egingo da, azpigeneroaren jatorria eta sorreraren arrazoiak aipatuko dira labur eta bere ezaugarriak nabarmenduko dira bereziki. Ez nuke aipatu gabe utzi nahi atal hori garatzeko orduan liburu bat izan dudala batik bat bide-lagun, hark lagundu baitit bildungsroman azpigeneroaren lehen irudi bat jasotzen hasieran, eta korrontean gehiago sakontzen, ondoren. Gauzak horrela, derrigorrezko iruditu zait laburpen honetan aipatzea bigarren atala idazteko María de los Ángeles Rodríguez Fontelaren La novela de autoformación. Una aproximación teórica e histórica al “bildungsroman” desde la narrativa hispánica liburua hartu dudala gehienbat oinarritzat, bildungsroman azpigeneroaren inguruko lan bereziki osatua baita. Lehen gerturatze horretatik abiatuz, nire lanarekin bat datorren marko teoriko zehatzago bati helduko diot. Hala, bigarren ataleko lehen azpiatalean emakumeek idatzitako bildungsromanen inguruan arituko naiz, maiz gizonezkoenen oso bestelakoak direlako. Ondoren, bigarren azpiatalean, eleberri autobiografikoa definitu eta haren ezaugarriak aipatuko dira labur, batetik, bildungsroman azpigeneroa era borobil eta bikainenean garatzea ahalbidetzen duen generoa delako eta, bestetik, aztertuko dugun obra genero horretan bertan koka dezakegulako. Behin bildungsroman azpigeneroa aurkeztuta, hirugarren atalean Oculto sendero eleberrian jarriko dut arreta. Hiru azpiatal desberdindu ditut: lehenengoan Elena Fortúnen biografia aztertuko da; ondoren, Oculto sendero eleberria aurkeztuko da eta, azkenik, eleberria autobiografikotzat jotzera bultzatzen gaituzten arrazoiak aurkeztuko dira, bigarren atalean azaldutako ezaugarriak oinarritzat hartuta eta eleberriko protagonistaren eta idazlea beraren biografia alderatuz. Lanaren bigarren atalean plazaratutako ideiak izan dira laugarren ataleko zati praktikoa aurrera ateratzeko euskarri, bertan aipatutako ezaugarrietan oinarritu bainaiz Oculto sendero bildungsroman bat baieztatzeko. Horretarako, laugarren atalean banaka aztertuko dut azpigeneroaren ezaugarri bakoitza eta ezaugarrion adibide gisa eleberriko hainbat esaldi ekarriko ditut lanera. Amaitzeko, ondorioen atalean, eginiko azterketatik ateratako ondorioak aurkeztuko dira. Bestalde, azken orrietan lana burutzeko erabili den bibliografia dago eskuragarri

Relativistic effects and fully spin-polarized Fermi surface at the Tl/Si(111) surface

We present a detailed analysis of the relativistic electronic structure and the momentum dependent spin-polarization of the Tl/Si(111) surface. Our first principle calculations reveal the existence of fully spin-polarized electron pockets associated to the huge spin-splitting of metallic surface bands. The calculated spin-polarization shows a very complex structure in the reciprocal space, strongly departing from simple theoretical model approximations. Interestingly, the electronic spin-state close to the Fermi surface is polarized along the surface perpendicular direction and reverses its orientation between different electron pockets

Self-consistent tight-binding description of Dirac points moving and merging in two dimensional optical lattices

We present an accurate ab initio tight-binding model, capable of describing the dynamics of Dirac points in tunable honeycomb optical lattices following a recent experimental realization [L. Tarruell et al., Nature 483, 302 (2012)]. Our scheme is based on first-principle maximally localized Wannier functions for composite bands. The tunneling coefficients are calculated for different lattice configurations, and the spectrum properties are well reproduced with high accuracy. In particular, we show which tight binding description is needed in order to accurately reproduce the position of Dirac points and the dispersion law close to their merging, for different laser intensities.Comment: 11 pages, 16 figure

Breakdown of the Peierls substitution for the Haldane model with ultracold atoms

We present two independent calculations of the tight-binding parameters for a specific realization of the Haldane model with ultracold atoms. The tunneling coefficients up to next-to-nearest neighbors are computed ab-initio by using the maximally localized Wannier functions, and compared to analytical expressions written in terms of gauge invariant, measurable properties of the spectrum. The two approaches present a remarkable agreement and evidence the breakdown of the Peierls substitution: (i) the phase acquired by the next-to-nearest tunneling amplitude $t_{1}$ presents quantitative and qualitative differences with respect to that obtained by the integral of the vector field A, as considered in the Peierls substitution, even in the regime of low amplitudes of A; (ii) for larger values, also $|t_{1}|$ and the nearest-neighbor tunneling $t_{0}$ have a marked dependence on A. The origin of this behavior and its implications are discussed.Comment: 5 pages, 5 figure

Tuning paramagnetic spin-excitations of single adatoms

Around 50 years ago, Doniach [Proc. Phys. Soc. 91, 86 (1967)] predicted the existence of paramagnons in nearly ferromagnetic materials, recently measured in bulk Pd [Phys. Rev. Lett. 105, 027207 (2010)]. Here we predict the analogous effect for single adatoms, namely paramagnetic spin-excitations (PSE). Based on time-dependent density functional theory, we demonstrate that these overdamped excitations acquire a well-defined peak structure in the meV energy region when the adatom's Stoner criterion for magnetism is close to the critical point. In addition, our calculations reveal a subtle tunability and enhancement of PSE by external magnetic fields, exceeding by far the response of bulk paramagnons and even featuring the atomic version of a quantum phase transition. We further demonstrate how PSE can be detected as moving steps in the $\mathrm{d}I/dV$ signal of state-of-the-art inelastic scanning tunneling spectroscopy, opening a potential route for experimentally accessing fundamental electronic properties of non-magnetic adatoms, such as the Stoner parameter.Comment: 6 pages, 3 figure