Hidden spin orbital texture at the Gamma over bar located valence band maximum of a transition metal dichalcogenide semiconductor

Finding stimuli capable of driving an imbalance of spin polarised electrons within a solid is the central challenge in the development of spintronic devices. However, without the aid of magnetism, routes towards this goal are highly constrained with only a few suitable pairings of compounds and driving mechanisms found to date. Here, through spin and angle resolved photoemission along with density functional theory, we establish how the p derived bulk valence bands of semiconducting 1T HfSe2 possess a local, ground state spin texture spatially confined within each Se sublayer due to strong sublayer localised electric dipoles orientated along the c axis. This hidden spin polarisation manifests in a coupled spin orbital texture with in equivalent contributions from the constituent p orbitals. While the overall spin orbital texture for each Se sublayer is in strict adherence to time reversal symmetry TRS , spin orbital mixing terms with net polarisations at time reversal invariant momenta are locally maintained. These apparent TRS breaking contributions dominate, and can be selectively tuned between with a choice of linear light polarisation, facilitating the observation of pronounced spin polarisations at the Brillouin zone centre for all kz. We discuss the implications for the generation of spin polarised populations from 1T structured transition metal dichalcogenides using a fixed energy, linearly polarised light sourc

Robust behavior and spin texture stability of the topological surface state in Bi2Se3 upon deposition of gold

The Dirac point of a topological surface state TSS is protected against gapping by time reversal symmetry. Conventional wisdom stipulates, therefore, that only through magnetisation may a TSS become gapped. However, non magnetic gaps have now been demonstrated in Bi2Se3 systems doped with Mn or In, explained by hybridisation of the Dirac cone with induced impurity resonances. Recent photoemission experiments suggest that an analogous mechanism applies even when Bi2Se3 is surface dosed with Au. Here, we perform a systematic spin and angle resolved photoemission study of Au dosed Bi2Se3. Although there are experimental conditions wherein the TSS appears gapped due to unfavourable photoemission matrix elements, our photon energy dependent spectra unambiguously demonstrate the robustness of the Dirac cone against high Au coverage. We further show how the spin textures of the TSS and its accompanying surface resonances remain qualitatively unchanged following Au deposition, and discuss the mechanism underlying the suppression of the spectral weigh

Ultrafast Thermalization Pathways of Excited Bulk and Surface States in the Ferroelectric Rashba Semiconductor GeTe

A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time and angle resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron electron and electron phonon interactions is responsible for the counterintuitive temperature dependence, with spin selection constrained interband electron electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron electron and electron phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronic

AdS Branes Corresponding to Superconformal Defects

We investigate an AdS_4 x L_2 D5-brane in AdS_5 x X_5 space-time, in the context of AdS/dCFT correspondence. Here, X_5 is a Sasaki-Einstein manifold and L_2 is a submanifold of X_5. This brane has the same supersymmetry as the 3-dimensional N=1 superconformal symmetry if L_2 is a special Legendrian submanifold in X_5. In this case, this brane is supposed to correspond to a superconformal wall defect in 4-dimensional N=4 super Yang-Mills theory. We construct these new string backgrounds and show they have the correct supersymmetry, also in the case with non-trivial gauge flux on L_2. The simplest new example is AdS_4 x T^2 brane in AdS_5 x S^5. We construct the brane solution expressing the RG flow between two different defects. We also perform similar analysis for an AdS_3 x L_3 M5-brane in AdS_4 x X_7, for a weak G_2 manifold X_7 and its submanifold L_3. This system has the same supersymmetry as 2-dimensional N=(1,0) global superconformal symmetry, if L_3 is an associative submanifold.Comment: 22 pages, LaTeX, 3 figures. v2: typos corrected, references added. v3: typos correcte

Monetary Policy Rules and Directions of Causality: a Test for the Euro Area

Using a VAR model in first differences with quarterly data for the euro zone, the study aims to ascertain whether decisions on monetary policy can be interpreted in terms of a “monetary policy rule” with specific reference to the so-called nominal GDP targeting rule (Hall and Mankiw, 1994; McCallum, 1988; Woodford, 2012). The results obtained indicate a causal relation proceeding from deviation between the growth rates of nominal gross domestic product (GDP) and target GDP to variation in the three-month market interest rate. The same analyses do not, however, appear to confirm the existence of a significant inverse causal relation from variation in the market interest rate to deviation between the nominal and target GDP growth rates. Similar results were obtained on replacing the market interest rate with the European Central Bank refinancing interest rate. This confirmation of only one of the two directions of causality does not support an interpretation of monetary policy based on the nominal GDP targeting rule and gives rise to doubt in more general terms as to the applicability of the Taylor rule and all the conventional rules of monetary policy to the case in question. The results appear instead to be more in line with other possible approaches, such as those based on post Keynesian analyses of monetary theory and policy and more specifically the so-called solvency rule (Brancaccio and Fontana, 2013, 2015). These lines of research challenge the simplistic argument that the scope of monetary policy consists in the stabilization of inflation, real GDP, or nominal income around a “natural equilibrium” level. Rather, they suggest that central banks actually follow a more complex purpose, which is the political regulation of the financial system with particular reference to the relations between creditors and debtors and the related solvency of economic units

On the problem of Dirac cones in fullerenes on gold

Artificial graphene based on molecular networks enables the creation of novel 2D materials with unique electronic and topological properties. Landau quantization has been demonstrated by CO molecules arranged on the two dimensional electron gas on Cu 111 and the observation of electron quantization may succeed based on the created gauge fields. Recently, it was reported that instead of individual manipulation of CO molecules, simple deposition of nonpolar C60 molecules on Cu 111 and Au 111 produces artificial graphene as evidenced by Dirac cones in photoemission spectroscopy. Here, we show that C60 induced Dirac cones on Au 111 have a different origin. We argue that those are related to umklapp diffraction of surface electronic bands of Au on the molecular grid of C60 in the final state of photoemission. We test this alternative explanation by precisely probing the dimensionality of the observed conical features in the photoemission spectra, by varying both the incident photon energy and the degree of charge doping via alkali adatoms. Using density functional theory calculations and spin resolved photoemission we reveal the origin of the replicating Au 111 bands and resolve them as deep leaky surface resonances derived from the bulk Au sp band residing at the boundary of its surface projection. We also discuss the manifold nature of these resonances which gives rise to an onion like Fermi surface of Au 11

Lifetime of quasiparticles in the nearly free electron metal sodium

We report a high resolution angle resolved photoemission ARPES study of the prototypical nearly free electron metal sodium. The observed mass enhancement is slightly smaller than that derived in previous studies. The new results on the lifetime broadening increase the demand for theories beyond the random phase approximation. Our results do not support the proposed strong enhancement of the scattering rates of the charge carriers due to a coupling to spin fluctuations. Moreover, a comparison with earlier electron energy loss data on sodium yields a strong reduction of the mass enhancement of dipolar electron hole excitations compared to that of monopole hole excitations, measured by ARPE