Violation of critical universality at the antiferromagnetic phase transition of YbRh2Si2

We report on precise low-temperature specific-heat measurements, C(T), of YbRh2Si2 in the vicinity of the antiferromagnetic phase transition on a single crystal of superior quality (RRR 150). We observe a very sharp peak at T_N=72mK with absolute values as high as C/T=8J/molK^2. A detailed analysis of the critical exponent \alpha around T_N reveals \alpha=0.38 which differs significantly from those of the conventional universality classes in the Ginzburg-Landau theory, where \alpha<0.11. Thermal-expansion measurements corroborate this large positive critical exponent. These results provide insight into the nature of the critical magnetic fluctuations at a temperature-driven phase transition close to a quantum critical point.Comment: Accepted for PR

'When I click "ok" I become Sassy – I become a girl.' Young people and gender identity: Subverting the ‘body’ in massively multi-player online role-playing games

This article is available open access through the publisher’s website through the link below. Copyright @ 2012 Taylor & Francis.This article explores young people's practices in the virtual spaces of online gaming communities. Based on a five-year ethnographic study of virtual worlds, it considers how young people construct and maintain identities within virtual social systems. In particular, the article discusses digital gender practices and considers the potential that these games offer for their young users to engage in alternate gender identities. We argue that these digital spaces offer spaces for the imagination and can enhance agency and, potentially, resistance. However, digital identity is simultaneously no ‘liberated space’ and it incorporates norms and practices that often mirror those of the material world. We argue that this ‘porosity’ is an important tool through which young people come to understand gender identity

Controller Synthesis for Autonomous Systems Interacting With Human Operators

We propose an approach to synthesize control protocols for autonomous systems that account for uncertainties and imperfections in interactions with human operators. As an illustrative example, we consider a scenario involving road network surveillance by an unmanned aerial vehicle (UAV) that is controlled remotely by a human operator but also has a certain degree of autonomy. Depending on the type (i.e., probabilistic and/or nondeterministic) of knowledge about the uncertainties and imperfections in the operatorautonomy interactions, we use abstractions based on Markov decision processes and augment these models to stochastic two-player games. Our approach enables the synthesis of operator-dependent optimal mission plans for the UAV, highlighting the effects of operator characteristics (e.g., workload, proficiency, and fatigue) on UAV mission performance; it can also provide informative feedback (e.g., Pareto curves showing the trade-offs between multiple mission objectives), potentially assisting the operator in decision-making

Pressure dependence of the Néel and the superconducting transition temperature of CeCo(In0.9Cd0.1)5 studied by thermal expansion

We present low-temperature thermal expansion measurements on the nominally 10% Cd doped CeCoIn5. While the superconducting transition temperature is monotonically suppressed, an antiferromagnetic phase evolves in CeCoIn5 by Cd-doping. For the uniaxial pressure dependence of the Néel temperature along c, we find (∂ TN / ∂ p)∥ c = 0.206 K / GPa. The magnetic field dependence (for B ∥ c) of TN is stronger compared to CeRhIn5. As no traces of a superconducting transition are resolved in thermal expansion along the c-axis, we estimate a lower limit of the in-plane pressure dependence to (∂ Tc / ∂ p)⊥ c = 0.38 K / GPa. © 2007 Elsevier B.V. All rights reserved

Spin Structure of K Valleys in Single-Layer WS2 on Au(111)

The spin structure of the valence and conduction bands at the K\u304 and K\u304\u2032 valleys of single-layer WS2 on Au(111) is determined by spin- and angle-resolved photoemission and inverse photoemission. The bands confining the direct band gap of 1.98 eV are out-of-plane spin polarized with spin-dependent energy splittings of 417 meV in the valence band and 16 meV in the conduction band. The sequence of the spin-split bands is the same in the valence and in the conduction bands and opposite at the K\u304 and the K\u304\u2032 high-symmetry points. The first observation explains "dark" excitons discussed in optical experiments; the latter points to coupled spin and valley physics in electron transport. The experimentally observed band dispersions are discussed along with band structure calculations for a freestanding single layer and for a single layer on Au(111)

Evolution of Quantum Criticality in CeNi_{9-x}Cu_xGe_4

Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold degenerate ground state in CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ \chi ~ -ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.Comment: 15 pages, 9 figure

Electron-correlation effects in appearance-potential spectra of Ni

Spin-resolved and temperature-dependent appearance-potential spectra of ferromagnetic Nickel are measured and analyzed theoretically. The Lander self-convolution model which relates the line shape to the unoccupied part of the local density of states turns out to be insufficient. Electron correlations and orbitally resolved transition-matrix elements are shown to be essential for a quantitative agreement between experiment and theory.Comment: LaTeX, 6 pages, 2 eps figures included, Phys. Rev. B (in press

Quantum criticality in layered CeRhIn_{5-x}Sn_x compared with cubic CeIn$_{3-x}Sn_x

We report low-temperature thermal-expansion measurements on single crystals of the {\it layered} heavy fermion system \cri ($0.3\leq x \leq 0.6$) and compare it with a previous study on the related {\it cubic} system \ci [R. K\"{u}chler {\it et al.}, Phys. Rev. Lett. {\bf 96}, 256403 (2006)]. Both systems display a quantum critical point as proven by a divergent Gr\"uneisen ratio. Most remarkably, the three-dimensional itinerant model explains quantum criticality in {\it both} systems, suggesting that the crystalline anisotropy in \cri is unimportant. This is ascribed to the effect of weak disorder in these doped systems.Comment: Eur. Phys. Lett., to be publishe

The temperature dependent bandstructure of a ferromagnetic semiconductor film

The electronic quasiparticle spectrum of a ferromagnetic film is investigated within the framework of the s-f model. Starting from the exact solvable case of a single electron in an otherwise empty conduction band being exchange coupled to a ferromagnetically saturated localized spin system we extend the theory to finite temperatures. Our approach is a moment-conserving decoupling procedure for suitable defined Green functions. The theory for finite temperatures evolves continuously from the exact limiting case. The restriction to zero conduction band occupation may be regarded as a proper model description for ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a simple cubic film cut parallel to the (100) crystal plane, we find some marked correlation effects which depend on the spin of the test electron, on the exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure