Spin-½

About: Spin-½ is a research topic. Over the lifetime, 40423 publications have been published within this topic receiving 796639 citations.

Emergent quantum connement at topological insulator surfaces

Abstract: Here, we demon-strate that a conceptually simple model, imple-menting a semiconductor-like band bending ina parameter-free tight-binding supercell calcula-tion, can quantitatively explain the entire mea-sured hierarchy of electronic states. In combi-nation with circular dichroism in angle-resolvedphotoemission (ARPES) experiments, we furtheruncover a rich three-dimensional spin texture ofthis surface electronic system, resulting from thenon-trivial topology of the bulk band structure.Moreover, our study reveals how the full surface-bulk connectivity in topological insulators is mod-i ed by quantum con nement.Topological insulators (TIs) are an exotic state ofquantum matter, guaranteed to have metallic edge orsurface states due to an inverted ordering of their bulkelectronic bands. The corresponding topological invari-ants dictate that there must be an odd number of suchstates intersecting the Fermi level between each pair ofsurface time-reversal invariant momenta (TRIM). In themost widely investigated bismuth-chalchogenide familyof TIs, there is just one of these so-called topological sur-face states (TSSs) creating a single Dirac cone around theBrillouin zone centre.

Strong Interplay between Stripe Spin Fluctuations, Nematicity and Superconductivity in FeSe

Abstract: Elucidating the microscopic origin of nematic order in iron-based superconducting materials is important because the interactions that drive nematic order may also mediate the Cooper pairing. Nematic order breaks fourfold rotational symmetry in the iron plane, which is believed to be driven by either orbital or spin degrees of freedom. However, as the nematic phase often develops at a temperature just above or coincides with a stripe magnetic phase transition, experimentally determining the dominant driving force of nematic order is difficult. Here, we use neutron scattering to study structurally the simplest iron-based superconductor FeSe, which displays a nematic (orthorhombic) phase transition at $T_s=90$ K, but does not order antiferromagnetically. Our data reveal substantial stripe spin fluctuations, which are coupled with orthorhombicity and are enhanced abruptly on cooling to below $T_s$. Moreover, a sharp spin resonance develops in the superconducting state, whose energy (~4 meV) is consistent with an electron boson coupling mode revealed by scanning tunneling spectroscopy, thereby suggesting a spin fluctuation-mediated sign-changing pairing symmetry. By normalizing the dynamic susceptibility into absolute units, we show that the magnetic spectral weight in FeSe is comparable to that of the iron arsenides. Our findings support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.

Ultrafast optical rotations of electron spins in quantum dots

Abstract: The spin state of electrons trapped in a quantum dot only lasts a few microseconds. Before this information is lost, it is useful to controllably rotate the spin as many times as possible. Laser pulses can now rotate electron spins in an ensemble of quantum dots in just a few picoseconds. Coherent manipulation of quantum bits (qubits) on timescales much shorter than the coherence time1,2 is a key prerequisite for quantum information processing. Electron spins in quantum dots are particularly attractive for implementations of qubits, and efficient optical methods for initialization and readout of spins have been developed in recent years3,4. Spin coherence times in the microsecond range have been demonstrated5. Therefore, spin control by picosecond optical pulses would be highly desirable so that a large number of spin rotations could be carried out while coherence is maintained. A major remaining challenge is demonstration of such rotations with high fidelity. Here, we use an ensemble of quantum-dot electron spins focused into a small number of precession modes about a magnetic field by periodic optical pumping. We demonstrate ultrafast optical rotations of spins about arbitrary axes on a picosecond timescale using laser pulses as control fields.

Density matrix description of spin-selective radical pair reactions

Abstract: (1976). Density matrix description of spin-selective radical pair reactions. Molecular Physics: Vol. 32, No. 5, pp. 1491-1493.

Sub-projection-noise sensitivity in broadband atomic magnetometry.

Abstract: We demonstrate sub-projection-noise sensitivity of a broadband atomic magnetometer using quantum nondemolition spin measurements. A cold, dipole-trapped sample of rubidium atoms provides a long-lived spin system in a nonmagnetic environment, and is probed nondestructively by paramagnetic Faraday rotation. The calibration procedure employs as known reference state, the maximum-entropy or "thermal" spin state, and quantitative imaging-based atom counting to identify electronic, quantum, and technical noise in both the probe and spin system. The measurement achieves a sensitivity 1.6 dB (2.8 dB) better than projection-noise (thermal state quantum noise) and will enable squeezing-enhanced broadband magnetometry.