Nonlinear optical probe of tunable surface electrons on a topological insulator

We use ultrafast laser pulses to experimentally demonstrate that the second-order optical response of bulk single crystals of the topological insulator Bi$_2$Se$_3$ is sensitive to its surface electrons. By performing surface doping dependence measurements as a function of photon polarization and sample orientation we show that second harmonic generation can simultaneously probe both the surface crystalline structure and the surface charge of Bi$_2$Se$_3$. Furthermore, we find that second harmonic generation using circularly polarized photons reveals the time-reversal symmetry properties of the system and is surprisingly robust against surface charging, which makes it a promising tool for spectroscopic studies of topological surfaces and buried interfaces

A parity-breaking electronic nematic phase transition in the spin-orbit coupled metal Cd2_2Re2_2O7_7

Strong electron interactions can drive metallic systems toward a variety of well-known symmetry-broken phases, but the instabilities of correlated metals with strong spin-orbit coupling have only recently begun to be explored. We uncovered a multipolar nematic phase of matter in the metallic pyrochlore Cd$_2$Re$_2$O$_7$ using spatially resolved second-harmonic optical anisotropy measurements. Like previously discovered electronic nematic phases, this multipolar phase spontaneously breaks rotational symmetry while preserving translational invariance. However, it has the distinguishing property of being odd under spatial inversion, which is allowed only in the presence of spin-orbit coupling. By examining the critical behavior of the multipolar nematic order parameter, we show that it drives the thermal phase transition near 200 kelvin in Cd$_2$Re$_2$O$_7$ and induces a parity-breaking lattice distortion as a secondary order.Comment: 9 pages main text, 4 figures, 10 pages supplementary informatio

Charge collective modes in an incommensurately modulated cuprate

We report the first measurement of collective charge modes of insulating Sr14Cu24O41 using inelastic resonant x-ray scattering over the complete Brillouin zone. Our results show that the intense excitation modes at the charge gap edge predominantly originate from the ladder-containing planar substructures. The observed ladder modes (E vs. Q) are found to be dispersive for momentum transfers along the "legs" but nearly localized along the "rungs". Dispersion and peakwidth characteristics are similar to the charge spectrum of 1D Mott insulators, and we show that our results can be understood in the strong coupling limit (U >> t_{ladder}> t_{chain}). The observed behavior is in marked contrast to the charge spectrum seen in most two dimensional cuprates. Quite generally, our results also show that momentum-tunability of inelastic scattering can be used to resolve mode contributions in multi-component incommensurate systems.Comment: 4+ pages, 5 figure

Analytical technique for simplification of the encoder-decoder circuit for a perfect five-qubit error correction

Simpler encoding and decoding networks are necessary for more reliable quantum error correcting codes (QECCs). The simplification of the encoder-decoder circuit for a perfect five-qubit QECC can be derived analytically if the QECC is converted from its equivalent one-way entanglement purification protocol (1-EPP). In this work, the analytical method to simplify the encoder-decoder circuit is introduced and a circuit that is as simple as the existent simplest circuits is presented as an example. The encoder-decoder circuit presented here involves nine single- and two-qubit unitary operations, only six of which are controlled-NOT (CNOT) gates

The double life of electrons in magnetic iron pnictides, as revealed by NMR

We present a phenomenological, two-fluid approach to understanding the magnetic excitations in Fe pnictides, in which a paramagnetic fluid with gapless, incoherent particle-hole excitations coexists with an antiferromagnetic fluid with gapped, coherent spin wave excitations. We show that this two-fluid phenomenology provides an excellent quantitative description of NMR data for magnetic "122" pnictides, and argue that it finds a natural justification in LSDA and spin density wave calculations. We further use this phenomenology to estimate the maximum renormalisation of the ordered moment that can follow from low-energy spin fluctuations in Fe pnictides. We find that this is too small to account for the discrepancy between ab intio calculations and neutron scattering measurements.Comment: Accepted for publication in Europhys. Lett. 6 pages, 4 figure

Emergence of Fermi pockets in an excitonic CDW melted novel superconductor

A superconducting (SC) state (Tc ~ 4.2K) has very recently been observed upon successful doping of the CDW ordered triangular lattice TiSe$_2$, with copper. Using high resolution photoemission spectroscopy we identify, for the first time, the momentum space locations of the doped electrons that form the Fermi sea of the parent superconductor. With doping, we find that the kinematic nesting volume increases whereas the coherence of the CDW order sharply drops. In the superconducting doping, we observe the emergence of a large density of states in the form of a narrow electron pocket near the \textit{L}-point of the Brillouin Zone with \textit{d}-like character. The \textit{k}-space electron distributions highlight the unconventional interplay of CDW to SC cross-over achieved through non-magnetic copper doping.Comment: 4+ pages, 5 figures; Accepted for publication in Phys. Rev. Lett. (2007

Fermi surface topology and low-lying quasiparticle structure of magnetically ordered Fe1+xTe

We report the first photoemission study of Fe1+xTe - the host compound of the newly discovered iron-chalcogenide superconductors. Our results reveal a pair of nearly electron- hole compensated Fermi pockets, strong Fermi velocity renormalization and an absence of a spin-density-wave gap. A shadow hole pocket is observed at the "X"-point of the Brillouin zone which is consistent with a long-range ordered magneto-structural groundstate. No signature of Fermi surface nesting instability associated with Q= pi(1/2, 1/2) is observed. Our results collectively reveal that the Fe1+xTe series is dramatically different from the undoped phases of the high Tc pnictides and likely harbor unusual mechanism for superconductivity and quantum magnetic order.Comment: 5 pages, 4 Figures; Submitted to Phys. Rev. Lett. (2009