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How the fluctuation-exchange (FLEX) approximation and the Fermi-liquid theory fail to explain the anomalous behavior of the Hall coefficient in the normal state of high-Tc superconductors is clarified.
Rubidium adsorption on the surface of the topological insulator Bi$_2$Se$_3$ is found to induce a strong downward band bending, leading to the appearance of a quantum-confined two dimensional electron gas states (2DEGs) in the conduction band. The 2DEGs shows a strong Rashba-type spin-orbit splitting, and it has previously been pointed out that this has relevance to nano-scale spintronics devices. The adsorption of Rb atoms, on the other hand, renders the surface very reactive and exposure to oxygen leads to a rapid degrading of the 2DEGs. We show that intercalating the Rb atoms, presumably into the van der Waals gaps in the quintuple layer structure of Bi$_2$Se$_3$, drastically reduces the surface reactivity while not affecting the promising electronic structure. The intercalation process is observed above room temperature and accelerated with increasing initial Rb coverage, an effect that is ascribed to the Coulomb interaction between the charged Rb ions. Coulomb repulsion is also thought to be responsible for a uniform distribution of Rb on the surface.
Trying to dope graphite flakes we found that the magnetization of pure, several tens of micrometers grain size graphite powder and after a simple treatment with pure water shows clear and reproducible granular superconducting behavior with a critical temperature above 300K. The observed magnetic characteristics as a function of temperature, magnetic field and time, provide evidence for weakly coupled grains through Josephson interaction, revealing the existence of superconducting vortices.
A few organic conductors show a diversity of exciting properties like Mott insulating behaviour, spin liquid, antiferromagnetism, bad metal or unconventional superconductivity controlled by small changes in temperature, pressure or chemical substitution. While such a behaviour can be technologically relevant for functional switches, a full understanding of its microscopic origin is still lacking and poses a challenge in condensed matter physics since these phases may be a manifestation of electronic correlation. Here we determine from first principles the microscopic nature of the electronic phases in the family of organic systems kappa-(ET)2Cu[N(CN)2]BrxCl1-x by a combination of density functional theory calculations and the dynamical mean field theory approach in a new form adapted for organic systems. By computing spectral and optical properties we are able to disentangle the origin of the various optical transitions in these materials and prove that correlations are responsible for relevant features. Remarkably, while some transitions are inherently affected by correlations, others are completely uncorrelated. We discuss the consequences of our findings for the phase diagram in these materials.
In Bi$_2$Te$_2$Se, the period of quantum oscillations arising from surface Dirac fermions can be increased 6-fold using ionic liquid gating. At large gate voltages, the Fermi energy reaches the $N$ = 1 Landau level in a 14-Tesla field. This enables the $\frac12$-shift predicted for the Dirac spectrum to be measured accurately. A surprising result is that liquid gating strongly enhances the surface mobility. By analyzing the Hall conductivity, we show that the enhancement occurs on only one surface. We present evidence that the gating process is fully reversible (hence consistent with band-bending by the $E$-field from the anion layer accumulated). In addition to the surface carriers, the experiment yields the mobility and density of the bulk carriers in the impurity band. By analyzing the charge accumulation vs. gate voltage, we also obtain estimates of the depletion width and the areal depletion capacitance $C_d/A$. The value of $C_d/A$ implies an enhanced electronic polarizability in the depletion region.
We report the experimental demonstration of femtosecond electron diffraction using high-brightness MeV electron beams. High-quality, single-shot electron diffraction patterns for both polycrystalline aluminum and single-crystal 1T-TaS2 are obtained utilizing a 5 femto-Coulomb (~3x10^4 electrons) pulse of electrons at 2.8 MeV. The high quality of the electron diffraction patterns confirm that electron beam has a normalized emittance of ~50 nm-rad. The corresponding transverse and longitudinal coherence length are ~11 nm and ~2.5 nm, respectively. The timing jitter between the pump laser and probe electron beam was found to be ~ 100 fs (rms). The temporal resolution is demonstrated by observing the evolution of Bragg and superlattice peaks of 1T-TaS2 following an 800 nm optical pump and was found to be 130 fs. Our results demonstrate the advantages of MeV electron diffraction: such as longer coherent lengths, large scattering cross-section and larger signal-to-noise ratio, and the feasibility of ultimately realizing 10 fs time-resolved electron diffraction.
We have investigated the temperature and magnetic field dependence of magnetization, specific heat ($C_p$), and relative sample length change ($ΔL/L_0$) for understanding the field-induced spin-structural change in quasi-one-dimensional spin chain $α$-CoV$_2$O$_6$ which undergoes antiferromagnetic (AFM) transition below $T_N$$=$15 K. Analysis of $C_p$($T$) shows that an effective $S$$=$1/2 Ising state is realized below 20 K, though the magnetic fluctuations persist well above $T_N$. $C_p$ and the coefficient of linear thermal expansion ($α$) exhibit strong $H$ dependence in the AFM state. We also observe a huge positive magnetostriction [$ΔL$($H$)/$L_0$] below 20 K which does not show any tendency of saturation up to 9 T. With increasing field, a sharp and symmetric peak emerges below $T_N$ in both $C_p$($T$) and $α$($T$) due to field-induced first order ferrimagnetic/ferromagnetic-paramagnetic transitions. The large value of magnetostriction below $T_N$ suggests strong spin-lattice coupling in $α$-CoV$_2$O$_6$.
We introduce a type of quantum dissipation -- local quantum friction -- by adding to the Hamiltonian a local potential that breaks time-reversal invariance so as to cool the system. Unlike the Kossakowski-Lindblad master equation, local quantum friction directly effects unitary evolution of the wavefunctions rather than the density matrix: it may thus be used to cool fermionic many-body systems with thousands of wavefunctions that must remain orthogonal. In addition to providing an efficient way to simulate quantum dissipation and non-equilibrium dynamics, local quantum friction coupled with adiabatic state preparation significantly speeds up many-body simulations, making the solution of the time-dependent Schrödinger equation significantly simpler than the solution of its stationary counterpart.
We describe a simple method to find the ground state energy without calculating the expectation value of the Hamiltonian in the time-evolving block decimation algorithm with tensor network states. For example, we consider quantum many-fermion systems with matrix product states, which are updated consistently in a way that accounts for fermion exchange effects. This method can be applied to a wide class of fermion systems. We test this method in spinless fermion system where the exact ground state energy is known. We analyze finite size effects to determine the ground state energy in the thermodynamic limit that is compared to the exact value.
We present the Fermi-surface map of the spin-chiral bulk states for the non-centrosymmetric semiconductor BiTeI using de Haas-van Alphen and Shubnikov-de Haas oscillations. We identify two distinct Fermi surfaces with a unique spindle-torus-type topology and the non-trivial Berry phases, confirming the spin chirality with oppositely circulating spin-texture. Near the quantum limit at high magnetic fields, we find a substantial Zeeman effect with an effective g-factor of ~ 60 for the Rashba-split Fermi surfaces. These findings provide clear evidence of strong Rashba and Zeeman coupling in the bulk states of BiTeI, suggesting that BiTeI is a good platform hosting the spin-polarized chiral states.
SrHo2O4 is a geometrically frustrated magnet in which the magnetic Ho3+ ions form honeycomb layers connected through a network of zigzag chains. At low-temperature two distinct types of short-range magnetic order can be inferred from single crystal diffraction data, collected using both polarized and unpolarized neutrons. In the (hk0) plane the diffuse scattering is most noticeable around the k = 0 positions and its intensity rapidly increases at temperatures below 0.7 K. In addition, planes of diffuse scattering at Q = (hk +/- l/2) are visible at temperatures as high as 4.5 K. These planes coexist with the broad peaks of diffuse scattering in the (hk0) plane at low-temperatures. Correlation lengths associated with the broad peaks are L ~ 150 Å in the a-b plane and L ~ 190 Å along c axis, while the correlation length associated with the diffuse scattering planes is L ~ 230 Å along the c axis at the lowest temperature. Both types of diffuse scattering are elastic in nature. The highly unusual coexistance of the two types of diffuse scattering in SrHo2O4 is likely to be the result of the presence of two crystallographically inequivalent sites for Ho3+ in the unit cell.
We propose and analyze the effect of anyonic interferometers that are designed such that the probe anyons traveling in a given path through the interferometer twist or braid around each other. These "twisted" interferometers are found to provide operational utility that may not be available from anyon braiding operations and standard (untwisted) anyonic interferometry measurements. In particular, it enables Ising anyons to generate "magic states," which can be used to implement pi/8-phase gates. We consider the possible implementations of such twisted interferometers in quantum Hall systems, 2D p_x + i p_y superconductors, and 2D Majorana heterostructures, and discuss obstacles and challenges associated with implementation.
We demonstrate interface energy level engineering, exploiting the modification in energy band structure across Verwey phase transition of $Fe_{3}O_{4}$ electrode, in a $Fe_{3}O_{4}$(111)/$Alq_{3}$/Co vertical spin-valve (SV) device. Experimental results on device characteristics I-V) study exhibit a transition in conduction mode from carrier injection to tunneling across Verwey transition temperature ($T_{V}$) of $Fe_{3}O_{4}$ electrode. Both giant magneto-resistance (GMR) and tunneling MR (TMR) have been observed in a single SV device as a function of temperature, below and above $T_{V}$, respectively. Appearance of GMR, accompanied by injection limited natural Schottky-like I-V characteristics, provide evidences of spin injection at electrode/$Alq_{3}$ interface and transport through molecular orbitals in this SV device. Features of TMR exhibit significant differences from that of GMR. This is due to the dominant hyperfine-field interaction in the multi-step tunneling regime. We have achieved room-temperature SV operation in our device. A phenomenological model for device operation has been proposed to explain the transition in the conduction mode and associated MR features across $T_{V}$. We propose that the tuning of charge gap at Fermi level across Verwey transition due to charge ordering on the octahedral iron sites of $Fe_{3}O_{4}$ results in a corresponding tuning of conduction mode causing this unique cross over from GMR to TMR in this ferrite-based organic SV.
Correlation factors are constructed that are consistent with the permutation symmetry group of N Fermions at given value of the filling factor.
We analyze properties of field theories dual to extremal black branes in (4+1) dimensions with anisotropic near-horizon geometries. Such gravity solutions were recently shown to fall into nine classes which align with the Bianchi classification of real three-dimensional Lie algebras. As a warmup we compute constraints on critical exponents from energy conditions in the bulk and scalar two point functions for a general type I metric, which has translation invariance but broken rotations. We also comment on the divergent nature of tidal forces in general Bianchi-type metrics. Then we come to our main focus: extremal branes whose near-horizon isometries are those of the Heisenberg algebra (type II). We find hyperscaling-violating solutions with type II isometries in (4+1)-dimensions. We show that scale invariant (4+1)-dimensional type II metrics are related by Kaluza-Klein reduction to more symmetric $AdS_2\times R^2$ and (3+1)-dimensional hyperscaling-violating spacetimes. These solutions generically have $θ\leq 0$. We discuss how one can obtain flows from UV CFTs to Bianchi-type spacetimes in the IR via the Higgs mechanism, as well as potential inhomogeneous instabilities of type II. Finally, we compute two-point functions of massive and massless scalar operators in the dual field theory and find that they exhibit the behavior of a theory with Landau levels.
The band structures describing non-interacting particles in one-dimensional superlattices of arbitrary periodicity are obtained by an analytical diagonalization of the Hamiltonian without adopting the popular tight-binding approximation. The results are compared with those of the tight-binding approximation. In this way, a quantitative prediction of the validity and failure of the tight-binding approximation becomes possible. In particular, it is demonstrated that in contrast to the prediction of the tight-binding approximation the central energy bands do not touch for periodicities $τ$ of the lattice where $τ=4n$ and $n$ is an integer.
In this paper we study a system of $N$ coupled quantum oscillators interacting with each other directly with varying coupling strengths and indirectly through linear couplings to a scalar massless quantum field as its environment. The influence of the quantum field on the system is calculated with the use of the influence functional formalism. We take the direct route of seeking solutions to the evolutionary operator of the reduced density matrix for the derivation of the correlation functions. They are then used to construct the covariance matrix which we use to perform an analysis of the structure of quantum entanglement in the open system at a stationary state. To see the physical features more explicitly we specialize to a system of three quantum coupled oscillators placed at the vertices of a equilateral triangle and allowed to have disparate pairwise couplings. We analyze the entanglement between one oscillator and the other two with equal (symmetric) and unequal (asymmetric) coupling strengths. As an illustration we use the results for these two different configurations to address two representative issues in macroscopic quantum phenomena. We also mention possible extensions of our work and applications of our analysis and results to issues in some current areas of research in quantum thermodynamics and mesoscopic quantum systems.
The Kondo insulator SmB6 has long been known to exhibit low temperature (T < 10K) transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the (001)-surface of this material. The low-lying states are found to form electron-like Fermi surface pockets that enclose the X and the Gamma points of the surface Brillouin zone. These states disappear as temperature is raised above 15K in correspondence with the complete disappearance of the 2D conductivity channels in SmB6. While the topological nature of the in-gap metallic states cannot be ascertained without spin (spin-texture) measurements our bulk and surface measurements carried out in the transport-anomaly-temperature regime (T < 10K) are consistent with the first-principle predicted Fermi surface behavior of a topological Kondo insulator phase in this material.