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Cadmium arsenide (Cd3As2) has recently became conspicuous in solid-state physics due to several reports proposing that it hosts a pair of symmetry-protected 3D Dirac cones. Despite vast investigations, a solid experimental insight into the band structure of this material is still missing. Here we fill one of the existing gaps in our understanding of Cd3As2, and based on our Landau level spectroscopy study, we provide an estimate for the energy scale of 3D Dirac electrons in this system. We find that the appearance of such charge carriers is limited - contrary to a widespread belief in the solid-state community - to a relatively small energy scale (below 40 meV)., Comment: to be published in Phys. Rev. B

We investigate the effects of long-ranged Coulomb interactions in a tilted Dirac semimetal in two dimensions by using the perturbative renormalization-group (RG) method. Depending on the magnitude of the tilting parameter, the undoped system can have either Fermi points (type I) or Fermi lines (type II). Previous studies usually performed the renormalization-group transformations by integrating out the modes with large momenta. This is problematic when the Fermi surface is open, like type-II Dirac fermions. In this work we study the effects of Coulomb interactions, following the spirit of Shankar [Rev. Mod. Phys. 66, 129 (1994)], by introducing a cutoff in the energy scale around the Fermi surface and integrating out the high-energy modes. For type-I Dirac fermions, our result is consistent with that of the previous work. On the other hand, we find that for type-II Dirac fermions, the magnitude of the tilting parameter increases monotonically with lowering energies. This implies the stability of type-II Dirac fermions in the presence of Coulomb interactions, in contrast with previous results. Furthermore, for type-II Dirac fermions, the velocities in different directions acquire different renormalization even if they have the same bare values. By taking into account the renormalization of the tilting parameter and the velocities due to the Coulomb interactions, we show that while the presence of a charged impurity leads only to charge redistribution around the impurity for type-I Dirac fermions, for type-II Dirac fermions, the impurity charge is completely screened, albeit with a very long screening length. The latter indicates that the temperature dependence of physical observables are essentially determined by the RG equations we derived. We illustrate this by calculating the temperature dependence of the compressibility and specific heat of the interacting tilted Dirac fermions.

We investigate the nature of the magnetic phase transition induced by the short-ranged electron-electron interactions in a Weyl semimetal by using the perturbative renormalization-group method. We find that the critical point associated with the quantum phase transition is characterized by a Gaussian fixed point perturbed by a dangerously irrelevant operator. Although the low-energy and long-distance physics is governed by a free theory, the velocities of the fermionic quasiparticles and the magnetic fluctuations suffer from nontrivial renormalization effects. In particular, their ratio approaches one at low energies, which indicates an emergent Lorentz symmetry at the quantum critical point. We further investigate the stability of the fixed point in the presence of weak disorder preserving the chiral symmetry. We show that while the fixed point is generally stable against weak disorder, a moderately strong random chemical potential and/or random vector potential may induce a quantum phase transition towards a disorder-dominated phase. We propose a global phase diagram of the Weyl semimetal in the presence of both electron-electron interactions and disorder based on our results.

We study the quantum point contact between the topological superconductor and the helical Luttinger liquid. The effects of the electron-electron interactions in the helical Luttinger liquid on the low-energy physics of this system are analyzed by the renormalization group. Among the various couplings at the point contact which arise from the tunneling via the Majorana edge channel, the induced backscattering in the helical Luttinger liquid is the most relevant for repulsive interactions. Hence, at low temperatures, the helical Luttinger liquid is effectively cut into two separated half wires. As a result, the low-temperature physics is described by a fixed point consisting of two leads coupled to the topological superconductor, and the electrical transport properties through the point contact at low temperature and low bias are dominated by the tunneling via the Majorana edge channel. We compute the temperature dependence of the zero-bias tunneling conductance and study the full counting statistics for the tunneling current at zero temperature., 7 pages, 2 figure

We propose a low energy effective theory of lightly doped two-leg t-J ladders with the help of slave fermion technique. The continuum limit of this model consists of two kinds of Dirac fermions which are coupled to the O(3) non-linear sigma model in terms of the gauge coupling with opposite sign of "charges". In addition to the gauge interaction, there is another kind of attractive force between these Dirac fermions, which arises from the short-ranged antiferromagnetic order. We show that the latter is essential to determine the low energy properties of lightly doped two-leg t-J ladders. The effective Hamiltonian we obtain is a bosonic Gaussian model and the boson field basically describes the particle density fluctuation. We also find two types of gapped spin excitations. Finally, we discuss the possible instabilities: charge density wave (CDW) and singlet superconductivity (SC). We find that the SC instability dominates in our approximation. Our results indicate that lightly doped ladders fall into the universality class of Luther-Emery model., Comment: 16 pages, Revtex, no figures

We studied the defects of Bi2Se3 generated from Bridgman growth of stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size, and transport properties are strongly affected by the types of defects generated. Major defect types of the BiSe antisite and partial Bi2-layer intercalation are identified through combined studies of direct atomic-scale imaging with scanning transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Hall effect measurements. We propose a consistent explanation to the origin of defect type, growth morphology, and transport property.

Electronic phase diagram of LixCoO2 has been re-examined using potentiostatically de-intercalated single crystal samples. Stable phases of x ~ 0.87, 0.72, 0.53, 0.50, 0.43, and 0.33 were found and isolated for physical property studies. A-type and chain-type antiferromagnetic orderings have been suggested from magnetic susceptibility measurement results in x ~ 0.87 and 0.50 below ~ 10 K and 200 K, respectively, similar to those found in NaxCoO2 system. There is no Li vacancy superlattice ordering observed at room temperature for the electronically stable phase Li0.72CoO2 as revealed by synchrotron X-ray Laue diffraction. The peculiar magnetic anomaly near ~ 175 K as often found in powder samples of x ~ 0.46-0.78 cannot be isolated through this single crystal potentiostatic method, which supports the previously proposed explanation to be surface stabilized phase of significant thermal hysteresis and aging character.

$\ensuremath{\beta}$-pyrochore KOs${}_{2}$O${}_{6}$, which shows superconductivity below $~9.7$ K, has been converted into ${\mathrm{K}}_{x}$Os${}_{2}$O${}_{6}$ ($x\ensuremath{\lesssim}\frac{2}{3}$--$\frac{1}{2}$) electrochemically to show spin-glass-like behavior below $~6.1$ K. A room-temperature sample surface potential versus charge transfer scan indicates that there are at least two two-phase regions for $x$ between 1 and 0.5. A rattling model of superconductivity for the title compound has been examined using electrochemical potassium de-intercalation. The significant reduction of superconducting volume fraction due to minor potassium reduction suggests the importance of defect and phase coherence in the rattling model. Magnetic susceptibility, resistivity, and specific heat measurement results have been compared between the superconducting and spin-glass-like samples.

A complete phase diagram of Zn-substituted quantum quasi-two-dimensional helimagnet ${\text{LiCu}}_{2}{\text{O}}_{2}$ has been presented. Helical ordering transition temperature $({T}_{h})$ of the original ${\text{LiCu}}_{2}{\text{O}}_{2}$ follows finite size scaling for less than $\ensuremath{\sim}5.5\mathrm{%}$ Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution $\ensuremath{\ge}5.5\mathrm{%}$ quenches the long-range helical ordering and introduces an intriguing Zn-level-dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic field dependence for ${T}_{c}(\text{Zn}g0.055,\text{H})$. The magnetic coupling constants of nearest-neighbor ${J}_{1}$ and next-nearest-neighbor ${J}_{2}$ $(\ensuremath{\alpha}={J}_{2}/{J}_{1})$ are extracted from high temperature series expansion fitting and $N=16$ finite chain exact diagonalization simulation. We have also provided evidence of direct correlation between long-range helical spin ordering and the magnitude of electric polarization in this spin-driven multiferroic material.

The impact of oxygen deficiency on physical properties of ${\text{Na}}_{2/3}{\text{CoO}}_{2\ensuremath{-}\ensuremath{\delta}}$ has been investigated. From the combined thermogravimetric, magnetic susceptibility, and synchrotron x-ray Laue diffraction studies, it is demonstrated that ${\text{Na}}_{2/3}{\text{CoO}}_{2}$ shows no superlattice ordering due to Na ions; however ${\text{Na}}_{2/3}{\text{CoO}}_{1.98}$, which has the same Co valence as that of ${\text{Na}}_{0.71}{\text{CoO}}_{2}$, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of $\sqrt{12}a$. It is proposed that the Na ion ordering found in ${\text{Na}}_{2/3}{\text{CoO}}_{1.98}$ is identical to the ideal ${\text{Na}}_{0.71}{\text{CoO}}_{2}$ of large $\sqrt{12}a\ifmmode\times\else\texttimes\fi{}\sqrt{12}a\ifmmode\times\else\texttimes\fi{}3c$ superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling.

Hexagonal superlattice formed by sodium multivacancy-cluster ordering in Na0.77CoO2 has been proposed based on synchrotron x-ray Laue-diffraction study on electrochemically fine-tuned single crystals. The title compound sits closely to the proposed lower end of the miscibility gap of x0.77‐0.82 phase-separated range. The average sodium vacancy-cluster size is estimated to be 4.5 Na vacancies per layer within a large superlattice size of 19a19a3c. The exceptionally large Na vacancy-cluster size favors large twinned simple hexagonal superlattice of 19a, in competition with the smaller divacancy, trivacancy, and quadrivacancy clusters formed superlattices of 12a and 13a. Competing electronic correlations are revealed by the observed spin-glasslike magnetic hysteresis below 3 K and the twin domain, triple domain, and monodomain transformations during thermal cycling between 273 and 373 K.

We study the low-energy properties of the one-orbital quantum double-exchange model by using the slave fermion formulation. We construct a mean-field theory which gives a simple explanation for the magnetic and thermodynamic properties of the ferromagnetic metallic phase in manganites at low energy. The resulting electron spectral function and tunneling density of states show an incoherent asymmetric peak with weak temperature dependence, in addition to a quasiparticle peak. We also show that the gauge fluctuations in the ferromagnetic metallic phase are completely screened due to the Anderson-Higgs mechanism. Therefore, the mean-field state is robust against gauge fluctuations and exhibits spin-charge separation at low energy.

We study the quantum phase transition in an atomic Bose gas near a Feshbach resonance in terms of the renormalization group. This quantum phase transition is characterized by an Ising order parameter. We show that in the low temperature regime where the quantum fluctuations dominate the low-energy physics this phase transition is of first order because of the coupling between the Ising order parameter and the Goldstone mode existing in the bosonic superfluid. However, when the thermal fluctuations become important, the phase transition turns into the second order one, which belongs to the three-dimensional Ising universality class. We also calculate the damping rate of the collective mode in the phase with only a molecular Bose-Einstein condensate near the second-order transition line, which can serve as an experimental signature of the second-order transition., 8 pages, 2 figures, published version in Phys. Rev. B

We study the Josephson-like spin currents between two ferromagnetic metals by deriving the effective action of the junction. A DC spin Josephson current with the full O(3) symmetry is obtained. By the analogy to the superconducting junctions, we have shown that a time-independent uniform magnetic field can serve as the source of the AC spin Josephson effect. That is, the spin current becomes a periodic function of the time with the period proportional to the inverse of the magnitude of the external magnetic field., Phys. Rev. B 68, 184413 (2003)