Synthesis, characterization, phase diagrams and superconducting and normal state magnetic properties of La[subscript 2-x]Sr[subscript x]CuO4 (0 [less than or equal to] x [less than or equal to] 0.08) and electrochemically oxidized La[subscript 2-x]Sr[subscript x]CuO[subscript 4 + delta] (0 [less than or equal to] x [less than or equal to] 0.33, 0 [less than or equal to delta less than or equal to] 0.12)

La[subscript] 2-xSr[subscript] xCuO[subscript]4 (0 ≤ x ≤ 0.15) can all be intercalated with oxygen by an novel electrochemical oxidation method. Bulk superconductivity is found with an onset T[subscript] c ≈ 40 K for the whole range 0.01 ≤ x ≤ 0.15; for x = 0.25 and 0.33, the electrochemical oxidation did not improve the superconducting properties. The magnetic susceptibility [chi](T = 50-320 K) data for La[subscript]2CuO[subscript]4.11 and La[subscript]1.92Sr[subscript]0.08CuO[subscript]4.07 are nearly identical with those of conventionally prepared La[subscript]1.85Sr[subscript]0.15CuO[subscript]4, indicating that the hole doping level (p) in the CuO[subscript]2 planes of the three compounds is nearly the same. Combined thermogravimetric analysis and iodometric titration experiments indicate that part of the intercalated oxygen has a formal valence close to -1. The maximum doped-hole concentration in the CuO[subscript]2 planes that can be achieved from combined Sr-doping and electrochemical oxygen doping for 0 ≤ x ≤ 0.15 is p ≈ 0.16 holes/formula unit;Oxygen can also intercalate into single crystal La[subscript]2CuO[subscript]4 through a slow electrochemical oxidation process. The required low current and long time for the charging process reflects that the oxygen intercalation for a single crystal is limited by its small specific surface area and long diffusion distance. The anisotropic superconducting, magnetic and transport properties are summarized and compared with those of polycrystalline La[subscript]2CuO[subscript]4+[delta] as well as of YBa[subscript]2Cu[subscript]3O[subscript]7-[delta] and La[subscript] 2-xSr[subscript] xCuO[subscript]4 single crystals. The single crystal La[subscript]2CuO[subscript]4+[delta] has a maximum T[subscript] c ≈ 40 K, which is lower than that (T[subscript] c ≈ 42-45) of the corresponding polycrystalline samples;The magnetic phase diagram of La[subscript] 2-xSr[subscript] xCuO[subscript]4 in the antiferromagnetic (AF) regime (0 ≤ x ≤ 0.02) has been derived from [superscript]139La NQR studies from 4 to 250 K. The data demonstrate localization of the doped holes\u27 charge below ~30 K, followed by freezing of the holes\u27 effective spin degrees of freedom below T[subscript] f ≈ (815 x) K into a spin-glass-like (SG) state which is superimposed on the AF background. These and previous results allow a detailed magnetic phase diagram to be constructed for x ≤ 0.05 and reveal a distinct cross-over at x ≈ 0.02 in the nature of the SG transition

Doping effects on charge density instability in non-centrosymmetric PbxTaSe2

We report on the investigation of vibrational and electronic properties of the Pb doped dichalcogenide PbxTaSe2 using Raman scattering experiments. We observe a marked variation of the main vibrational modes with Pb concentration x. The concentration dependence of the vibrational modes resembles the dependence of the vibrational modes in TaSe2 on the number of crystallographic layers along the c axis direction [1]. The temperature and polarization dependence of Raman spectra of PbxTaSe2 revealed additional broad modes in the low frequency regime which are discussed in context of remnant charge density wave, induced disorder, or PbSe phase formed in the interface of Pb and TaSe2 layers.Comment: 14 page

Observation of Ultrahigh Mobility Surface States in a Topological Crystalline Insulator by Infrared Spectroscopy

Topological crystalline insulators (TCIs) possess metallic surface states protected by crystalline symmetry, which are a versatile platform for exploring topological phenomena and potential applications. However, progress in this field has been hindered by the challenge to probe optical and transport properties of the surface states owing to the presence of bulk carriers. Here we report infrared (IR) reflectance measurements of a TCI, (001) oriented $Pb_{1-x}Sn_{x}Se$ in zero and high magnetic fields. We demonstrate that the far-IR conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small IR penetration depth. Moreover, our experiments yield a surface mobility of 40000 $cm^{2}/(Vs)$, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin TCI crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for TCIs

Atomic-Scale Strain Manipulation of a Charge Density Wave

A charge density wave (CDW) is one of the fundamental instabilities of the Fermi surface occurring in a wide range of quantum materials. In dimensions higher than one, where Fermi surface nesting can play only a limited role, the selection of the particular wave vector and geometry of an emerging CDW should in principle be susceptible to controllable manipulation. In this work, we implement a simple method for straining materials compatible with low-temperature scanning tunneling microscopy/spectroscopy (STM/S), and use it to strain-engineer new CDWs in 2H-NbSe2. Our STM/S measurements combined with theory reveal how small strain-induced changes in the electronic band structure and phonon dispersion lead to dramatic changes in the CDW ordering wave vector and geometry. Our work unveils the microscopic mechanism of a CDW formation in this system, and can serve as a general tool compatible with a range of spectroscopic techniques to engineer novel electronic states in any material where local strain or lattice symmetry breaking plays a role.Comment: to appear in PNAS (2018

Magnetization Measurements of Antiferromagnetic Domains in Sr\u3csub\u3e2\u3c/sub\u3eCu\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3eCl\u3csub\u3e2\u3c/sub\u3e

The Cu3O4 layer in Sr2Cu3O4Cl2 is a variant of the square CuO2 lattice of the high-temperature superconductors, in which the center of every second plaquette contains an extra Cu2+ ion. Whereas the ordering of the spins in the ground-state and the spin-wave excitations of this frustrated spin system are both well understood, we find peculiar behavior resulting from antiferromagnetic domain walls. Pseudodipolar coupling between the two sets of Cu2+ ions results in a ferromagnetic moment, the direction of which reflects the direction of the antiferromagnetic staggered moment, allowing us to probe the antiferromagnetic domain structure. After an excursion to the high fields (\u3e1 T), as the field is lowered, we observe the growth of domains with ferromagnetic moment perpendicular to the field. This gives rise to a finite domain wall susceptibility at small fields, which diverges near 100 K, indicating a phase transition. We also find that the shape of the sample influences the domain-wall behavior