Chao Zhang

Phase Competition in Trisected Superconducting Dome

A detailed phenomenology of low energy excitations is a crucial starting point for microscopic understanding of complex materials, such as the cuprate high-temperature superconductors. Because of its unique momentum-space discrimination, angle-resolved photoemission spectroscopy (ARPES) is ideally suited for this task in the cuprates, where emergent phases, particularly superconductivity and the pseudogap, have anisotropic gap structure in momentum space. We present a comprehensive doping- and temperature-dependence ARPES study of spectral gaps in Bi2Sr2CaCu2O8+δ, covering much of the superconducting portion of the phase diagram. In the ground state, abrupt changes in near-nodal gap phenomenology give spectroscopic evidence for two potential quantum critical points, p = 0.19 for the pseudogap phase and p = 0.076 for another competing phase. Temperature dependence reveals that the pseudogap is not static below Tc and exists p > 0.19 at higher temperatures. Our data imply a revised phase diagram that reconciles conflicting reports about the endpoint of the pseudogap in the literature, incorporates phase competition between the superconducting gap and pseudogap, and highlights distinct physics at the edge of the superconducting dome.

Doping-Dependent Nodal Fermi Velocity of the High-Temperature Superconductor Bi 2 Sr 2 CaCu 2 O 8 + δ Revealed Using High-Resolution Angle-Resolved Photoemission Spectroscopy

The improved resolution of laser-based angle-resolved photoemission spectroscopy (ARPES) allows reliable access to fine structures in the spectrum. We present a systematic, doping-dependent study of a recently discovered low-energy kink in the nodal dispersion of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi-2212), which demonstrates the ubiquity and robustness of this kink in underdoped Bi-2212. The renormalization of the nodal velocity due to this kink becomes stronger with underdoping, revealing that the nodal Fermi velocity is non-universal, in contrast to assumed phenomenology. This is used together with laser-ARPES measurements of the gap velocity, v{sub 2}, to resolve discrepancies with thermal conductivity measurements.

Phase transformations and vibrational properties of coronene under pressure

Both the vibrational and structural properties of coronene have been investigated upon compression up to 30.5 GPa at room temperature by a combination of Raman scattering and synchrotron x-ray diffraction measurements. The spectroscopic and crystallographic results demonstrate that two pressure-induced structural phase transitions take place at 1.5 GPa and 12.2 GPa where the high-pressure phases are identified as monoclinic and orthorhombic crystal structures with space groups of P2/m and Pmmm, respectively. A kink in the slope of the cell parameters as a function of pressure is associated with the disappearance of several internal Raman modes, which suggests the existence of structural distortions or reorganizations at approximately 6.0 GPa. Above 17.1 GPa, almost no evidence of crystallinity can be observed, indicating a possible transformation of coronene into an amorphous phase.

Constraint on the potassium content for the superconductivity of potassium-intercalated phenanthrene

Raman-scattering measurements were performed on K(x)phenanthrene (0 ⩽ x ⩽ 6.0) at room temperature. Three phases (x = 3.0, 3.5, and 4.0) are identified based on the obtained Raman spectra. Only the K3phenanthrene phase is found to exhibit the superconducting transition at 5 K. The C-C stretching modes are observed to broaden and become disordered in K(x)phenanthrene with x = 2.0, 2.5, 6.0, indicating some molecular disorder in the metal intercalation process. This disorder is expected to influence the nonmetallic nature of these materials. The absence of metallic character in these nonsuperconducting phases is found from the calculated electronic structures based on the local density approximation.

Structural and vibrational properties of phenanthrene under pressure

The structural and vibrational properties of phenanthrene are measured at high pressures up to 30.2 GPa by Raman spectroscopy and synchrotron X-ray diffraction techniques. Two phase transitions are observed in the Raman spectra at pressures of 2.3 GPa and 5.4 GPa which correspond to significant changes of intermolecular and intramolecular vibrational modes. Above 10.2 GPa, all the Raman peaks are lost within the fluorescence background; however, upon further compression above 20.0 GPa, three broad peaks are observed at 1600, 2993, and 3181 cm(-1), indicating that phenanthrene has transformed into amorphous phase. Using X-ray diffraction, the structures of corresponding phases observed from Raman spectra are indexed with space groups of P2(1) for phase I (0-2.2 GPa), P2/m for phase II (2.2-5.6 GPa), P2/m+Pmmm for phase III (5.6-11.4 GPa) which has a coexistence of structures, and above 11.4 GPa the structure is indexed with space group of Pmmm. Although phenanthrene has transformed to a hydrogenated amorphous carbon structure above 20.0 GPa, these amorphous clusters still show characteristic crystalline behavior based on our X-ray diffraction patterns. Our results suggest that the long-range periodicity and the local disorder state coexist in phenanthrene at high pressures.

Magnetic instability and pair binding in aromatic hydrocarbon superconductors.

Understanding magnetism and electron correlation in many unconventional superconductors is essential to explore mechanism of superconductivity. In this work, we perform a systematic numerical study of the magnetic and pair binding properties in recently discovered polycyclic aromatic hydrocarbon (PAH) superconductors including alkali-metal-doped picene, coronene, phenanthrene, and dibenzopentacene. The π-electrons on the carbon atoms of a single molecule are modelled by the one-orbital Hubbard model, and the energy difference between carbon atoms with and without hydrogen bonds is taking into account. We demonstrate that the spin polarized ground state is realized for charged molecules in the physical parameter regions, which provides a reasonable explanation of local spins observed in PAHs. In alkali-metal-doped dibenzopentacene, our results show that electron correlation may produce an effective attraction between electrons for the charged molecule with one or three added electrons.

Structural transitions of solid germane under pressure

We present a structural characterization of solid germane (GeH4) under pressure from first-principles calculations. We find that this material undertakes a structural transformation from its low-pressure P21/c phase to high-pressure Cmmm phase at about 15 GPa where insulator-metal transition occurs, followed by two other metallic phases having the P21/m and C2/c structure at up to 200 GPa. Our results indicate that the metallization of GeH4 can be realized through band overlap within the material itself.

Phase transitions and electron-phonon coupling in platinum hydride.

Structural phase transitions and superconducting properties of platinum hydride under pressure are explored through the first-principles calculations based on the density functional theory. Three new low-pressure phases (Pm3m, Cmmm and P4/nmm) are predicted, and all of them are metallic and stable relative to decomposed cases. The superconducting critical temperature of two high-pressure phases correlates with the electron-phonon coupling. The presence of soft modes induced by Kohn anomalies and the hybridization between H and Pt atoms result in the strong electron-phonon coupling. Our results have major implications for other transition metal hydrides under pressure.

First-principles investigations on the magnetic property in tripotassium doped picene

First-principles calculations are performed to investigate the magnetic characteristics in the tripotassium doped picene, especially for the effects induced by the volume variations. When changing volume, both crystal lattice constants and atomic positions are optimized. For the system with the experimental crystal volume, the doped picene shows a weak antiferromagnetic instability. When the volume expands from this experimental crystal volume, the antiferromagnetic spin ordering becomes clear. The electronic structures show that the magnetism comes from the spin unbalance on the π orbitals of the C atoms. On the contrary, both ferromagnetic and antiferromagnetic spin orderings are strongly suppressed as the volume is reduced. Our results indicate that the magnetism is sensitive to the variation of volume or pressure in the tripotassium doped picene. No metal-insulator transition is observed for several considered volumes.

Enhancement of the thermoelectric performance of bulk SnTe alloys via the synergistic effect of band structure modification and chemical bond softening

SnTe alloys, which have the same crystal structure as PbTe, have attracted increasing attention. Here, we demonstrate that the synergistic effect of band structure modification and chemical bond softening can be realized simultaneously in In & Mn doped SnTe bulk alloys. The Seebeck coefficient and power factor are synergistically improved by co-doping of In and Mn. In doping is known to introduce a resonance level. Mn doping reduces the separation of light- and heavy-valence bands. The combination of these effects significantly enhances the Seebeck coefficient at room temperature owing to around a factor of five times increase in the band effective mass. The reduction of thermal conductivity is from the decrease of both the electronic and phononic parts. The electronic thermal conductivity is decreased by the increase in defect scattering, as can be confirmed by the carrier mobility. The force constant of the bonds around the Te site is decreased due to the co-doping of In & Mn, which indicates that the chemical bonds are softened, which leads to low sound velocity and lower lattice thermal conductivity. As a result, the peak thermoelectric figure of merit, zT = 1.03 has been achieved for Sn0.89In0.01Mn0.1Te at 923 K. This strategy of using the synergistic effect of band structure modification and chemical bond softening could be applicable to other thermoelectric materials.