Lattice thermal conductivity of ZnO: experimental and theoretical studies.

ZnO has been explored using different approaches such as doping, nanostructuring, 2D confinement, and introduction of interface effects for improving thermoelectric performance and lowering thermal conductivity. Herein, the lattice thermal conductivity (κL) of ZnO determined from Raman thermometry, the 3ω-method and simulation using three-phonon scattering is presented. The average Debye temperature (θD) of A1(TO) and E2(high) modes estimated utilizing bond-order-length-strength correlation with local bond averaging effects is ∼422 K. The average κL of ZnO calculated using the theoretical coefficient of Slack's equation, θD, and Grüneisen parameter (γ) in Slack's equation is 2.75 W m-1 K-1, which is significantly lower than the κL ∼ 50.9 W m-1 K-1 simulated by considering the Perdew-Burke-Ernzerhof functional with Hubbard and non-analytical corrections. The coefficient of Slack's equation is determined from the κL ∼ 31 W m-1 K-1 of ZnO measured using the 3ω-method for the accurate estimation of the κL. The experimental coefficient of Slack's equation with Raman thermometry data yields κL ∼ 50.6 W m-1 K-1, which is higher than the value obtained using the 3ω-method but consistent with the theoretical value. Thus, three-phonon anharmonicity describes the κL of ZnO associated with Raman scattering. The method adopted to calculate κL will help for the in-depth analysis of phonon dynamics and the design of wurtzite-ZnO-related power electronics.

Reformation of La0.7Sr0.3MnO3 properties by using ZnO in La0.7Sr0.3MnO3-ZnO heterostructures grown on (001) oriented Si.

Study of the available density of states (DOS) close-to-zero bias for conduction in strongly correlated electron systems, such as half-metallic La0.7Sr0.3MnO3 (LSMO) and its heterostructures, is important for fundamental and application reasons. As the DOS is proportional to the differential conductance (dI/dV), the dI/dV of a 120 ŠLSMO film and its reformation in LSMO/ZnO heterostructures was investigated for different ZnO thicknesses. Unlike in conventional metals, the dI/dV of LSMO exhibits a power-law dependent zero-bias anomaly, i.e., dI/dV ∝ Vm (m ∼ 1) near zero bias in the ferromagnetic metallic state at 10 K. The growth of ZnO on LSMO reforms the linear dI/dVvs. V of LSMO near zero bias to non-linear. The exponent 'm' becomes ∼0.5 for a higher ZnO thickness, revealing increased electron-electron interactions and suppression of Kondo-like, double and superexchange interactions, which are responsible for the depression of the DOS of LSMO near zero bias. In a magnetically disordered state, i.e., around the Curie temperature, ZnO reforms the linear V-shaped dI/dV vs. V of LSMO to parabolic U-shaped dI/dVvs.V and controls the electron concentrations in the t2g-orbitals of Mn realized from the DOS simulations. Additionally, ZnO introduces a peak in the dI/dV vs. V due to Fowler-Nordheim tunnelling, and the peak voltage can be tuned by varying the ZnO thickness or temperature from 300 K to 360 K. Such functions of ZnO yield major perspectives for novel applications in thin-film-based devices.

Lattice thermal conductivity of topological insulator Bi2Se3 nanocrystals: comparison from theoretical and experimental.

Lattice thermal conductivity (κL) calculations using the Wiedemann-Franz law involve electrical conductivity, which introduces an error in the actual value of κL. We have adopted a non-contact measurement technique and calculated the κL from the temperature and power-dependent Raman spectra of the Bi2Se3 nanocrystals with truncated hexagon plate morphology stabilized in a hexagonal crystal structure. The hexagon plates of Bi2Se3 are 37 to 55 nm thick with lateral dimensions around 550 nm. These Bi2Se3 nanocrystals show three Raman lines, which agree with the theoretical prediction of A11g, E2g and A21g modes. Although the first-order thermal coefficient (-0.016) of Bi2Se3 nanocrystals is quite low, the room temperature κL ∼1.72 W m-1 K-1 is close to the value obtained from the simulation adopting a three-phonon process. The phonon lifetime of Bi2Se3 nanocrystals observed between ∼0.2 ps and 2 ps confirmed carrier-carrier thermalization with a small contribution from electron-electron and intraband electron-longitudinal-optical-phonon relaxation. The variations of phonon lifetime, Gruneisen parameter and κL of the mode frequencies outline the crucial role of the anharmonicity and acoustic-optical phonon scattering in reducing the κL of Bi2Se3. The non-contact measurements and relevant thermal property parameters open up exciting opportunities to address the anharmonic effects in other thermoelectric materials for obtaining a high figure of merit.

The suppression of spin-orbit coupling effect by the ZnO layer of La0.7Sr0.3MnO3/ZnO heterostructures grown on (001) oriented Si restores the negative magnetoresistance.

Dual sign magnetoresistance (MR) and spin-glass state are achieved by stabilizing 120 Å thick La0.7Sr0.3MnO3 (LSMO) film on a (001) oriented Si substrate using pulsed sputtered plasma deposition method. The growth of the ZnO film on top of LSMO suppresses the Curie temperature around 30 K, and reduces the out-of-plane positive MR to zero. On increasing the paramagnetic ZnO film thickness, the out-of-plane negative MR and net magnetic moment increase with the same Curie temperature. At the same time, the band gap decreases, and is attributed to the grain size. The existence of the spin-glass state designates the presence of the non-collinear Mn ion spins, which formed because of the competing double exchange and superexchange interactions. The spin-glass state in the LSMO film is rich in the charge transfer driven localized strong antiferromagnetic coupling at the Si-LSMO interface. The localized strong antiferromagnetic coupling and spin-orbit coupling induced weak antilocalization favor positive MR and reduce the Curie temperature in LSMO. In contrast, the strong magnetic scattering and the loss of the 2D confinement of the charge carrier in LSMO-ZnO heterostructures favor the negative MR. Our investigations show that the technologically important interfacial magnetic coupling and magnetoresistance could be achieved in a bottom interface, and can be manipulated by the top interface of the semiconducting-ferromagnetic-semiconducting heterostructures.

The effect of mechanical strain on the Dirac surface states in the (0001) surface and the cohesive energy of the topological insulator Bi2Se3.

The band gap (E g) engineering and Dirac point tuning of the (0001) surface of 8 QLs (quintuple layers) thick Bi2Se3 slab are explored using the first-principles density functional theory calculations by varying the strain. The strain on the Bi2Se3 slab primarily varies the bandwidth, modifies the p z - orbital population of Bi and moves the Dirac point of the (0001) surface of Bi2Se3. The Dirac cone feature of the (0001) surface of Bi2Se3 is preserved for the entire range of the biaxial strain. However, around 5% tensile uniaxial strain and even lower value of volume conservation strain annihilate the Dirac cone, which causes the loss of topological (0001) surface states of Bi2Se3. The biaxial strain provides ease in achieving the Dirac cone at the Fermi energy (E F) than the uniaxial and volume conservation strains. Interestingly, the transition from direct E g to indirect E g state of the (0001) surface of Bi2Se3 is observed in the volume conservation strain-dependent E g. The strain on Bi2Se3, significantly modifies the conduction band of Se2 atoms near E F compared to Bi and Se1, and plays a vital role in the conduction of the (0001) surface of Bi2Se3. The atomic cohesive energy of the Bi2Se3 slab is very close to that of (0001) oriented nanocrystals extracted from the Raman spectra. The strain-dependent cohesive energy indicates that at a higher value of strain, the uniaxial and volume conservation strain provides better stability than that of the biaxial strain (0001) oriented growth of the Bi2Se3 nanocrystals. Our study establishes the relationship between the strained lattice and electronic structures of Bi2Se3, and more generally demonstrates the tuning of the Dirac point with the mechanical strain.

Interfacial reconstruction in La0.7Sr0.3MnO3 thin films: giant low-field magnetoresistance.

Herein, interfacial reconstruction in a series of La0.7Sr0.3MnO3 (LSMO) films grown on a (001) oriented LaAlO3 (LAO) substrate using the pulsed plasma sputtering technique is demonstrated. X-ray diffraction studies suggested that the LSMO film on LAO was stabilized in a tetragonal structure, which was relaxed in-plane and strained along the out-of-plane direction. The interfacial reconstruction of the LSMO-LAO interface due to the reorientation of the Mn ion spin induced spin-glass behavior due to the presence of non-collinear Mn ion spins. Consequently, the interface effect was observed on the Curie temperature, temperature-dependent resistivity, metal-to-semiconductor transition temperature, and magnetoresistance (MR). At a magnetic field of 7 T, MR decreased from 99.8% to 7.69% as the LSMO film thickness increased from 200 Å to 500 Å. A unique characteristic of the LSMO films is the large low-field MR after a decrease in the field from the maximum field. The observed temperature-dependent magnetization and low-temperature resistivity upturn of the LSMO films grown on LAO provide direct evidence that the low-field MR is due to the non-collinear interfacial spins of Mn. The present work demonstrates the great potential of interface and large low-field MR, which might advance the fundamental applications of orbital physics and spintronics.

Ultrathin Scale Tailoring of Anisotropic Magnetic Coupling and Anomalous Magnetoresistance in SrRuO3-PrMnO3 Superlattices.

A strong perpendicular magnetocrystalline anisotropy (PMA) in antiferromagnetically coupled SrRuO3(17 uc (unit cell))/PrMnO3( n uc) superlattices effectively reconstructs the interfacial spin ordering. The occurrence of significant anisotropic interfacial antiferromagnetic coupling between the Ru and Mn ions is systematically tuned by varying the PrMnO3 layer thickness in ultrathin scale from 3 to 12 uc, which is associated with a rise in PMA energy from 0.28 × 106 to 1.60 × 106 erg/cm3. The analysis using the Stoner-Wohlfarth model and density functional theory confirm that the exchange anisotropy is the major contribution to the PMA. The superlattices with PrMnO3 layer thickness ≥7 uc exhibit the tunneling-like transport of Ru 4d electrons, which is rather expected in the stronger antiferromagnetically coupled superlattices with thinner PrMnO3 layer. Tunneling-like transport at thicker spacer layer in the SrRuO3-PrMnO3 superlattice system is an unique feature of two ferromagnet-based superlattices. Our investigations show that the technologically important interfacial magnetic coupling, PMA, and tunneling magnetoresistance could be achieved in a periodically stacked bilayer and can be precisely manipulated by the size effect in ultrathin scale.

Apparatus and method for the growth of epitaxial complex oxides on native amorphous SiO2 surface of (001) oriented single crystal silicon.

The design, fabrication, and performance of an apparatus for the deposition of complex oxides with highly uniform thicknesses at controllable deposition rates over large area, even on the native amorphous SiO2 layer of (001) oriented single crystal Si, are described. The apparatus makes use of the lateral port of a spherical chamber. The port is maintained at uniform temperature, and it houses a substrate heater. The deposition process is controlled by varying different parameters such as target-to-substrate distance, sputtering power, sputtering gas atmosphere, substrate temperature, and pulsed plasma growth. The system has been tested by growing a series of La0.7Sr0.3MnO3 thin films on Si. The systematic strain relaxation and thus the tunable magnetic properties along with the presence of high-quality surface morphology of the films indicate that the designed system could be used to fabricate different components of oxide electronics-based devices over larger area.

Interfacial Antiferromagnetic Coupling and Dual-Exchange Bias in Tetragonal SrRuO3-PrMnO3 Superlattices.

The functional properties of oxide heterostructures depend on the interfaces accommodating ions, their spins, and structural mismatches. Here, by stabilizing tetragonal symmetry, we achieve the in-plane antiferromagnetic (AFM) ordering and dual-exchange bias in the superlattices consisting of two ferromagnets SrRuO3 (SRO) and PrMnO3 (PMO). The tetragonal symmetry of this superlattice system achieved after the octahedral rotations yield an elongation of the c-axis parameter with Ru-O-Mn bond angle close to 180°, induces an interfacial antiferromagnetic ordering, which is suppressed as the ferromagnetic (FM) ordering in the PMO layer increases. The 0.1 T in-plane cooling field (Hcool) leads to the shift (ca. -0.04 T) of minor hysteresis loop along the negative field axis due to the presence of -0.87 erg/cm2 AFM interfacial exchange coupling energy density (ERu,Mn) at 20 K. The exchange bias field (HEB) switches from negative to positive value with the increase in Hcool. For 5 T Hcool, the HEB is positive, but the ERu,Mn is -1.25 erg/cm2 for n ≤ 8 (n = number of unit cells of PMO) and 1.52 erg/cm2 for n ≥ 8. The HEB and its switching from negative to positive with the increase in Hcool are explained by the interplay of strong antiferromagnetic coupling energy and Zeeman energy at the interfaces. The results demonstrate that the SRO-PMO superlattice could be a model system for the investigation of the interfacial exchange coupling in functional oxides.