Electrical Transport and Point Contact Spectroscopy in Core-Shell Bimetallic Nanostructures

Abstract

An extensively pursued strategy for realizing new materials is the creation of interfaces between two similar or dissimilar systems, which can invoke multiple processes like electronic reconstruction, strain-induced phenomena, spin-orbital interaction, formation of artificial superlattice and emergent spin and orbital ordering. The motivation behind the work done in this thesis was to fabricate a metal where nanoscale crystalline interfaces can be embedded in a controlled manner, and the density of the interfaces can be tuned such that the separation between the interfaces is of the order of the electronic wavelengths. To achieve this, we have adapted the strategy of creating core-shell metal nanoparticles (NPs) and assembling them to form a structure where ultra-small NPs (forming the core) are embedded inside a crystalline matrix (forming the shell). We have chosen Ag and Au NPs, two of the most widely studied metal NPs, to form this ‘cookie’-like structure. Ultra-small NPs of noble metals, in particular silver (Ag) or gold (Au), have been extensively investigated for their optical, magnetic, chemical, and physical properties, but assembling such structures in an electrically conducting metallic matrix, where the physical dimension of individual NPs plays a decisive role, has remained elusive. This is because true metallic conduction through individual or clusters of metallic NPs is often prevented by tunnel barriers due to surface-protecting ligands, oxidation, etc.

We established innovative synthesis protocols overcoming the existing challenges in fabrication and demonstrated a bimetallic hybrid where Ag nanoparticles (AgNPs) of diameter 1-3 nm are embedded inside an Au lattice at separations of 3-5 nm. Our synthesis protocols offer precise tunability over the radius and filling fraction, F, of the buried AgNPs. We show, for the first time, the formation of an all-metal composite via bottom-up assembly, where the electrical resistivity of the hybrid scales directly with the net area of the buried nanoscale interfaces of Au and Ag. Using a combination of electrical transport and point contact spectroscopy, we observed (1) a systematic increase in the electron-phonon coupling constant, λ by nearly two orders of magnitude compared to pristine Au or Ag with increasing density of AgNPs, (2) robust metallic transport over the entire experimental range of temperature (T ∼ 6−300 K) and λ, and (3) scaling of τ^(−1)_(e−ph) with λT for all devices for T > Θ_D, where Θ_D is the Debye temperature. With increasing AgNP density, the electrical resistivity deviates from T-linearity and approaches a saturation to the Mott-Ioffe-Regel scale ρ_MIR ∼ ha/e^2 for both disorder (T → 0) and phonon (T ≫ Θ_D)-dependent components of resistivity (here, a = 0.3 nm, is the lattice constant of Au). We show that λ as large as ≈ 20 can be obtained by tuning the density of AgNPs, which is almost ten times larger than that known for any metallic solid so far. Our results also indicate the depletion of conduction electrons at the fermi level due to polaronic localization. Both the electrical transport and point contact spectroscopy measurements suggest a unique coexistence of itinerant and localized electrons (polarons) in the system.

From quantum transport measurements, we show nearly ∼ 20 times increase in the spin-orbit coupling (SOC), quantified by SO scattering rate (1/τ_(soc)) with increasing interface density, F. We show compelling evidence that the spin-relaxation arises from a Rashba coupling at the buried Ag/Au interfaces. The resultant coupling strength can be tuned by an order of magnitude with varying F, reaching a maximum of ∼ 15 − 25 meV/Å at temperature ∼ 8 K. Accessing Rashba-like physics has only been possible until now, either in 2D interfaces or surfaces or in 3D noncentrosymmetric crystals with polar sites, which are mostly non-metallic. We report, for the first time, Rashba coupling in a bulk metal that also globally preserves the inversion symmetry. Using a combination of quantum transport and point contact spectroscopy measurements, we show that the polaronic trapping of electrons likely modulates the strength of the Rashba interaction by affecting the wavefunction asymmetry at the interfaces.

The work done in this thesis establishes that with a densely packed network of crystalline interfaces, one can tune the interactions among the lattice, charge, spin and orbital degrees of freedom by an unprecedented amount in noble metals like Au and Ag, that have been known to have the weakest electron-phonon and electron-electron interactions.