Shichao Yan

Purely Coherent Nonlinear Optical Response in Solution Dispersions of Graphene Sheets

We have developed an efficient chemical exfoliation approach for the high-throughput synthesis of solution-processable, high-quality graphene sheets that are noncovalently functionalized by alkylamine. Purely coherent nonlinear optical response of these graphene sheets has been investigated, using near-infrared, visible, and ultraviolet continous wave and ultrafast laser beams. Spatial self-phase modulation has been unambiguously observed in the solution dispersions. Our results suggest that this coherent light scattering is due to a broadband, ultrafast, and remarkably huge third-order optical nonlinearity χ(3), which is a manifestation of the graphenes cone-shaped large-energy-scale band structure. Our experimental findings endow graphene new potentials in nonlinear optical applications.

Control of quantum magnets by atomic exchange bias

Mixing of discretized states in quantum magnets has a radical impact on their properties. Managing this effect is key for spintronics in the quantum limit. Magnetic fields can modify state mixing and, for example, mitigate destabilizing effects in single-molecule magnets. The exchange bias field has been proposed as a mechanism for localized control of individual nanomagnets. Here, we demonstrate that exchange coupling with the magnetic tip of a scanning tunnelling microscope provides continuous tuning of spin state mixing in an individual nanomagnet. By directly measuring spin relaxation time with electronic pump-probe spectroscopy, we find that the exchange interaction acts analogously to a local magnetic field that can be applied to a specific atom. It can be tuned in strength by up to several tesla and cancel external magnetic fields, thereby demonstrating the feasibility of complete control over individual quantum magnets with atomically localized exchange coupling.

Magnetic fingerprint of individual Fe4 molecular magnets under compression by a scanning tunnelling microscope

Single-molecule magnets (SMMs) present a promising avenue to develop spintronic technologies. Addressing individual molecules with electrical leads in SMM-based spintronic devices remains a ubiquitous challenge: interactions with metallic electrodes can drastically modify the SMMs properties by charge transfer or through changes in the molecular structure. Here, we probe electrical transport through individual Fe4 SMMs using a scanning tunnelling microscope at 0.5 K. Correlation of topographic and spectroscopic information permits identification of the spin excitation fingerprint of intact Fe4 molecules. Building from this, we find that the exchange coupling strength within the molecules magnetic core is significantly enhanced. First-principles calculations support the conclusion that this is the result of confinement of the molecule in the two-contact junction formed by the microscope tip and the sample surface.

Turning on and off the Rotational Oscillation of a Single Porphine Molecule by Molecular Charge State

The rotation dynamics of single magnesium porphine molecules on an ultrathin NaCl bilayer is investigated with low-temperature scanning tunneling microscopy and density functional theory calculations. It is observed that the rotational oscillation between two stable orientations can be turned on and off by the molecular charge state, which can be manipulated with the tunneling electrons. The features of the charge states and the mechanism of molecular rotational on/off state control are revealed at the atomic scale. The dependence of molecular orientation switching rate on the tunneling electron energy and the current density illustrates the underlying resonant tunneling excitation and single-electron process. The drive and control of molecular motion with tunneling electrons demonstrated in this study suggests a novel approach toward electronically controlled molecular rotors and motors.

Influence of Domain Walls in the Incommensurate Charge Density Wave State of Cu Intercalated 1T-TiSe2

We report a low-temperature scanning tunneling microscopy study of the charge density wave (CDW) order in 1T-TiSe_{2} and Cu_{0.08}TiSe_{2}. In pristine 1T-TiSe_{2} we observe a long-range coherent commensurate CDW (CCDW) order. In contrast, Cu_{0.08}TiSe_{2} displays an incommensurate CDW (ICDW) phase with localized CCDW domains separated by domain walls. Density of states measurements indicate that the domain walls host an extra population of fermions near the Fermi level which may play a role in the emergence of superconductivity in this system. Fourier transform scanning tunneling spectroscopy studies suggest that the dominant mechanism for CDW formation in the ICDW phase may be electron-phonon coupling.

Nonlocally sensing the magnetic states of nanoscale antiferromagnets with an atomic spin sensor

A three-atom spin chain can sense the magnetic states of nano-antiferromagnets with micro–electron volt sensitivity. The ability to sense the magnetic state of individual magnetic nano-objects is a key capability for powerful applications ranging from readout of ultradense magnetic memory to the measurement of spins in complex structures with nanometer precision. Magnetic nano-objects require extremely sensitive sensors and detection methods. We create an atomic spin sensor consisting of three Fe atoms and show that it can detect nanoscale antiferromagnets through minute, surface-mediated magnetic interaction. Coupling, even to an object with no net spin and having vanishing dipolar stray field, modifies the transition matrix element between two spin states of the Fe atom–based spin sensor that changes the sensor’s spin relaxation time. The sensor can detect nanoscale antiferromagnets at up to a 3-nm distance and achieves an energy resolution of 10 μeV, surpassing the thermal limit of conventional scanning probe spectroscopy. This scheme permits simultaneous sensing of multiple antiferromagnets with a single-spin sensor integrated onto the surface.

Three-Dimensional Mapping of Single-Atom Magnetic Anisotropy

Magnetic anisotropy plays a key role in the magnetic stability and spin-related quantum phenomena of surface adatoms. It manifests as angular variations of the atoms magnetic properties. We measure the spin excitations of individual Fe atoms on a copper nitride surface with inelastic electron tunneling spectroscopy. Using a three-axis vector magnet we rotate the magnetic field and map out the resulting variations of the spin excitations. We quantitatively determine the three-dimensional distribution of the magnetic anisotropy of single Fe atoms by fitting the spin excitation spectra with a spin Hamiltonian. This experiment demonstrates the feasibility of fully mapping the vector magnetic properties of individual spins and characterizing complex three-dimensional magnetic systems.

Building Complex Kondo Impurities by Manipulating Entangled Spin Chains

The creation of molecule-like structures in which magnetic atoms interact controllably is full of potential for the study of complex or strongly correlated systems. Here, we create spin chains in which a strongly correlated Kondo state emerges from magnetic coupling of transition-metal atoms. We build chains up to ten atoms in length by placing Fe and Mn atoms on a Cu2N surface with a scanning tunneling microscope. The atoms couple antiferromagnetically via superexchange interaction through the nitrogen atom network of the surface. The emergent Kondo resonance is spatially distributed along the chain. Its strength can be controlled by mixing atoms of different transition metal elements and manipulating their spatial distribution. We show that the Kondo screening of the full chain by the electrons of the nonmagnetic substrate depends on the interatomic entanglement of the spins in the chain, demonstrating the prerequisites to build and probe spatially extended strongly correlated nanostructures.

Tip expansion in a laser assisted scanning tunneling microscope

The thermal expansion of a scanning tunneling microscope tip induced by femtosecond laser is investigated with various parameters including laser power, modulation frequency, illumination spot, and laser wavelength. The magnitude of tip expansion is measured to be proportional to the laser power. The response bandwidth is closely related to the length of the tip cone section, which is consistent with a two-rod model simulation. While visible lasers produce significant tip expansion, deep ultraviolet and near infrared lasers result in significantly reduced expansion magnitude, which can be explained with the tip induced surface plasmon in the tunneling junction.

Scanning probe microscopy: close-up on spin coherence.

Ultrafast, coherent spin dynamics in semiconductor heterostructures can be measured with a scanning tunnelling microscope by using femtosecond pulses of circularly polarized light.