Jeehoon Kim

Nanoscale imaging and control of resistance switching in VO2 at room temperature

We demonstrate controlled local phase switching of a VO2 film using a biased conducting atomic force microscope tip. After application of an initial, higher “training” voltage, the resistance transition is hysteretic with IV loops converging upon repeated voltage sweep. The threshold Vset to initiate the insulator-to-metal transition is on order ∼5 V at room temperature, and increases at low temperature. We image large variations in Vset from grain to grain. Our imaging technique opens up the possibility for an understanding of the microscopic mechanism of phase transition in VO2 as well as its potential relevance to solid state devices.

Violation of Ohm’s law in a Weyl metal

Ohms law is a fundamental paradigm in the electrical transport of metals. Any transport signatures violating Ohms law would give an indisputable fingerprint for a novel metallic state. Here, we uncover the breakdown of Ohms law owing to a topological structure of the chiral anomaly in the Weyl metal phase. We observe nonlinear I-V characteristics in Bi0.96Sb0.04 single crystals in the diffusive limit, which occurs only for a magnetic-field-aligned electric field (E∥B). The Boltzmann transport theory with the charge pumping effect reveals the topological-in-origin nonlinear conductivity, and it leads to a universal scaling function of the longitudinal magnetoconductivity, which completely describes our experimental results. As a hallmark of Weyl metals, the nonlinear conductivity provides a venue for nonlinear electronics, optical applications, and the development of a topological Fermi-liquid theory beyond the Landau Fermi-liquid theory.

Microcantilever Q control via capacitive coupling

We introduce a versatile method to control the quality factor Q of a conducting cantilever in an atomic force microscope (AFM) via capacitive coupling to the local environment. Using this method, Q may be reversibly tuned to within ∼10% of any desired value over several orders of magnitude. A point-mass oscillator model describes the measured effect. Our simple Q control module increases the AFM functionality by allowing greater control of parameters such as scan speed and force gradient sensitivity, which we demonstrate by topographic imaging of a VO2 thin film in high vacuum.

Charge ordering, ferroelectric, and magnetic domains in LuFe2O4 observed by scanning probe microscopy

LuFe2O4 is a multiferroic system which exhibits charge order, ferroelectricity, and ferrimagnetism simultaneously below ∼230 K. The ferroelectric/charge order domains of LuFe2O4 are imaged with both piezoresponse force microscopy (PFM) and electrostatic force microscopy (EFM), while the magnetic domains are characterized by magnetic force microscopy (MFM). Comparison of PFM and EFM results suggests that the proposed ferroelectricity in LuFe2O4 is not of usual displacive type but of electronic origin. Simultaneous characterization of ferroelectric/charge order and magnetic domains by EFM and MFM, respectively, on the same surface of LuFe2O4 reveals that both domains have irregular patterns of similar shape, but the length scales are quite different. The domain size is approximately 100 nm for the ferroelectric domains, while the magnetic domain size is much larger and gets as large as 1 μm. We also demonstrate that the origin of the formation of irregular domains in LuFe2O4 is not extrinsic but intrinsic.

Measurement of the magnetic penetration depth of a superconducting MgB2 thin film with a large intraband diffusivity

We report the temperature dependent magnetic penetration depth

Single-vortex pinning and penetration depth in superconducting NdFeAsO1−xFx

\lambda(T)

Spatially resolved penetration depth measurements and vortex manipulation in the ferromagnetic superconductor ErNi2B2C

and the superconducting critical field

Observation of Electronic Inhomogeneity and Charge Density Waves in a Bilayer La 2 − 2 x Sr 1 + 2 x Mn 2 O 7 Single Crystal

H_{c2}(T)

Construction of a 3He magnetic force microscope with a vector magnet

in a 500-nm MgB

Investigation of Superconductivity and Magnetism by Magnetic Force Microscope

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