arXiv subjects

Chang-Beom Eom

Publications and source records attributed to Chang-Beom Eom.

71 records · Page 4Linked to original sources

arXiv2016

Room-temperature quantum transport signatures in graphene/LaAlO3/SrTiO3 heterostructures

The pseudospin quantum degree of freedom is one of the most remarkable properties of graphene that distinguishes it from ordinary two-dimensional metals and semiconductors. Pseudospin quantum interference leads to weak antilocalization (WAL) and is influenced strongly by point defects and thermal perturbations that break chirality and destroy phase coherence. Preserving and manipulating quantum transport properties up to room temperature is key to realizing practical pseudospin-based graphene devices. Here we report fabrication and transport characterization of graphene field-effect devices on a complex-oxide heterostructure, LaAlO3/SrTiO3. Signatures of WAL, a consequence of pseudospin quantum interference, persist up to room temperature. The LaAlO3/SrTiO3 substrate plays a critical role in suppressing short-range impurity scattering and electron-phonon coupling. The observation of quantum transport signatures at room temperature is unique to this system and presents new opportunities for the development of pseudospin-based devices and novel multifunctional devices that couple to the complex-oxide interface.

cond-mat.mes-hall↗

arXiv2016

Method for Transferring High-Mobility CVD-Grown Graphene with Perfluoropolymers

The transfer of graphene grown by chemical vapor deposition (CVD) using amorphous polymers represents a widely implemented method for graphene-based electronic device fabrication. However, the most commonly used polymer, poly(methyl methacrylate) (PMMA), leaves a residue on the graphene that limits the mobility. Here we report a method for graphene transfer and patterning that employs a perfluoropolymer---Hyflon---as a transfer handle and to protect graphene against contamination from photoresists or other polymers. CVD-grown graphene transferred this way onto LaAlO$_3$/SrTiO$_3$ heterostructures is atomically clean, with high mobility (~30,000 cm$^2$V$^{-1}$s$^{-1}$) near the Dirac point at 2 K and clear, quantized Hall and magneto-resistance. Local control of the LaAlO$_3$/SrTiO$_3$ interfacial metal-insulator transition---through the graphene---is preserved with this transfer method. The use of perfluoropolymers such as Hyflon with CVD-grown graphene and other 2D materials can readily be implemented with other polymers or photoresists.

cond-mat.mtrl-sci↗

arXiv2016

Tunable electron-electron interactions in LaAlO3/SrTiO3 nanostructures

The interface between the two complex oxides LaAlO3 and SrTiO3 has remarkable properties that can be locally reconfigured between conducting and insulating states using a conductive atomic force microscope. Prior investigations of sketched quantum dot devices revealed a phase in which electrons form pairs, implying a strongly attractive electron-electron interaction. Here, we show that these devices with strong electron-electron interactions can exhibit a gate-tunable transition from a pair-tunneling regime to a single-electron (Andreev bound state) tunneling regime where the interactions become repulsive. The electron-electron interaction sign change is associated with a Lifshitz transition where the dxz and dyz bands start to become occupied. This electronically tunable electron-electron interaction, combined with the nanoscale reconfigurability of this system, provides an interesting starting point towards solid-state quantum simulation.

cond-mat.str-el↗

arXiv2016

Micrometer-scale ballistic transport of electron pairs in LaAlO3/SrTiO3 nanowires

High-mobility complex-oxide heterostructures and nanostructures offer new opportunities for extending the paradigm of quantum transport beyond the realm of traditional III-V or carbon-based materials. Recent quantum transport investigations with LaAlO$_3$/SrTiO$_3$-based quantum dots have revealed the existence of a strongly correlated phase in which electrons form spin-singlet pairs without becoming superconducting. Here we report evidence for micrometer-scale ballistic transport of electron pairs in quasi-one-dimensional (quasi-1D) LaAlO$_3$/SrTiO$_3$ nanowire cavities. In the paired phase, Fabry-Perot-like quantum interference is observed, in sync with conductance oscillations observed in the superconducting regime (at zero magnetic field). Above a critical magnetic field $B_p$, electron pairs unbind and conductance oscillations shift with magnetic field. These experimental observations extend the regime of ballistic electronic transport to strongly correlated phases.

cond-mat.mes-hall↗

arXiv2015

Ultrafast observation of electron hybridization and in-gap states formation in Kondo insulator SmB6

SmB6 is a promising candidate for topological Kondo insulator. In this letter, we report ultrafast carrier dynamics of SmB6. Two characteristic temperatures: T1=100 K and T2= 20 K are observed. T1 corresponds to the opening of the f-d hybridization gap revealed by an abrupt disappearance of terahertz f-band plasmon oscillations. Between T1 and T2, a phonon bottleneck effect dominates the photocarrier relaxation processes. Below T2, we observe the formation of in-gap states, which are strongly affected by optically injected hot electrons and the transient electron temperature change.

cond-mat.str-el↗

arXiv2014

Evidence of Josephson junction behavior in top-gated LaAlO$_3$-SrTiO$_3$

We demonstrate top-gate tunable Josephson junction like behavior in the two dimensional electron gas at the LaAlO$_3$-SrTiO$_3$ interface. A combination of global back-gating and local top-gating is used to define the junctions, providing an efficient way for much finer spatial control over the properties of the interface, as compared to back-gating alone. The variation of critical currents and zero bias resistances with temperature shows that the junctions behave like short, overdamped weak links. This technique could be an important tool to illuminate the nature of superconductivity in the LaAlO$_3$-SrTiO$_3$ interface system.

cond-mat.supr-con↗

arXiv2013

Room-Temperature Electronically-Controlled Ferromagnetism at the LaAlO3/SrTiO3 Interface

Reports of emergent conductivity, superconductivity, and magnetism at oxide interfaces have helped to fuel intense interest in their rich physics and technological potential. Here we employ magnetic force microscopy to search for room-temperature magnetism in the well-studied LaAlO3/SrTiO3 system. Using electrical top gating to deplete electrons from the oxide interface, we directly observe an in-plane ferromagnetic phase with sharply defined domain walls. Itinerant electrons, introduced by a top gate, align antiferromagnetically with the magnetization, at first screening and then destabilizing it as the conductive state is reached. Subsequent depletion of electrons results in a new, uncorrelated magnetic pattern. This newfound control over emergent magnetism at the interface between two non-magnetic oxides portends a number of important technological applications.

cond-mat.mes-hall↗

arXiv2013

Electro-Mechanical Response of Top-Gated LaAlO3/SrTiO3 Heterostructures

LaAlO3/SrTiO3 heterostructures are known to exhibit a sharp, hysteretic metal-insulator transition (MIT) with large enhanced capacitance near depletion. To understand the physical origin of this behavior, the electromechanical response of top-gated LaAlO3/SrTiO3 heterostructures is probed using two simultaneous measurement techniques: piezoforce microscopy (PFM) and capacitance spectroscopy. PFM measurements reveal local variations in the hysteretic response, which is directly correlated with capacitance measurements. The enhanced capacitance at the MIT is linked to charging/discharging dynamics of nanoscale conducting islands, which are revealed through PFM imaging and time-resolved capacitance and piezoresponse measurements.

cond-mat.mes-hall↗

arXiv2013

Anomalous Transport in Sketched Nanostructures at the LaAlO3/SrTiO3 Interface

The oxide heterostructure LaAlO3/SrTiO3 supports a two-dimensional electron liquid with a variety of competing phases including magnetism, superconductivity and weak antilocalization due to Rashba spin-orbit coupling. Further confinement of this 2D electron liquid to the quasi-one-dimensional regime can provide insight into the underlying physics of this system and reveal new behavior. Here we describe magnetotransport experiments on narrow LaAlO3/SrTiO3 structures created by a conductive atomic force microscope lithography technique. Four-terminal local transport measurements on ~10-nm-wide Hall bar structures yield longitudinal resistances that are comparable to the resistance quantum h/e2 and independent of the channel length. Large nonlocal resistances (as large as 10^4 ohms) are observed in some but not all structures with separations between current and voltage that are large compared to the 2D mean-free path. The nonlocal transport is strongly suppressed by the onset of superconductivity below ~200 mK. The origin of these anomalous transport signatures is not understood, but may arise from coherent transport defined by strong spin-orbit coupling and/or magnetic interactions.

cond-mat.mes-hall↗

arXiv2012

Superconducting LaAlO3/SrTiO3 Nanowires

We report superconductivity in quasi-1D nanostructures created at the LaAlO3/SrTiO3 interface. Nanostructures having line widths w~10 nm are formed from the parent two-dimensional electron liquid using conductive atomic force microscope lithography. Nanowire cross-sections are small compared to the superconducting coherence length in LaAlO3/SrTiO3 (w<<xi~100 nm), placing them in the quasi-1D regime. Broad superconducting transitions with temperature and finite resistances in the superconducting state well below Tc~200 mK are observed. V-I curves show switching between the superconducting and normal states that are characteristic of superconducting nanowires. The four-terminal resistance in the superconducting state shows an unusual dependence on the current path, varying by as much as an order of magnitude.

cond-mat.mes-hall↗

arXiv2012

Nonlocal Piezoresponse of LaAlO3/SrTiO3 Heterostructures

The hysteretic piezoelectric response in LaAlO3/SrTiO3 heterostructures can provide important insights into the mechanism for interfacial conductance and its metastability under various conditions. We have performed a variety of nonlocal piezoelectric force microscopy experiments on 3 unit cell LaAlO3/SrTiO3 heterostructures. A hysteretic piezoresponse is observed under various environmental and driving conditions. The hysteresis is suppressed when either the sample is placed in vacuum or the interface is electrically grounded. We present a simple physical model which can account for the observed phenomena.

cond-mat.mtrl-sci↗

arXiv2011

Cooper Pair Writing at the LaAlO3/SrTiO3 Interface

The LaAlO3/SrTiO3 interface provides a unique platform for controlling the electronic properties of the superconducting semiconductor SrTiO3. Prior investigations have shown that two-dimensional superconductivity can be produced at the LaAlO3/SrTiO3 interface and tuned electrostatically. The recently demonstrated reversible control of the metal-insulator transition at the same interface using conductive atomic force microscopy (c-AFM) raises the question of whether this room-temperature technique can produce structures that exhibit superconducting, normal metallic and insulating phases at sub-Kelvin temperatures. Here we report low-temperature magnetotransport experiments on conducting structures defined at an otherwise insulating LaAlO3/SrTiO3 interface. A quantum phase transition associated with the formation of Cooper pairs is observed in these predefined structures at sub-Kelvin temperatures. However, a finite resistance remains even at the lowest temperature. At higher magnetic fields, interfaces with high mobility also exhibit strong Shubnikov-de Haas oscillations as well as a larger Ginsburg-Landau coherence length. Cooper pair localization, spin-orbit coupling, and finite-size effects may factor into an explanation for some of the unusual properties observed.

cond-mat.mes-hall↗

arXiv2011

Nature of polarization fatigue in BiFeO3

As a room-temperature multiferroic, BiFeO3 has been intensively investigated for both magnetoelectric devices and non-volatile ferroelectric memory applications. Both magnetoelectric and ferroelectric memory devices have the same control knob: polarization switching by an applied electric field. Due to the rhombohedral symmetry of BiFeO3, there are four ferroelastic variances and three different polarization switching events: (1) 71° switching from r1- to r3+, (2) 109° switching from r1- to r2+ (or r4+), and (3) 180o switching from r1- to r1+ (the superscript + and - stand for up and down polarization, respectively). Each switching path is coupled to a different reorientation of the BiFeO3 unit cell, and hence different coupling to the magnetic order as well as different magnitudes of switchable polarization. A degradation of the ferroelectric properties of BiFeO3 will result in losing controllability of magnetic order switching in magnetoelectric devices and capacity for information storage in ferroelectric memory devices. Especially, polarization fatigue will directly restrict the reliability of the actual devices. Hence it is important to understand the intrinsic fatigue behavior of each polarization switching path in BiFeO3 thin films. In this communication, we report polarization fatigue in BiFeO3 depending on switching path, and propose a fatigue model which will broaden our understanding of the fatigue phenomenon in low-symmetry materials.

cond-mat.mtrl-sci↗

arXiv2010

"Water-cycle" mechanism for writing and erasing nanostructures at the LaAlO3/SrTiO3 interface

Nanoscale control of the metal-insulator transition in LaAlO3/ SrTiO3 heterostructures can be achieved using local voltages applied by a conductive atomic-force microscope probe. One proposed mechanism for the writing and erasing process involves an adsorbed H2O layer at the top LaAlO3 surface. In this picture, water molecules dissociates into OH- and H+ which are then selectively removed by a biased AFM probe. To test this mechanism, writing and erasing experiments are performed in a vacuum AFM using various gas mixtures. Writing ability is suppressed in those environments where H2O is not present. The stability of written nanostructures is found to be strongly associated with the ambient environment. The self-erasure process in air can be strongly suppressed by creating a modest vacuum or replacing the humid air with dry inert gas. These experiments provide strong constraints for theories of both the writing process as well as the origin of interfacial conductance.

cond-mat.mes-hall↗

arXiv2010

Rewritable nanoscale oxide photodetector

Nanophotonic devices seek to generate, guide, and/or detect light using structures whose nanoscale dimensions are closely tied to their functionality. Semiconducting nanowires, grown with tailored optoelectronic properties, have been successfully placed into devices for a variety of applications. However, the integration of photonic nanostructures with electronic circuitry has always been one of the most challenging aspects of device development. Here we report the development of rewritable nanoscale photodetectors created at the interface between LaAlO3 and SrTiO3. Nanowire junctions with characteristic dimensions 2-3 nm are created using a reversible AFM writing technique. These nanoscale devices exhibit a remarkably high gain for their size, in part because of the large electric fields produced in the gap region. The photoconductive response is gate-tunable and spans the visible-to-near-infrared regime. The ability to integrate rewritable nanoscale photodetectors with nanowires and transistors in a single materials platform foreshadows new families of integrated optoelectronic devices and applications.

cond-mat.mes-hall↗

arXiv2010

Nanoscale rectification at the LaAlO3/SrTiO3 interface

Nanoscale control over electron transport at scales that are comparable to the Fermi wavelength or mean-free path can lead to new families of electronic devices. Here we report electrical rectification in nanowires formed by nanoscale control of the metal-insulator transition at the interface between LaAlO3 and SrTiO3. Controlled in-plane asymmetry in the confinement potential produces electrical rectification in the nanowire, analogous to what occurs naturally for Schottky diodes or by design in structures with engineered structural inversion asymmetry. Nanostructures produced in this manner may be useful for electro-optic applications or in spintronic devices.

cond-mat.mes-hall↗

arXiv2005

Epitaxial magnetic perovskite nanostructures

Precise arrays of sub-100 nm diameter nanodots are fabricated from magnetic oxide perovskites with the total coverage area up to 5 mm2. Epitaxial single crystal structure of the nanodots is demonstrated by cross-sectional TEM. Low temperature MFM study confirmed the ferromagnetism of the produced particles.

cond-mat.mtrl-sci↗