Yoon Hee Jeong

Concurrent transition of ferroelectric and magnetic ordering near room temperature

Strong spin-lattice coupling in condensed matter gives rise to intriguing physical phenomena such as colossal magnetoresistance and giant magnetoelectric effects. The phenomenological hallmark of such a strong spin-lattice coupling is the manifestation of a large anomaly in the crystal structure at the magnetic transition temperature. Here we report that the magnetic Néel temperature of the multiferroic compound BiFeO(3) is suppressed to around room temperature by heteroepitaxial misfit strain. Remarkably, the ferroelectric state undergoes a first-order transition to another ferroelectric state simultaneously with the magnetic transition temperature. Our findings provide a unique example of a concurrent magnetic and ferroelectric transition at the same temperature among proper ferroelectrics, taking a step toward room temperature magnetoelectric applications.

The 3ω technique for measuring dynamic specific heat and thermal conductivity of a liquid or solid

We show how to measure dynamic specific heat and thermal conductivity of a solid or liquid sample using the 3ω technique, which is an ac‐modulation method where we use a heater simultaneously as the sensor. By varying the width of the heater relative to the thermal decay length, one can choose the proper regime to measure thermal conductivity or specific heat. The technique is applied to window glass and the results confirm the validity of the method. Experimental results for potassium dihydrogen phosphate crystal demonstrate the first‐order transition at the Curie point, and the dynamic specific heat of supercooled liquid potassium–calcium nitrate is shown.

Metal insulator transitions in perovskite SrIrO3 thin films

Understanding of metal insulator transitions in a strongly correlated system, driven by Anderson localization (disorder) and/or Mott localization (correlation), is a long standing problem in condensed matter physics. The prevailing fundamental question would be how these two mechanisms contrive to accomplish emergent anomalous behaviors. Here, we have grown high quality perovskite SrIrO3 thin films, containing a strong spin orbit coupled 5d element Ir, on various substrates such as GdScO3 (110), DyScO3 (110), SrTiO3 (001), and NdGaO3 (110) with increasing lattice mismatch, in order to carry out a systematic study on the transport properties. We found that metal insulator transitions can be induced in this system; by either reducing thickness (on best lattice matched substrate) or changing degree of lattice strain (by lattice mismatch between film and substrates) of films. Surprisingly these two pathways seek two distinct types of metal insulator transitions; the former falls into disorder driven Anderson type whereas the latter turns out to be of unconventional Mott-Anderson type with the interplay of disorder and correlation. More interestingly, in the metallic phases of SrIrO3, unusual non-Fermi liquid characteristics emerge in resistivity as Δρ ∝ Te with e evolving from 4/5 to 1 to 3/2 with increasing lattice strain. We discuss theoretical implications of these phenomena to shed light on the metal insulator transitions.

A Highly Efficient Three-Stage Doherty Power Amplifier with Flat Gain for WCDMA Applications

A three-stage Doherty power amplifier (DPA) with a flat gain is represented. The driving amplifier at an input of peaking cells is used to achieve high efficiency and flat gain to control the input power level of peaking cells. For the experimental validation, the proposed DPA is implemented using a GaN HEMT and Si LDMOSFETs. For a continuous wave signal, power-added efficiencies of 34.0% and 39.3% are obtained at 8.5-dB back-off power (BOP) and 4.0-dB BOP from the saturation output power, respectively. The measured one-carrier wideband code division multiple access results show high efficiency and flat gain characteristics above 10 dB over the entire output power range.

Ferromagnetic ZnO bicrystal nanobelts fabricated in low temperature

Zinc oxide bicrystal nanobelts were fabricated via a vapor phase transport of a powder mixture of Zn, BiI3, and MnCl2∙H2O at temperatures as low as 300°C. The bicrystal nanobelts, growing along the [011−3] direction, have the widths of 40–150nm and lengths of tens of microns. The energy dispersive x-ray spectroscopy result verifies that the bicrystal nanobelts contain higher concentration of both Bi and Mn along the grain boundary. The investigation of the growth mechanism proposes that MnBi may induce the formation of bicrystal nanobelts. Photoluminescence spectra show that the ultraviolet emission of the bicrystal nanobelts has a blueshift of 18meV as compared to Bi–ZnO nanowires at 10K. The bicrystal nanobelts also exhibit ferromagnetism at room temperature.

Effects of substrate temperature on the unusual non-Fermi liquid metal to insulator transition in perovskite SrIrO3 thin films

Electronic transport has been investigated for strong spin-orbit coupled perovskite SrIrO3 thin films grown at various substrate temperatures. The electronic transport of the SrIrO3 films is found to be very sensitive to the growth parameters; in particular, the film can either be a metal or an insulator depending upon the substrate growth temperature. While all the metallic films show unusual sublinear temperature dependent non-Fermi liquid behaviors in resistivity, the insulating film grown at a higher temperature stands out for its inhomogeneous Ir distribution, as analyzed by secondary ion mass spectrometry. This observation demonstrates that the inhomogeneous distribution of cations can be one of the fundamental factors in affecting the electronic transport in heavy element based oxide films and heterostructures.

Dynamic specific heat near the Curie point of Gd

We have developed a dynamic calorimeter, which can measure thermal properties of a wire or planar sample as a function of frequency over the range of five decades, 0.01 Hz–1 kHz. We have used this calorimeter to study the frequency dependence of the specific heat of gadolinium near the Curie point. There is an indication that the specific heat peak associated with the magnetic phase transition may depend on the measuring frequencies.

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.

Specific heat of YCo12B6 and GdCo12B6 intermetallics

The specific heat of polycrystalline intermetallics YCo12B6 and GdCo12B6 which crystallize in the hexagonal SrNi12B6 structure have been measured in the temperature range of 80 K<T<250 K. The results are successfully explained using the Weiss and the Neel molecular field models and the Ginzburg–Landau theory. The implications of the present results are also discussed.

High density array of multiferroic nanoislands in a large area

We report the innovative fabrication of a high density array of multiferroic BiFeO3 (BFO) nanoislands on a conductive substrate in a large area. The anodic aluminum oxide (AAO) template has widely been used to fabricate highly arranged nanostructures, because of easy control of pore size and perfect hexagonal pore packing. The existing AAO mask-assisted pulsed laser deposition (PLD) method is limited to fabricating a nanoisland array in a small area. To supplement the shortcoming of this method, a thick AAO membrane in a large area was electrochemically detached and floated on polystyrene (PS) film without crack. Then, a nanoporous polystyrene (PS) template was prepared by dry etching with the thick AAO membrane mask, followed by spin coating of the BFO precursor on the PS template. After removing the PS template at high temperature, we prepared a high density array of multiferroic BFO nanoislands in a large area epitaxially grown on an STO:Nb (100) substrate. A high density array of BFO nanoislands in a large area showed both ferroelectricity of individual nanoislands obtained by piezoresponse force microscopy (PFM) and macroscopic magnetism measured by a superconducting quantum interference device (SQUID) based magnetic property measurement system (MPMS). A high density array of BFO nanoislands could be employed as a next-generation memory device capable of electric writing and magnetic reading (or vice versa).