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K. H. Kim

Publications and source records attributed to K. H. Kim.

At least 55 records · Page 3Linked to original sources
arXiv2006

Interplay between carrier and impurity concentrations in annealed Ga$_{1-x}$Mn$_{x}$As intrinsic anomalous Hall Effect

Investigating the scaling behavior of annealed Ga$_{1-x}$Mn$_{x}$As anomalous Hall coefficients, we note a universal crossover regime where the scaling behavior changes from quadratic to linear, attributed to the anomalous Hall Effect intrinsic and extrinsic origins, respectively. Furthermore, measured anomalous Hall conductivities when properly scaled by carrier concentration remain constant, equal to theoretically predicated values, spanning nearly a decade in conductivity as well as over 100 K in T$_{C}$. Both the qualitative and quantitative agreement confirms the validity of new equations of motion including the Berry phase contributions as well as tunablility of the intrinsic anomalous Hall Effect.

cond-mat.mtrl-sci↗
arXiv2006

Evidence of metallic clustering in annealed Ga1-xMnxAs from atypical scaling behavior of the anomalous Hall coefficient

We report on the anomalous Hall coefficient and longitudinal resistivity scaling relationships on a series of annealed Ga1-xMnxAs epilayers (x~0.055). As-grown samples exhibit scaling parameter n of ~ 1. Near the optimal annealing temperature, we find n ~ 2 to be consistent with recent theories on the intrinsic origins of anomalous Hall Effect in Ga1-xMnxAs. For annealing temperatures far above the optimum, we note n > 3, similar behavior to certain inhomogeneous systems. This observation of atypical behavior agrees well with characteristic features attributable to spherical resonance from metallic inclusions from optical spectroscopy measurements.

cond-mat.mtrl-sci↗
arXiv2005

Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8

We present powder and single-crystal neutron diffraction and bulk measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to 8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This system contains two types of Ni ions, which we call "spine" and "cross-tie". Our neutron measurements can be described with the paramagnetic space group Cmca for T < 15K and each observed magnetically ordered phase is characterized by the appropriate irreducible representation(s). Our zero-field measurements show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order which is dominated by a longitudinally-modulated structure with the spine spins mainly parallel to the a-axis. Upon further cooling, a transition is induced at T_HL=6.3K to an elliptically polarized incommensurate structure with both spine and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a first-order phase transition, below which the magnetic structure is a commensurate antiferromagnet with the staggered magnetization primarily along the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat peak at T_CC'=2.3K indicates an additional transition, which we were however not able to relate to a change of the magnetic structure. Neutron, specific heat, and magnetization measurements produce a comprehensive temperature-field phase diagram. The symmetries of the two incommensurate magnetic phases are consistent with the observation that only one phase has a spontaneous ferroelectric polarization. All the observed magnetic structures are explained theoretically using a simplified model Hamiltonian, involving competing nearest- and next-nearest-neighbor exchange interactions, spin anisotropy, Dzyaloshinskii-Moriya and pseudo-dipolar interactions.

cond-mat.str-el↗
arXiv2005

Phonon Thermal Transport of URu2Si2: Broken Translational Symmetry and Strong-Coupling of the Hidden Order to the Lattice

A dramatic increase in the total thermal conductivity (k) is observed in the Hidden Order (HO) state of single crystal URu2Si2. Through measurements of the thermal Hall conductivity, we explicitly show that the electronic contribution to k is extremely small, so that this large increase in k is dominated by phonon conduction. An itinerant BCS/mean-field model describes this behavior well: the increase in kappa is associated with the opening of a large energy gap at the Fermi Surface, thereby decreasing electron-phonon scattering. Our analysis implies that the Hidden Order parameter is strongly coupled to the lattice, suggestive of a broken symmetry involving charge degrees of freedom.

cond-mat.str-el↗
arXiv2005

Irreversible dynamics of the phase boundary in U(Ru0.9)Rh0.04)2Si2 and implications for ordering

We report measurements and analysis of the specific heat and magnetocaloric effect at the phase boundary into the single magnetic field-induced phase (phase II) of U(Ru{0.96}Rh{0.04})2Si2, which yield striking similarities to the valence transition of Yb{1-x}YxInCu4. To explain these similarities, we propose a bootstrap mechanism by which a structural distortion causes an electric quadrupolar order parameter within phase II to become coupled to the 5f-electron hybridization, giving rise to a valence change at the transition.

cond-mat.str-el↗
arXiv2005

Inhomogeneous level splitting in Pr$_{2-x}$Bi$_x$Ru$_2$O$_7$

We report that Bi doping drives Pr$_x$Bi$_{2-x}$Ru$_2$O$_7$ from an antiferromagnetic insulator ($x=2$) to a metallic paramagnet ($x\approx 1$) with a broad low $T$ maximum in $C/T$. Neutron scattering reveals local low energy spin excitations ($\hbarω\approx 1$ meV) with a spectrum that is unaffected by heating to $k_BT>>\hbarω$. We show that a continuous distribution of splittings of the non-Kramers Pr$^{3+}$ ground state doublet such as might result from various types of lattice strain can account for all the data.

cond-mat.mtrl-sci↗
arXiv2005

Kapranov rank vs. tropical rank

We show that determining Kapranov rank of tropical matrices is not only NP-hard over any infinite field but also if solving Diophantine equations over the rational numbers is undecidable, then determining Kapranov rank over the rational numbers is undecidable. We prove that Kapranov rank of tropical matrices is not bounded in terms of tropical rank, answering a question of Develin, Santos, and Sturmfels.

math.CO↗
arXiv2005

Muon-Spin-Rotation Measurements of the Penetration Depth in the Infinite-Layer Electron-Doped Cuprate Superconductor Sr0.9La0.1CuO2

Muon spin rotation (mSR) measurements of the in-plane penetration depth lambda_ab have been performed in the electron-doped infinite layer high-Tc superconductor (HTS) Sr0.9La0.1CuO2. Absence of the magnetic rare-earth ions in this compound allowed to measure for the first time the absolute value of lambda_ab(0) in electron-doped HTS using mSR. We found lambda_ab(0)=116(2) nm. The zero-temperature depolarization rate sigma(0)?1/lambda_ab(0)^2=4.6(1) MHz is more than four times higher than expected from the Uemura line. Therefore this electron-doped HTS does not follow the Uemura relation found for hole-doped HTS.

cond-mat.supr-con↗
arXiv2004

Resistivity and magnetization of U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$, $x=$ 2, 2.5, 3 and 4 % in magnetic fields of up to 45 T

This posting contains raw data used for extracting features of the phase diagram and quantum criticality versus magnetic field $B$, temperature $T$ and Rh-doping $x$ that will soon appear in Physical Review Letters, 2004. All of the raw resistivity data is shown while only a selection of the magnetization data is shown for which a sample-out background subtraction was made.

cond-mat.str-el↗
arXiv2004

Competing Magnetic Phases on a "Kagome Staircase"

We present thermodynamic and neutron data on Ni_3V_2O_8, a spin-1 system on a kagome staircase. The extreme degeneracy of the kagome antiferromagnet is lifted to produce two incommensurate phases at finite T - one amplitude modulated, the other helical - plus a commensurate canted antiferromagnet for T ->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni_3V_2O_8 thus provides an elegant example of order from sub leading interactions in a highly frustrated system

cond-mat.str-el↗
arXiv2004

Dimensionality of superconductivity in the infinite-layer high-temperature cuprate Sr0.9M0.1CuO2 (M = La, Gd)

The high magnetic field phase diagram of the electron-doped infinite layer high-temperature superconducting (high-T_c) compound Sr_{0.9}La_{0.1}CuO_2 was probed by means of penetration depth and magnetization measurements in pulsed fields to 60 T. An anisotropy ratio of 8 was detected for the upper critical fields with H parallel (H_{c2}^{ab}) and perpendicular (H_{c2}^c) to the CuO_2 planes, with H_{c2}^{ab} extrapolating to near the Pauli paramagnetic limit of 160 T. The longer superconducting coherence length than the lattice constant along the c-axis indicates that the orbital degrees of freedom of the pairing wavefunction are three dimensional. By contrast, low-field magnetization and specific heat measurements of Sr_{0.9}Gd_{0.1}CuO_2 indicate a coexistence of bulk s-wave superconductivity with large moment Gd paramagnetism close to the CuO_2 planes, suggesting a strong confinement of the spin degrees of freedom of the Cooper pair to the CuO_2 planes. The region between H_{c2}^{ab} and the irreversibility line in the magnetization, H_{irr}^{ab}, is anomalously large for an electron-doped high-T_c cuprate, suggesting the existence of additional quantum fluctuations perhaps due to a competing spin-density wave order.

cond-mat.supr-con↗
arXiv2003

Charge-density Waves Survive the Pauli Paramagnetic Limit

Measurements of the resistance of single crystals of (Per)$_2$Au(mnt)$_2$ have been made at magnetic fields $B$ of up to 45 T, exceeding the Pauli paramagnetic limit of $B_{\rm P}\approx 37$ T. The continued presence of non-linear charge-density wave electrodynamics at $B \geq 37$ T unambiguously establishes the survival of the charge-density wave state above the Pauli paramagnetic limit, and the likely emergence of an inhomogeneous phase analogous to that anticipated to occur in superconductors.

cond-mat.str-el↗
arXiv2003

Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical field $H_{c2}$ of different $MgB_2$ samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities $ρ$, we show a clear trend of $H_{c2}$ increase as $ρ$ increases. One particular high resistivity film had zero-temperature $H_{c2}(0)$ well above the $H_{c2}$ values of competing non-cuprate superconductors such as $Nb_3Sn$ and Nb-Ti. Our high-field transport measurements give record values $H_{c2}^\perp (0) \approx 34T$ and $H_{c2}\|(0) \approx 49 T$ for high resistivity films and $H_{c2}(0)\approx 29 T$ for untextured bulk polycrystals. The highest $H_{c2}$ film also exhibits a significant upward curvature of $H_{c2}(T)$, and temperature dependence of the anisotropy parameter $γ(T) = H_{c2}\|/ H_{c2}^\perp$ opposite to that of single crystals: $γ(T)$ decreases as the temperature decreases, from $γ(T_c) \approx 2$ to $γ(0) \approx 1.5$. This remarkable $H_{c2}$ enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in $MgB_2$, which offers special opportunities for further $H_{c2}$ increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of $H_{c2}(T)$ observed suggest that $MgB_2$ can be made into a versatile, competitive high-field superconductor.

cond-mat.supr-con↗
arXiv2003

Photoemission and x-ray absorption spectroscopy study of electron-doped colossal magnetoresistance manganite: La0.7Ce0.3MnO3 film

The electronic structure of La0.7Ce0.3MnO3 (LCeMO) thin film has been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Ce 3d core-level PES and XAS spectra of LCeMO are very similar to those of CeO2, indicating that Ce ions are far from being trivalent. A very weak 4f resonance is observed around the Ce 4d $\to$ 4f absorption edge, suggesting that the localized Ce 4f states are almost empty in the ground state. The Mn 2p XAS spectrum reveals the existence of the Mn(2+) multiplet feature, confirming the Mn(2+)-Mn(3+) mixed-valent states of Mn ions in LCeMO. The measured Mn 3d PES/XAS spectra for LCeMO agrees reasonably well with the calculated Mn 3d PDOS using the LSDA+U method. The LSDA+U calculation predicts a half-metallic ground state for LCeMO.

cond-mat.str-el↗
arXiv2003

Two-band Effects in the Angular Dependence of Hc2 of MgB2 Single Crystals

The angular dependence of the upper critical field Hc2 of MgB2 single crystals is studied at various temperatures by means of specific heat and transport measurements in magnetic fields up to 17 T. Clear deviations from Ginzburg-Landau behavior are observed at all temperatures and are explained by two-band effects. The angular- and temperature dependence of the deviations are in qualitative agreement with theoretical predictions based on band-structure calculations. Quantitative agreement is obtained with an interband coupling slightly stronger than the calculated, enabling band-structure anisotropies and interband coupling strength to be experimentally estimated. This provides a new pathway to the study of disorder and doping effects in MgB2.

cond-mat.supr-con↗
arXiv2003

Electronic structures of Zn$_{1-x}$Co$_x$O using photoemission and x-ray absorption spectroscopy

Electronic structures of Zn$_{1-x}$Co$_x$O have been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Co 3d states are found to lie near the top of the O $2p$ valence band, with a peak around $\sim 3$ eV binding energy. The Co $2p$ XAS spectrum provides evidence that the Co ions in Zn$_{1-x}$Co$_{x}$O are in the divalent Co$^{2+}$ ($d^7$) states under the tetrahedral symmetry. Our finding indicates that the properly substituted Co ions for Zn sites will not produce the diluted ferromagnetic semiconductor property.

cond-mat.mtrl-sci↗
arXiv2003

Anisotropies of the lower and upper critical fields in MgB$_2$ single crystals

The temperature dependence of the London penetration depth ($λ$) and coherence length ($ξ$) has been deduced from Hall probe magnetization measurements in high quality MgB$_2$ single crystals in the two main crystallographic directions. We show that, in contrast to conventional superconductors, MgB$_2$ is characterized by two different anisotropy parameters ($Γ_λ= λ_c/λ_{ab}$ and $Γ_ξ= ξ_{ab}/ξ_c$) which strongly differ at low temperature and merge at $T_c $. These results are in very good agreement with recent calculations in weakly coupled two bands suprerconductors (Phys. Rev. B, 66, 020509(R) (2002).

cond-mat.supr-con↗