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Je-Geun Park

Publications and source records attributed to Je-Geun Park.

At least 127 records · Page 7Linked to original sources
arXiv2017

Charge-spin correlation in van der Waals antiferromagenet NiPS3

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS3, a van derWaals antiferromagnet, from our study of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption and photoemission spectroscopy, and density-functional calculations. NiPS3 displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting a strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission and optical spectra support a self-doped ground state in NiPS3. Our work demonstrates that layered transition-metal trichalcogenide magnets are a useful candidate for the study of correlated-electron physics in two-dimensional magnetic material.

cond-mat.str-el↗
arXiv2017

The low-temperature highly correlated quantum phase in the charge-density-wave 1T-TaS_2 compound

A prototypical quasi-2D metallic compound, 1T-TaS_2 has been extensively studied due to an intricate interplay between a Mott-insulating ground state and a charge density-wave (CDW) order. In the low-temperature phase, 12 out of 13 Ta_{4+} 5\textit{d}-electrons form molecular orbitals in hexagonal star-of-David patterns, leaving one 5\textit{d}-electron with \textit{S} = 1/2 spin free. This orphan quantum spin with a large spin-orbit interaction is expected to form a highly correlated phase of its own. And it is most likely that they will form some kind of a short-range order out of a strongly spin-orbit coupled Hilbert space. In order to investigate the low-temperature magnetic properties, we performed a series of measurements including neutron scattering and muon experiments. The obtained data clearly indicate the presence of the short-ranged phase and put the upper bound on ~ 0.4 \textitμ_B for the size of the magnetic moment, consistent with the orphan-spin scenario.

cond-mat.str-el↗
arXiv2017

Properties of spin 1/2 triangular lattice antiferromagnets: CuRE2Ge2O8 (RE=Y, La)

We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg = 2 eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering.

cond-mat.mtrl-sci↗
arXiv2017

Doping effects on trimerization and magnetoelectric coupling of single crystal multiferroic (Y,Lu)MnO3

Hexagonal RMnO3 is a multiferroic compound with a giant spin-lattice coupling at an antiferromagnetic transition temperature [1]. Despite extensive studies over the past two decades, however, the origin and underlying microscopic mechanism of the strong spin-lattice coupling remain still very much elusive. In this study, we have tried to address this problem by measuring the thermal expansion and dielectric constant of doped single crystals Y1-xLuxMnO3 with x = 0, 0.25, 0.5, 0.75, and 1.0. From these measurements, we confirm that there is a progressive change in the physical properties with doping. At the same time, all our samples exhibit clear anomalies at TN, even in the samples with x = 0.5 and 0.75 as opposed to some earlier ideas, which suggests an unusual doping dependence of the anomaly. Our work reveals yet another interesting facet of the spin lattice coupling issue in hexagonal RMnO3.

cond-mat.str-el↗
arXiv2017

Spin glass behavior in frustrated quantum spin system CuAl2O4 with a possible orbital liquid state

CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu2+. It is a rather unique feature that the Cu2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = Theta_CW/Tm ~67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.

cond-mat.str-el↗
arXiv2017

Magnetic Transitions in the Chiral Armchair-Kagome System Mn$_2$Sb$_2$O$_7$

The competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn$_2$Sb$_2$O$_7$ forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armchair unit and link adjacent kagome planes. This structural motif may be viewed as intermediate between the triangles of the kagome network and the tetrahedra in the pyrochlore lattice. Mn$_2$Sb$_2$O$_7$ exhibits two distinct magnetic phase transitions, at 11.1 and 14.2K, at least one of which has a weak ferromagnetic component. The magnetic propagation vector does not change through the lower transition, suggesting a metamagnetic transition or a transition involving a multi-component order parameter. Although previously reported in the $P3_121$ space group, Mn$_2$Sb$_2$O$_7$ actually crystallizes in $P2$, which allows ferroelectricity, and we show clear evidence of magnetoelectric coupling indicative of multiferroic order. The quasi-two-dimensional `armchair-kagome' lattice presents a promising platform for probing chiral magnetism and the effect of dimensionality in highly frustrated systems.

cond-mat.str-el↗
arXiv2016

Ising-Type Magnetic Ordering in Atomically Thin FePS3

Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order-disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with TN ~118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering.

cond-mat.mes-hall↗
arXiv2016

The $3d$-Electron Heisenberg Pyrochlore Mn$_2$Sb$_2$O$_7$

In frustrated magnetic systems, geometric constraints or the competition amongst interactions introduce extremely high degeneracy and prevent the system from readily selecting a low-temperature ground state. The most frustrated known spin arrangement is on the pyrochlore lattice, but nearly all magnetic pyrochlores have unquenched orbital angular momentum, constraining the spin directions through spin-orbit coupling. Pyrochlore Mn$_2$Sb$_2$O$_7$ is an extremely rare Heisenberg pyrochlore system, with directionally-unconstrained spins and low chemical disorder. We show that it undergoes a spin-glass transition at 5.5K, which is suppressed by disorder arising from Mn vacancies, indicating this ground state to be a direct consequence of the spins' interactions. The striking similarities to $3d$ transition metal pyrochlores with unquenched angular momentum suggests that the low spin-orbit coupling in the $3d$ block makes Heisenberg pyrochlores far more accessible than previously imagined.

cond-mat.str-el↗
arXiv2016

Spontaneous structural distortion of metallic Shastry-Sutherland system DyB4 by quadrupole-spin-lattice coupling

DyB4 has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, surprisingly, it undergoes two successive transitions: a magnetic ordering at TN = 20K, and a quadrupole ordering at TQ=12.5 K. From high-resolution neutron and synchrotron X-ray powder diffraction studies, we have obtained full structural information on this material in all phases, and demonstrate that structural modifications occurring at quadrupolar transition lead to the lifting of frustrations inherent in the Sh-S model. Our study thus provides a complete experimental picture of how the intrinsic frustration of the Sh-S lattice can be lifted by the coupling to quadrupole moments. We show that two other factors, i.e. strong spin-orbit coupling and long-range RKKY interaction in metallic DyB4, play an important role in this behavior.

cond-mat.str-el↗
arXiv2016

Spontaneous decays of magneto-elastic excitations in noncollinear antiferromagnet (Y,Lu)MnO3

When magnons and phonons, the fundamental quasiparticles of the solid, are coupled to one another, they form a new hybrid quasi-particle, leading to novel phenomena and interesting applications. Despite its wide-ranging importance, however, detailed experimental studies on the underlying Hamiltonian is rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remains largely unexplored although it is essential for a proper understanding of their diverse thermodynamic behaviour as well as intrinsic zero-temperature decay. Here we show that in noncollinear antiferromagnets, a strong magnon-phonon coupling can significantly enhance the anharmonicity, resulting in the creation of magnetoelastic excitations and their spontaneous decay. By measuring the spin waves over the full Brillouin zone and carrying out anharmonic spin wave calculations using a Hamiltonian with an explicit magnon-phonon coupling, we have identified a hybrid magnetoelastic mode in (Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling term required to produce it. Our work has wide implications for understanding of the spin-phonon coupling and the resulting excitations of the broad classes of noncollinear magnets.

cond-mat.mtrl-sci↗
arXiv2016

Magnon-phonon coupling and two-magnon continuum in two-dimensional triangular antiferromagnet CuCrO2

CuCrO2 is a manifestation of a two-dimensional triangular antiferromagnet which exhibits an incommensurate noncollinear magnetic structure similar to a classical 120 degree ordering. Using the inelastic neutron scattering technique, direct evidence of a magnon-phonon coupling in CuCrO2 is revealed via the mixed magnon-phonon character of the excitation mode at 12.5 meV as well as a minimum at the zone boundary. A simple model Hamiltonian that incorporates an exchange striction type magnon-phonon coupling reproduces the observed features accurately. Also, continuum excitations originating from the interaction between quasiparticles are observed with strong intensity at the zone boundary. These features of the magnetic excitations are key to an understanding of the emergent excitations in noncollinear antiferromagnetic compounds.

cond-mat.str-el↗
arXiv2016

Jahn-Teller distortion driven magnetic polarons in magnetite

The first known magnetic mineral, magnetite (Fe$_3$O$_4$), has unusual properties which have fascinated mankind for centuries; it undergoes the Verwey transition at $T_{\rm V}$ $\sim$120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition however remains contentious. Here we use resonant inelastic X-ray scattering (RIXS) over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe$^{2+}$ and Fe$^{3+}$ states. Comparison of the RIXS results with crystal-field multiplet calculations shows that the spin-orbital $dd$ excitons of the Fe$^{2+}$ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe$^{2+}$O$_6$ octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and best explained as magnetic polarons.

cond-mat.str-el↗
arXiv2016

Tunneling transport of mono- and few-layers magnetic van der Waals MnPS$_3$

We have investigated the tunneling transport of mono- and few-layers of MnPS3 by using conductive atomic force microscopy. Due to the band alignment of indium tin oxide/MnPS3/Pt-Ir tip junction, the key features of both Schottky junction and Fowler-Nordheim tunneling (FNT) were observed for all the samples with varying thickness. Using the FNT model and assuming the effective electron mass (0.5 me) of MnPS3, we estimate the tunneling barrier height to be 1.31 eV and the dielectric breakdown strength as 5.41 MV/cm.

cond-mat.mes-hall↗
arXiv2016

Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li$_2$RuO$_3$

When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.

cond-mat.str-el↗
arXiv2015

Hexagonal RMnO$_3$: a model system for 2D triangular lattice antiferromagnets

The hexagonal RMnO$_3$ (h-RMnO$_3$) are multiferroic materials, which exhibit the coexistence of a magnetic order and ferroelectricity. Their distinction is in their geometry that both results in an unusual mechanism to break inversion symmetry, and also produces a 2D triangular lattice of Mn spins, which is subject to geometrical magnetic frustration due to the antiferromagnetic interactions between nearest neighbour Mn ions. This unique combination makes the h-RMnO$_3$ a model system to test ideas of spin-lattice coupling, particularly when the both the improper ferroelectricity and the Mn trimerisation that appears to determine the symmetry of the magnetic structure arise from the same structure distortion. In this review, we demonstrate how the use of both neutron and X-ray diffraction and inelastic neutron scattering techniques have been essential to paint this comprehensive and coherent picture of the h-RMnO$_3$.

cond-mat.str-el↗
arXiv2015

Size Dependence of Metal-Insulator Transition in Stoichiometric Fe3O4 Nanocrystals

Magnetite (Fe3O4) is one of the most actively studied materials with a famous metal-insulator transition (MIT), so-called the Verwey transition at around 123 K. Despite the recent progress in synthesis and characterization of Fe3O4 nanocrystals (NCs), it is still an open question how the Verwey transition changes on a nanometer scale. We herein report the systematic studies on size dependence of the Verwey transition of stoichiometric Fe3O4 NCs. We have successfully synthesized stoichiometric and uniform-sized Fe3O4 NCs with sizes ranging from 5 to 100 nm. These stoichiometric Fe3O4 NCs show the Verwey transition when they are characterized by conductance, magnetization, cryo-XRD, and heat capacity measurements. The Verwey transition is weakly size-dependent and becomes suppressed in NCs smaller than 20 nm before disappearing completely for less than 6 nm, which is a clear, yet highly interesting indication of a size effect of this well-known phenomena. Our current work will shed new light on this ages-old problem of Verwey transition.

cond-mat.str-el↗
arXiv2015

High-resolution structure studies and magnetoelectric coupling of relaxor multiferroic Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$

Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$ (PFN), one of the few relaxor multiferroic systems, has a $G$-type antiferromagnetic transition at $T_N$ = 143 K and a ferroelectric transition at $T_C$ = 385 K. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range structures of PFN: (i) a clear sign of short-range structural correlation above $T_C$, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below $T_N$. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder between Fe$^{3+}$ and Nb$^{5+}$ atoms. We argue that this disorder gives rise to short-range structural correlations arising from O disorder in addition to Pb displacement.

cond-mat.mtrl-sci↗