arXiv subjects

Young S. Lee

Publications and source records attributed to Young S. Lee.

At least 37 records · Page 2Linked to original sources

arXiv2020

Site-Specific Structure at Multiple Length Scales in Kagome Quantum Spin Liquid Candidates

Realizing a quantum spin liquid (QSL) ground state in a real material is a leading issue in condensed matter physics research. In this pursuit, it is crucial to fully characterize the structure and influence of defects, as these can significantly affect the fragile QSL physics. Here, we perform a variety of cutting-edge synchrotron X-ray scattering and spectroscopy techniques, and we advance new methodologies for site-specific diffraction and L-edge Zn absorption spectroscopy. The experimental results along with our first-principles calculations address outstanding questions about the local and long-range structures of the two leading kagome QSL candidates, Zn-substituted barlowite Cu$_3$Zn$_{x}$Cu$_{1-x}$(OH)$_6$FBr and herbertsmithite Cu$_3$Zn(OH)$_6$Cl$_2$. On all length scales probed, there is no evidence that Zn substitutes onto the kagome layers, thereby preserving the QSL physics of the kagome lattice. Our calculations show that antisite disorder is not energetically favorable and is even less favorable in Zn-barlowite compared to herbertsmithite. Site-specific X-ray diffraction measurements of Zn-barlowite reveal that Cu$^{2+}$ and Zn$^{2+}$ selectively occupy distinct interlayer sites, in contrast to herbertsmithite. Using the first measured Zn L-edge inelastic X-ray absorption spectra combined with calculations, we discover a systematic correlation between the loss of inversion symmetry from pseudo-octahedral (herbertsmithite) to trigonal prismatic coordination (Zn-barlowite) with the emergence of a new peak. Overall, our measurements suggest that Zn-barlowite has structural advantages over herbertsmithite that make its magnetic properties closer to an ideal QSL candidate: its kagome layers are highly resistant to nonmagnetic defects while the interlayers can accommodate a higher amount of Zn substitution.

cond-mat.str-el↗

arXiv2020

Materializing Rival Ground States in the Barlowite Family of Kagome Magnets: Quantum Spin Liquid, Spin Ordered, and Valence Bond Crystal States

The spin-$\frac{1}{2}$ kagome antiferromagnet is considered an ideal host for a quantum spin liquid ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu$_4$(OH)$_6$FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin-$\frac{1}{2}$ kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a quantum spin liquid (QSL). The presence of interlayer spins eventually leads to an interesting pinwheel $q=0$ magnetic order. Partially Zn-substituted barlowite (Cu$_{3.44}$Zn$_{0.56}$(OH)$_6$FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is similar to that of herbertsmithite, even though the Cu$^{2+}$ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.

cond-mat.str-el↗

arXiv2019

High resolution time- and angle-resolved photoemission spectroscopy with 11 eV laser pulses

Performing time and angle resolved photoemission spectroscopy (tr-ARPES) at high momenta necessitates extreme ultraviolet laser pulses, which are typically produced via high harmonic generation (HHG). Despite recent advances, HHG-based setups still require large pulse energies (hundreds of $μ$J to mJ) and their energy resolution is limited to tens of meV. Here, we present a novel 11 eV tr-ARPES setup that generates a flux of $5\times10^{10}$ photons/s and achieves an unprecedented energy resolution of 16 meV. It can be operated at high repetition rates (up to 250 kHz) while using input pulse energies down to 3 $μ$J. We demonstrate these unique capabilities by simultaneously capturing the energy and momentum resolved dynamics in two well-separated momentum space regions of a charge density wave material ErTe$_3$. This novel setup offers opportunity to study the non-equilibrium band structure of solids with exceptional energy and time resolutions at high repetition rates.

cond-mat.mtrl-sci↗

arXiv2018

Synthesis-Dependent Properties of Barlowite and Zn-Substituted Barlowite

The mineral barlowite, Cu$_4$(OH)$_6$FBr, has been the focus of recent attention due to the possibility of substituting the interlayer Cu$^{2+}$ site with non-magnetic ions to develop new quantum spin liquid materials. We re-examine previous methods of synthesizing barlowite and describe a novel hydrothermal synthesis method that produces large single crystals of barlowite and Zn-substituted barlowite (Cu$_3$Zn$_x$Cu$_{1-x}$(OH)$_6$FBr). The two synthesis techniques yield barlowite with indistinguishable crystal structures and spectroscopic properties at room temperature; however, the magnetic ordering temperatures differ by 4 K and the thermodynamic properties are clearly different. The dependence of properties upon synthetic conditions implies that the defect chemistry of barlowite and related materials is complex and significant. Zn-substituted barlowite exhibits a lack of magnetic order down to T = 2 K, characteristic of a quantum spin liquid, and we provide a synthetic route towards producing large crystals suitable for neutron scattering.

cond-mat.str-el↗

arXiv2017

High pressure x-ray study of spin-Peierls physics in the quantum spin chain material TiOCl

The application of pressure can induce transitions between unconventional quantum phases in correlated materials. The inorganic compound TiOCl, composed of chains of S=1/2 Ti ions, is an ideal realization of a spin-Peierls system with a relatively simple unit cell. At ambient pressure, it is an insulator due to strong electronic interactions (a Mott insulator). Its resistivity shows a sudden decrease with increasing pressure, indicating a transition to a more metallic state which may coincide with the emergence of charge density wave order. Therefore, high pressure studies of the structure with x-rays are crucial in determining the ground-state physics in this quantum magnet. In ambient pressure, TiOCl exhibits a transition to an incommensurate nearly dimerized state at $T_{c2}=92$ K and to a commensurate dimerized state at $T_{c1}=66$ K. Here, we discover a rich phase diagram as a function of temperature and pressure using x-ray diffraction on a single crystal in a diamond anvil cell down to $T=4$ K and pressures up to 14.5 GPa. Remarkably, the magnetic interaction scale increases dramatically with increasing pressure, as indicated by the high onset temperature of the spin-Peierls phase. At $\sim$7 GPa, the extrapolated onset of the spin-Peierls phase occurs above $T=300$ K, indicating a quantum singlet state exists at room temperature. Further comparisons are made with the phase diagrams of related spin-Peierls systems that display metallicity and superconductivity under pressure.

cond-mat.str-el↗

arXiv2017

Infrared phonons as a probe of a spin-liquid states in herbertsmithite ZnCu3(OH)6Cl2

We report on temperature dependence of the infrared reflectivity spectra of a single crystalline herbertsmithite in two polarizations --- parallel and perpendicular to the kagome plane of Cu atoms. We observe anomalous broadening of the low frequency phonons possibly caused by fluctuations in the exotic dynamical magnetic order of the spin liquid.

cond-mat.str-el↗

arXiv2015

Correlated impurities and intrinsic spin liquid physics in the kagome material Herbertsmithite

Low energy inelastic neutron scattering on single crystals of the kagome spin liquid compound ZnCu3(OD)6Cl2 (Herbertsmithite) reveals antiferromagnetic correlations between impurity spins for energy transfers E < 0.8 meV (~J/20). The momentum dependence differs significantly from higher energy scattering which arises from the intrinsic kagome spins. The low energy fluctuations are characterized by diffuse scattering near wavevectors (1 0 0) and (0 0 3/2), which is consistent with antiferromagnetic correlations between pairs of nearest neighbor Cu impurities on adjacent triangular (Zn) interlayers. The corresponding impurity lattice resembles a simple cubic lattice in the dilute limit below the percolation threshold. Such an impurity model can describe prior neutron, NMR, and specific heat data. The low energy neutron data are consistent with the presence of a small spin-gap (Delta ~ 0.7 meV) in the kagome layers, similar to that recently observed by NMR. The ability to distinguish the scattering due to Cu impurities from that of the planar kagome Cu spins provides a new avenue for probing intrinsic spin liquid physics.

cond-mat.str-el↗

arXiv2015

The thermal Hall effect of spin excitations in a Kagome magnet

At low temperatures, the thermal conductivity of spin excitations in a magnetic insulator can exceed that of phonons. However, because they are charge neutral, the spin waves are not expected to display a thermal Hall effect in a magnetic field. Recently, this semiclassical notion has been upended in quantum magnets in which the spin texture has a finite chirality. In the Kagome lattice, the chiral term generates a Berry curvature. This results in a thermal Hall conductivity $κ_{xy}$ that is topological in origin. Here we report observation of a large $κ_{xy}$ in the Kagome magnet Cu(1-3, bdc) which orders magnetically at 1.8 K. The observed $κ_{xy}$ undergoes a remarkable sign-reversal with changes in temperature or magnetic field, associated with sign alternation of the Chern flux between magnon bands. We show that thermal Hall experiments probe incisively the effect of Berry curvature on heat transport.

cond-mat.str-el↗

arXiv2014

Thermodynamic Properties of the Quantum Spin Liquid Candidate ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ in High Magnetic Fields

We report measurements of the specific heat and magnetization of single crystal samples of the spin-1/2 kagome compound ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ (herbertsmithite), a promising quantum spin-liquid candidate, in high magnetic fields and at low temperatures. The magnetization was measured up to $μ_{0}H$ = 55 T at $T$ = 0.4 K, showing a saturation of the weakly interacting impurity moments in fields above $\sim10$ T. The specific heat was measured down to $T < 0.4$ K in magnetic fields up to 18 T, revealing $T$-linear and $T$-squared contributions. The $T$-linear contribution is surprisingly large and indicates the presence of gapless excitations in large applied fields. These results further highlight the unusual excitation spectrum of the spin liquid ground state of herbertsmithite.

cond-mat.str-el↗

arXiv2013

Imaging the Nanoscale Band Structure of Topological Sb

Many promising building blocks of future electronic technology - including non-stoichiometric compounds, strongly correlated oxides, and strained or patterned films - are inhomogeneous on the nanometer length scale. Exploiting the inhomogeneity of such materials to design next-generation nanodevices requires a band structure probe with nanoscale spatial resolution. To address this demand, we report the first simultaneous observation and quantitative reconciliation of two candidate probes - Landau level spectroscopy and quasiparticle interference imaging - which we employ here to reconstruct the multi-component surface state band structure of the topological semimetal antimony(Sb). We thus establish the technique of band structure tunneling microscopy (BSTM), whose unique advantages include nanoscale access to non-rigid band structure deformation, empty state dispersion, and magnetic field dependent states. We use BSTM to elucidate the relationship between bulk conductivity and surface state robustness in topological materials, and to quantify essential metrics for spintronics applications.

cond-mat.mes-hall↗

arXiv2013

Fractionalized excitations in the spin liquid state of a kagomé lattice antiferromagnet

New physics can emerge in magnetic materials where quantum fluctuations are enhanced due to reduced dimensionality and strong frustration. One long sought example is the resonating-valence-bond (RVB) state, where atomic magnetic moments are strongly correlated but do not order or freeze even in the limit of T -> 0. The RVB ground state does not break conventional symmetries, such as lattice translation or spin-rotation. The realization of such a quantum spin liquid in two-dimensions would represent a new state of matter. It is believed that spin liquid physics plays a role in the phenomenon of high-Tc superconductivity, and the topological properties of the spin liquid state may have applications in the field of quantum information. We present neutron scattering measurements of the spin excitations on single crystal samples of the spin-1/2 kagomé lattice antiferromagnet ZnCu3(OD)6Cl2 (also called herbertsmithite). Our observation of a spinon continuum in a two-dimensional magnet is remarkable first. The results serve as a key fingerprint of the quantum spin liquid state in herbertsmithite.

cond-mat.str-el↗

arXiv2013

Spin Induced Optical Conductivity in the Spin Liquid Candidate Herbertsmithite

A quantum spin liquid (QSL) is a state of matter in which magnetic spins interact strongly, but quantum fluctuations inhibit long-range magnetic order even at zero temperature. A QSL has been predicted to have a host of exotic properties, including fractionalized excitations and long-range quantum entanglement. Despite the numerous theoretical studies, experimental realization of a QSL has proved to be challenging due to the lack of candidate materials. The triangular organic salts EtMe3Sb[Pd(dmit)2]2 and κ-(BEDT-TTF)2Cu2(CN)3, and kagome ZnCu3(OH)6Cl2 (Herbertsmithite) have recently emerged as promising candidates of exhibiting a QSL state, but the nature of their ground states is still elusive. Here we studied a large-area high-quality single crystal of Herbertsmithite by means of time-domain terahertz (THz) spectroscopy. We observed in the low-frequency (0.6-2.2 THz) optical conductivity evidence for the nature of the spin system. In particular, the in-plane absorption spectrum exhibits a unique frequency dependence that can be described by a power-law with an exponent of approximately 1.4, in sharp contrast with the ω^4 dependence expected for an ordered Mott insulator. The absorption is also found to increase as the temperature decreases, a behavior unexpected for conventional insulators. Such features are consistent with recent theory based on the interactions between the charge and spin degrees of freedom in a QSL system.

cond-mat.str-el↗

arXiv2012

Refining the Spin Hamiltonian in the Spin-1/2 Kagome Lattice Antiferromagnet ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ using Single Crystals

We report thermodynamic measurements of the S=1/2 kagome lattice antiferromagnet ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$, a promising candidate system with a spin-liquid ground state. Using single crystal samples, the magnetic susceptibility both perpendicular and parallel to the kagome plane has been measured. A small, temperature-dependent anisotropy has been observed, where $χ_{z}/ χ_{p} > 1$ at high temperatures and $χ_{z}/ χ_{p} < 1$ at low temperatures. Fits of the high-temperature data to a Curie-Weiss model also reveal an anisotropy. By comparing with theoretical calculations, the presence of a small easy-axis exchange anisotropy can be deduced as the primary perturbation to the dominant Heisenberg nearest neighbor interaction. These results have great bearing on the interpretation of theoretical calculations based on the kagome Heisenberg antiferromagnet model to the experiments on ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$.

cond-mat.str-el↗

arXiv2011

Synthesis and Characterization of Single Crystal Samples of Spin-$1/2$ Kagome Lattice Antiferromagnets in the Zn-Paratacamite Family Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$Cl$_{2}$

The Zn-paratacamite family, Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$Cl$_{2}$ for $x \, \geq$ 0.33, is an ideal system for studying spin-1/2 frustrated magnetism in the form of antiferromagnetic Cu$^{2+}$ kagome planes. Here we report a new synthesis method by which high quality millimeter-sized single crystals of Zn-paratacamite have been produced. These crystals have been characterized by metal analysis, x-ray diffraction, neutron diffraction, and thermodynamic measurements. The $x$ = 1 member of the series displays a magnetic susceptibility that is slightly anisotropic at high temperatures with $χ_{c} \, > \, χ_{ab}$. Neutron and synchrotron x-ray diffraction experiments confirm the quality of these $x$ = 1 single crystals and indicate no obvious structural transition down to temperatures of T=2 K.

cond-mat.str-el↗

arXiv2010

The Herbertsmithite Hamiltonian: $μ$SR measurements on single crystals

We present transverse field muon spin rotation/relaxation measurements on single crystals of the spin-1/2 kagome antiferromagnet Herbertsmithite. We find that the spins are more easily polarized when the field is perpendicular to the kagome plane. We demonstrate that the difference in magnetization between the different directions cannot be accounted for by Dzyaloshinksii-Moriya type interactions alone, and that anisotropic axial interaction is present.

cond-mat.str-el↗

arXiv2010

Surface State Transport and Ambipolar Electric Field Effect in Bi2Se3 Nanodevices

Electronic transport experiments involving the topologically protected states found at the surface of Bi2Se3 and other topological insulators require fine control over carrier density, which is challenging with existing bulk-doped material. Here we report on electronic transport measurements on thin (<100 nm) Bi2Se3 devices and show that the density of the surface states can be modulated via the electric field effect by using a top-gate with a high-k dielectric insulator. The conductance dependence on geometry, gate voltage, and temperature all indicate that transport is governed by parallel surface and bulk contributions. Moreover, the conductance dependence on top-gate voltage is ambipolar, consistent with tuning between electrons and hole carriers at the surface.

cond-mat.mes-hall↗

arXiv2010

Ambipolar Electric Field Effect in Metallic Bi2Se3

Topological insulators (TIs) constitute a new class of materials with unique properties resulting from the relativistic-like character and topological protection of their surface states. Theory predicts these to exhibit a rich variety of physical phenomena such as anomalous magneto-electric coupling and Majorana excitations. Although TI surface states have been detected in Bi-based compounds by ARPES and STM techniques, electrical control over their density, required for most transport experiments, remains a challenge. Existing materials are heavily doped in the bulk, thus preventing electrical tunability of the surface states and their integration into topological quantum electronic devices. Here we show that electronic transport in metallic Bi2Se3 nanoscale devices can be controlled by tuning the surface density via the electric field effect. By choosing an appropriate high-k dielectric, we are able to shift the Fermi energy through the charge neutrality point of the surface states, resulting in ambipolar transport characteristics reminiscent of those observed in graphene. Combining magnetotransport, field effect, and geometry dependent experiments, we provide transport measurements of the surface state mobility, and identify likely scattering mechanisms by measuring its temperature dependence.

cond-mat.mes-hall↗

arXiv2010

Interplay of thermal and quantum spin fluctuations on the kagome lattice

We present a Raman spectroscopic investigation of the Herbertsmithite ZnCu3(OH)6Cl2, the first realization of a Heisenberg s=1/2 antiferromagnet on a perfect kagome lattice. The magnetic excitation spectrum of this compound is dominated by two components, a high temperature quasi elastic signal and a low temperature, broad maximum. The latter has a linear low energy slope and extends to high energy. We have investigated the temperature dependence and symmetry properties of both signals. Our data agree with previous calculations and point to a spin liquid ground state.

cond-mat.str-el↗