Young S. Lee's research while affiliated with Stanford University and other places
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Publications (84)
A recent experiment has unveiled an anomalously strong electron-electron attraction in the one-dimensional copper-oxide chain Ba2−xSrxCuO3+δ. While the effect of the near-neighbor electron attraction V in the one-dimensional extended Hubbard chain has been examined recently, its effect in the Hubbard model beyond the one-dimensional chain remains u...
A quantum spin liquid is a novel ground state that can support long-range entanglement between magnetic moments, resulting in exotic spin excitations involving fractionalized S=12 spinons. Here, we measure the excitations in single crystals of the spin liquid candidate Zn-barlowite using resonant inelastic x-ray scattering. By analyzing the inciden...
Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in δ-CsSnI3. With the application of pressure up to 40 GPa, the initially insulating δ-CsSnI3 transforms to a meta...
Kagome lattice Heisenberg antiferromagnets are known to be highly sensitive to perturbations caused by the structural disorder. NMR is a local probe ideally suited for investigating such disorder-induced effects, but in practice, large distributions in the conventional one-dimensional NMR data make it difficult to distinguish the intrinsic behavior...
Electron-phonon coupling was believed to govern the carrier transport in halide perovskites and related phases. Here we demonstrate that electron-electron interaction enhanced by Cs-involved electron redistribution plays a direct and prominent role in the low-temperature electrical transport of compressed CsPbI3 and renders Fermi liquid (FL)-like b...
Kagome lattice Heisenberg antiferromagnets are known to be highly sensitive to perturbations caused by structural disorder. NMR is a local probe ideally suited for investigating such disorder-induced effects, but in practice large distributions in the conventional one-dimensional NMR data make it difficult to distinguish the intrinsic behavior expe...
We report results of low-temperature heat-capacity, magnetocaloric-effect, and neutron-diffraction measurements of TmVO4, an insulator that undergoes a continuous ferroquadrupolar phase transition associated with local partially filled 4f orbitals of the thulium (Tm[Formula: see text]) ions. The ferroquadrupolar transition, a realization of Ising n...
We explore spin dynamics in Cu(1,3-bdc), a quasi-2D topological magnon insulator. The results show that the thermal evolution of the Landé g factor (g) is anisotropic: gin-plane decreases while gout-of-plane increases with increasing temperature T. Moreover, the anisotropy of the g factor (Δg) and the anisotropy of saturation magnetization (ΔMs) ar...
We explore spin dynamics in Cu(1,3-bdc), a quasi-2D topological magnon insulator. The results show that the thermal evolution of Land\'e $g$-factor ($g$) is anisotropic: $g_\textrm{in-plane}$ reduces while $g_\textrm{out-plane}$ increases with increasing temperature $T$. Moreover, the anisotropy of the $g$-factor ($\Delta g$) and the anisotropy of...
We use ^{79}Br nuclear quadrupole resonance (NQR) to demonstrate that ultraslow lattice dynamics set in below the temperature scale set by the Cu-Cu superexchange interaction J (≃160 K) in the kagome lattice Heisenberg antiferromagnet Zn-barlowite. The lattice completely freezes below 50 K, and ^{79}Br NQR line shapes become twice broader due to in...
The kagome Heisenberg antiferromagnet formed by frustrated spins arranged in a lattice of corner-sharing triangles is a prime candidate for hosting a quantum spin liquid (QSL) ground state consisting of entangled spin singlets. But the existence of various competing states makes a convincing theoretical prediction of the QSL ground state difficult,...
Electron-phonon coupling was believed to govern the carrier transport in halide perovskites and related phases. Here we demonstrate that electron-electron interaction plays a direct and prominent role in the low-temperature electrical transport of compressed CsPbI3 and renders Fermi liquid (FL)-like behavior. By compressing {\delta}-CsPbI3 to 80 GP...
We use $^{79}$Br nuclear quadrupole resonance (NQR) to demonstrate that ultra slow lattice dynamics set in below the temperature scale set by the Cu-Cu super-exchange interaction $J$($\simeq160$ K) in the kagome lattice Heisenberg antiferromagnet Zn-barlowite. The lattice completely freezes below 50 K, and $^{79}$Br NQR lineshapes become twice broa...
We report results of low-temperature heat capacity, magnetocaloric effect and neutron diffraction measurements of TmVO$_{4}$, an insulator that undergoes a continuous ferroquadrupolar phase transition associated with local partially-filled $4f$ orbitals of the thulium (Tm$^{3+}$) ions. The ferroquadrupolar transition, a realization of Ising nematic...
The kagome Heisenberg antiferromagnet formed by frustrated spins arranged in a lattice of corner-sharing triangles is a prime candidate for hosting a quantum spin liquid (QSL) ground state consisting of entangled spin singlets1. However, the existence of various competing states makes a convincing theoretical prediction of the QSL ground state diff...
A quantum spin liquid is a novel ground state that can support long-range entanglement between magnetic moments, resulting in exotic spin excitations involving fractionalized $S=\frac{1}{2}$ spinons. Here, we measure the excitations in single crystals of the spin liquid candidate Zn-barlowite using resonant inelastic X-ray scattering. By analyzing...
Spin- and charge- stripe order has been extensively studied in the superconducting cuprates, among which underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) is an archetype which has static spin density wave (SDW) order at low temperatures. An intriguing, but not completely understood, phenomenon in LSCO is that the stripes a...
Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskit...
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...
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...
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
Performing time- and angle-resolved photoemission (tr-ARPES) spectroscopy 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 (from hundreds of μJ to mJ) and their energy resolution is limited to t...
Performing time- and angle-resolved photoemission (tr-ARPES) spectroscopy 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 (from hundreds of μJ to mJ) and their energy resolution is limited to t...
The spin-
1
2
kagome antiferromagnet is considered an ideal host for a quantum spin liquid (QSL) 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 (Cu4(OH)6FBr) and Zn-substituted barlowite demonstrate the delicate inter...
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 $\mu$J to mJ) and their energy resolution is limited to ten...
When the bonds of a quantum magnet are modulated with a periodic pattern, exotic quantum ground states may 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. Our new variant of barlowite ma...
Fermi surface (FS) topology is a fundamental property of metals and superconductors. In electron-doped cuprate Nd2-x Ce
x
CuO4 (NCCO), an unexpected FS reconstruction has been observed in optimal- and overdoped regime (x = 0.15-0.17) by quantum oscillation measurements (QOM). This is all the more puzzling because neutron scattering suggests that t...
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 sin...
The application of pressure reveals a rich phase diagram for the quantum S=1/2 spin chain material TiOCl. We performed x-ray diffraction on single-crystal samples 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 on...
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 Mot...
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.
The search for the hypothetical state has been a 43-year-long slog, one whose end may now be in sight.
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 coherent optical manipulation of solids is emerging as a promising way to
engineer novel quantum states of matter. The strong time periodic potential of
intense laser light can be used to generate hybrid photon-electron states.
Interaction of light with Bloch states leads to Floquet-Bloch states which are
essential in realizing new photo-induce...
The kagome Heisenberg antiferromagnet is a leading candidate in the search for a spin system with a quantum spin-liquid ground
state. The nature of its ground state remains a matter of active debate. We conducted oxygen-17 single-crystal nuclear magnetic
resonance (NMR) measurements of the spin-1/2 kagome lattice in herbertsmithite [ZnCu3(OH)6Cl2],...
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...
Herbertsmithite ZnCu[subscript 3](OH)[subscript 6]Cl[subscript 2] is a kagome lattice antiferromagnet with spin-1/2 and has been demonstrated to be a likely candidate of spin liquid by a number of recent experiments. The high-field magnetization of the kagome lattice is complicated due to the presence of a few percent of extra Cu impurities sitting...
Herbertsmithite ZnCu3(OH)(6)Cl-2 is a kagome lattice antiferromagnet with spin-1/2 and has been demonstrated to be a likely candidate of spin liquid by a number of recent experiments. The high-field magnetization of the kagome lattice is complicated due to the presence of a few percent of extra Cu impurities sitting on the interlayer metallic sites...
The cooperative Jahn-Teller effect (CJTE) refers to the correlation of distortions arising from individual Jahn-Teller centres in complex compounds. The effect usually induces strong coupling between the static or dynamic charge, orbital and magnetic ordering, which has been related to many important phenomena such as colossal magnetoresistance and...
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 $\mu_{0}H$
= 55 T at $T$ = 0.4 K, showing a saturatio...
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 sp...
We report a direct measurement of the low-frequency optical conductivity of large-area single-crystal herbertsmithite, a promising spin-liquid candidate material, by means of terahertz time-domain spectroscopy. In the spectral range below 1.4 THz, we observe a contribution to the real part of the in-plane conductivity σ_{ab}(ω) from the spin degree...
The experimental realization of quantum spin liquids is a long-sought goal in physics, as they represent new states of matter. Quantum spin liquids cannot be described by the broken symmetries associated with conventional ground states. In fact, the interacting magnetic moments in these systems do not order, but are highly entangled with one anothe...
We develop a theoretical model that describes the second harmonic generation of light from the surface of the topological insulator Bi2Se3 and experimentally demonstrate that the technique is sensitive to the surface electrons. By performing a crystal symmetry analysis of Bi2Se3(111) we determine the nonlinear electric susceptibility tensor element...
We report thermodynamic measurements of the S=1/2 kagome lattice antiferromagnet ZnCu[subscript 3](OH)[subscript 6]Cl[subscript 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-depend...
Gapless surface states on topological insulators are protected from elastic scattering on nonmagnetic impurities which makes them promising candidates for low-power electronic applications. However, for widespread applications, these states should have to remain coherent at ambient temperatures. Here, we studied temperature dependence of the electr...
We report thermodynamic measurements of the S=1/2 kagome lattice antiferromagnet ZnCu3(OH)6Cl2, 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, wh...
Dirac-like surface states on surfaces of topological insulators have a chiral spin structure that suppresses backscattering and protects the coherence of these states in the presence of nonmagnetic scatterers. In contrast, magnetic scatterers should open the backscattering channel via the spin-flip processes and degrade the state's coherence. We pr...
The new two-dimensional magnetically frustrated title compound is synthesized from an aqueous NH4OH solution containing Ni(OH)2, V2O5, an Ba(OAc)2 (reflux, 4 h).
Topological insulators (TL) have attracted much attention because of
their exotic properties. Bi2Se3 is a model TL with
a relative large bulk gap and a simple surface state structure. By
depositing various impurities on the surface, we were able to fill the
topological surface state and higher lying Rashba splitting surface
states. The spin texture...
Herbertsmithite ZnCu3(OD)6Cl2 is one of
the most promising examples for a quantum spin liquid state. Despite the
remarkable absence of long range magnetic order down to at least 50mK,
understanding the magnetic properties of
ZnCu3(OD)6Cl2 remains a challenge. This
is mainly due to the difficulty in locating the defects, and in
understanding the pos...
Frustrated magnets with integer spin are predicted to have exotic physical properties including spin nematicity, yet few are known to exist. We report a new, frustrated S = 1 magnet, BaNi(3)(OH)(2)(VO(4))(2), which is the structural analogue of the mineral vesignieite. Magnetic frustration arises from a competition between ferromagnetic and antifer...
The Zn-paratacamite family, ZnxCu4-x(OH)6Cl2 for x [greater than or equal to]0.33, is an ideal system for studying spin-1/2 frustrated magnetism in the form of antiferromagnetic Cu2+ 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...
Topological insulators possess completely different spin-orbit coupled bulk and surface electronic spectra that are each predicted to exhibit exotic responses to light. Here we report time-resolved fundamental and second harmonic optical pump-probe measurements on the topological insulator Bi(2)Se(3) to independently measure its photoinduced charge...
We performed high-resolution spin- and angle-resolved photoemission spectroscopy studies of the electronic structure and the spin texture on the surface of Bi2Se3, a model TI. By tuning the photon energy, we found that the topological surface state is well separated from the bulk states in the vicinity of kz = Z plane of the bulk Brillouin zone. Th...
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 accoun...
Structural characterization, exploiting X-ray scattering differences at elemental absorption edges, is developed to quantitatively determine crystallographic site-specific metal disorder. We apply this technique to the problem of Zn-Cu chemical disorder in ZnCuâ(OH)âClâ. This geometrically frustrated kagome antiferromagnet is one of the best candid...
We report single-crystal 2-D NMR investigation of the nearly ideal spin S=1/2
kagome lattice ZnCu3(OD)6Cl2. We successfully identify 2-D NMR signals
originating from the nearest-neighbors of Cu2+ defects occupying Zn sites. From
the 2-D Knight shift measurements, we demonstrate that weakly interacting Cu2+
spins at these defects cause the large Cur...
We present a hydrothermal method for growing millimeter-sized crystals of the quantum magnets with formula Cu4-xZnx(OH)(6)Cl-2: clinoatacamite (x=0), paratacamite (0.33 < x < 1) and herbertsmithite (x=1). These highly pure single crystals have been characterized by x-ray diffraction, chemical analysis, Raman spectroscopy, and magnetic susceptibilit...
Magnetization, specific heat, and neutron scattering measurements were
performed to study a magnetic transition in jarosite, a spin-5/2 kagome lattice
antiferromagnet. When a magnetic field is applied perpendicular to the kagome
plane, magnetizations in the ordered state show a sudden increase at a critical
field H_c, indicative of the transition f...
We use ultrafast laser pulses to experimentally demonstrate that the second-order optical response of bulk single crystals of the topological insulator Bi(2)Se(3) is sensitive to its surface electrons. By performing surface doping dependence measurements as a function of photon polarization and sample orientation we show that second harmonic genera...
We have observed a softening of phonons and a structural phase transition in a superconducting Ba[subscript 0.59]K[subscript 0.41]BiO[subscript 3] (T[subscript c] = 31 K) single crystal using elastic and inelastic neutron scattering measurements. The soft phonon occurs for the [1 1 1] transverse acoustic mode at the zone boundary. The phonon energi...
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 account...
Electronic transport experiments involving the topologically protected states found at the surface of Bi(2)Se(3) 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) Bi(2)Se(3) devices and show that th...
Structural characterization, exploiting X-ray scattering differences at elemental absorption edges, is developed to quantitatively determine crystallographic site-specific metal disorder. We apply this technique to the problem of Zn-Cu chemical disorder in ZnCu(3)(OH)(6)Cl(2). This geometrically frustrated kagomé antiferromagnet is one of the best...
We present a Raman spectroscopic investigation of the herbertsmithite ZnCu3(OH)6Cl2, a realization of a Heisenberg s=1/2 antiferromagnet on a perfect kagome lattice. Focusing on its magnetic excitation spectrum we find two components, a high-temperature quasielastic signal and a low temperature, broad maximum. The latter has a linear low-energy slo...
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...
The spin-1/2 kagome lattice antiferromagnet herbertsmithite, ZnCu(3)(OH)(6)Cl(2), is a candidate material for a quantum spin liquid ground state. We show that the magnetic response of this material displays an unusual scaling relation in both the bulk ac susceptibility and the low energy dynamic susceptibility as measured by inelastic neutron scatt...
Spin-frustrated systems are one avenue for inducing macroscopic quantum states in materials. However, experimental realization of this goal has been difficult because of the lack of simple materials and, if available, the separation of the unusual magnetic properties arising from exotic magnetic states from behavior associated with chemical disorde...
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 sta...
We report an electrical transport study of the topological insulator Bi2Se3 Topological insulators are materials characterized by a gap in the bulk and gapless surface states. The surface states have Dirac dispersions and are protected from back-scattering. The latter protection arises from the fundamental symmetries of the material, and has far re...
Using muon spin resonance we examine the organometallic hybrid compound
Cu(1,3-benzenedicarboxylate) [Cu(1,3-bdc)], which has structurally perfect spin
1/2 copper kagome planes separated by pure organic linkers. This compound has
antiferromagnetic interactions with Curie-Weiss temperature of -33 K. We found
slowing down of spin fluctuations startin...
The ground state of the spin-Peierls system TiOCl was probed using
synchrotron x-ray diffraction on a single-crystal sample at T = 6 K. We tracked
the evolution of the structural superlattice peaks associated with the
dimerized ground state as a function of pressure. The dimerization along the b
axis is rapidly suppressed in the vicinity of a first...
We report polarized neutron scattering studies of spin-wave excitations and spin fluctuations in the S = 5/2 kagome lattice antiferromagnet KFe[subscript 3](OH)[subscript 6](SO[subscript 4])[subscript 2] (jarosite). Inelastic polarized neutron scattering measurements at 10 K on a single crystal sample reveal two spin gaps, associated with in-plane...
We have performed thermodynamic and neutron scattering measurements on the S=1/2 three-dimensional antiferromagnet clinoatacamite, Cu2(OH)3Cl. The crystal lattice feature Cu^2+ ions arranged on a distorted kagom'e lattice with weak magnetic coupling between adjacent planes. Long range magnetic order with a weak ferromagnetic moment emerges at the N...
The spin wave excitations of the S = 5/2 kagomé lattice antiferromagnet KFe3(OH)6(SO4)2 have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted ‘‘zero energy mode,’’ verifying a fundamental prediction for the kagomé lattice. A simple Heisenberg spin Hamiltonian provides an...
The collective behaviour of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report a study of the spin chirality on a two-dimensional geometric...
