Refining the Spin Hamiltonian in the Spin-1/2 Kagome Lattice Antiferromagnet ZnCu3_{3}(OH)6_{6}Cl2_{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 $\chi_{z}/ \chi_{p} > 1$ at high temperatures and $\chi_{z}/ \chi_{p} < 1$ 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$_{2}$.Comment: 4 pages, 4 figure

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.Comment: 5 figure

Synthesis and characterization of single crystal kagomé lattice antiferromagnets

Thesis: S.M., Massachusetts Institute of Technology, Department of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (pages 67-69).An ideal spin-% kagomé lattice has been a long sought material. This system is characterized by strong magnetic frustration and is a likely candidate for a spin-liquid ground state. The spin-liquid state was originally proposed to exist in the parent compounds of the high temperature superconducting cuprates as originally proposed by Anderson. However, the lack of ideal samples have hampered experimental tests of the theories. A few years ago, a kagomé lattice material called herbertsmithite (ZnCu3(OH)6Cl2) has been successfully synthesized and studied. Since then, many experiments have been performed which have produced a lot of new guidance for our theoretical understanding of this frustrated magnetic system. However, single crystals are crucial for further progress. We have successfully produced high quality single crystals ZnCu3(OH)6Cl2 . These crystals are large enough for measurements, such as x-ray diffraction, magnetism, heat capacity, neutron scattering, thermal conductivity, muon-scattering and optical measurement. In this thesis, I will summarize the current state of knowledge for herbertsmithite and its family, the single crystal growth technique, and characterization of the resulting samples. A discussion of further directions of growth and measurement is at the end.by Tianheng Han.S.M

Thermodynamic and neutron scattering study of the spin- 1/2 kagome antiferromagnet ZnCu₃(OH)₆Cl₂ : a quantum spin liquid system

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 183-200).New physics, such as a quantum spin liquid, can emerge in systems where quantum fluctuations are enhanced due to reduced dimensionality and strong frustration. The realization of 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. The Zn-paratacamite family, ZnCu₃(OH)₆Cl₂ for x > 0.33, is an ideal system to look for such an exotic state in the form of antiferromagnetic Cu2+ kagome planes. The x = 1 end member, named herbertsmithite, has shown promising spin liquid properties from prior studies on powder samples. Here we show a new synthesis by which high-quality centimeter-sized single crystals of Zn-paratacamite have been produced for the first time. Neutron and synchrotron x-ray diffraction experiments indicate no structural transition down to T = 2 K. The magnetic susceptibility both perpendicular and parallel to the kagome plane has been measured for the x = 1 sample. A small, temperature-dependent anisotropy has been observed, where Xz / Xp > 1 at high temperatures and Xz / Xp < 1 at low temperatures. Fits of the high-temperature data to a Curie-Weiss model also reveal anisotropies for [Theta]cw's and g-factors. By comparing with theoretical calculations, the presence of a small easy-axis exchange anisotropy can be deduced as a 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 ZnCu₃(OH)₆Cl₂. Specific heat measurements down to dilution temperatures and under strong applied magnetic fields show a super linear temperature dependence with a finite linear term. Most importantly, we present neutron scattering measurements of the spin excitations on a large deuterated single crystal sample of herbertsmithite. Our observation of a spinon continuum in a two-dimensional magnet is unprecedented. The results serve as a key fingerprint of the quantum spin liquid state in herbertsmithite.by Tianheng Han.Ph.D

The Herbertsmithite Hamiltonian: μ\muSR 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.Comment: 8 pages, 3 figures, accepted to JPCM special issue on geometrically frustrated magnetis

Synthesis and Characterization of Single Crystal Samples of Spin-1/21/2 Kagome Lattice Antiferromagnets in the Zn-Paratacamite Family Znx_{x}Cu4−x_{4-x}(OH)6_{6}Cl2_{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 $\chi_{c} \, > \, \chi_{ab}$. 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.Comment: 4 pages, 3 figures, accepted by PRB rapid communicatio

Liquid Scintillators Loaded with up to 40 Weight Percent Cesium Lead Bromide Quantum Dots for Gamma Scintillation

Cesium lead bromide quantum dot (CsPbBr3 QD) is emerging as a promising luminescent material for gamma ray detection. However, its intensive self-absorption traps the luminescent photons generated and diminishes the scintillation light yield. The self-absorption is magnified at high CsPbBr3 QD loading and large scintillator volume, which are required to effectively attenuate gamma photons. We report a liquid scintillator loaded with up to 40 wt % of CsPbBr3 QDs. Pyrromethene 580 (PM-580), a fluorescent dye, was co-dissolved in the solution as a fluorescence resonance energy-transfer acceptor to overcome the self-absorption of the QDs. The rapid energy transfer from the QDs to PM-580 also accelerates the scintillation decay kinetics. The scintillation light yield of the liquid containing 20 wt % QDs and 0.75 wt % PM-580 was 8800 photons/MeV. The decay lifetime is 24.3 ns, faster than most inorganic crystal scintillators. The light yield was 7300 photons/MeV at 40 wt % QD loading. Gamma pulse spectroscopy of the 40 wt % QD liquid produced the 662 keV gamma photopeak with a 27% energy resolution, demonstrating the potential of the CsPbBr3 QD loaded organic scintillators for spectroscopic gamma detection

Observation of superconductivity in granular bi nanowires fabricated by electrodeposition

Bulk rhombohedral Bi at ambient pressure is a well-known semimetal, and its transition to a superconductor has not been observed, at least down to 50 mK. We report that, unlike bulk rhombohedral Bi, granular Bi nanowires with well-defined rhombohedral grains of similar to 10 nm diameter, fabricated by electrochemically depositing Bi into porous polycarbonate membranes at ambient pressure, are superconducting with two transition temperatures, T-c, of 7.2 and 8.3 K. These T-c values coincide with T-c values of the high-pressure phases Bi-III and Bi-V, respectively. Analysis of our structural and transport data indicates that the superconductivity in granular Bi nanowires probably arises from grain boundary areas where there are structural reconstructions between the grains showing a preferred orientation within a small angular distribution

High-field magnetic ground state in S = 1/2 kagome lattice antiferromagnet ZnCu[subscript 3](OH)[subscript 6]Cl[subscript 2<]

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 on the interlayer metallic sites. To determine the magnetic ground state of the kagome lattice, we measured the magnetization of a single crystalline ZnCu[subscript 3](OH)[subscript 6]Cl[subscript 2] using torque magnetometry down to the base temperatures 20 mK in intense magnetic field as high as 31 T. The high-field intrinsic magnetization from the kagome lattice turns out to be linear with magnetic field, and the magnetic susceptibility is independent of temperature at 20 mK ≤ T ≤ 5 K. Moreover, below 2 K, several field-induced anomalies are observed in between 7 T and 15 T.United States. Dept. of Energy (Grant DE-FG02-07ER46134