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ABSTRACT: A micro-calorimeter working at ultra-low temperature down to 75 mK and high magnetic field up to 17.5 T has been developed.
A typical sample weight is as small as 100 μg. A magnetic phase diagram of magnon Bose-Einstein condensation (BEC) in Ba3Cr2O8 is determined. The scaling index of the phase boundary is very close to that of the isotropic three dimensional BEC at low
temperature, in spite of the existence of Dzyaloshinskii-Moriya (DM) interaction which is confirmed in the previous ESR measurement.
KeywordsCalorimeter-Magnon BEC
Journal of Low Temperature Physics 04/2012; 159(1):118-121. · 1.19 Impact Factor
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S. E. Dissanayake,
C. Chan,
S. Ji,
J. Lee,
Y. Qiu,
K. C. Rule,
B. Lake,
M. Green,
M. Hagihala,
X. G. Zheng,
T. K. Ng, S. -H. Lee
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ABSTRACT: Using elastic and inelastic neutron scattering techniques with and without
application of an external magnetic field $H$, the magnetic ground states of
Zn$_x$Co$_{4-x}$(OD)$_6$Cl$_2$ ($x=0,1$) were studied. Our results show that
for $x=0$, the ground state is a magnetic long-range ordered (LRO) state where
each tetrahedron forms an "umbrella"-type structure. On the other hand, for
$x=1$, no static ordering was observed down to 1.5 K, which resembles the
behavior found in the isostructural quantum system
Zn$_x$Cu$_{4-x}$(OD)$_6$Cl$_2$. When $H$ field is applied, however the $x=1$
system develops the same LRO state as $x=0$. This indicates that the $x=1$
disordered state is in the vicinity of the $x=0$ ordered state.
03/2012;
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ABSTRACT: Using synchrotron x-ray and neutron diffraction, we disentangle spin-lattice order in highly frustrated ZnCr2O4 where magnetic chromium ions occupy the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of antialigning spins surrounding the triangular faces of tetrahedra is resolved by establishing weak interactions on each triangle through an intricate lattice distortion. However, the resulting spin order is not simply a Néel state on strong bonds, but rather a complex coplanar spin structure, indicating that antisymmetric and/or further neighbor exchange interactions also play a role as ZnCr2O4 resolves conflicting magnetic interactions.
Physical Review Letters 08/2009; 103(3):037201. · 7.37 Impact Factor
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M Kofu,
H Ueda,
H Nojiri,
Y Oshima,
T Zenmoto,
K C Rule,
S Gerischer,
B Lake,
C D Batista,
Y Ueda, S-H Lee
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ABSTRACT: By using bulk magnetization, electron spin resonance (ESR), heat capacity, and neutron scattering techniques, we characterize the thermodynamic and quantum phase diagrams of Ba3Cr2O8. Our ESR measurements indicate that the low field paramagnetic ground state is a mixed state of the singlet and the Sz=0 triplet for H perpendicular c. This suggests the presence of an intradimer Dzyaloshinsky-Moriya (DM) interaction with a DM vector perpendicular to the c axis.
Physical Review Letters 06/2009; 102(17):177204. · 7.37 Impact Factor
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M. Kofu,
H Ueda,
H Nojiri,
Y Oshima,
T. Zenmoto,
K. Rule,
S. Gerischer,
B. Lake,
C. D. Batista,
Y Ueda, S H Lee
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ABSTRACT: By using bulk magnetization, electron spin resonance (ESR), heat capacity, and neutron scattering techniques, we characterize the thermodynamic and quantum phase diagrams of Ba$_3$Cr$_2$O$_8$. Our ESR measurements indicate that the low field paramagnetic ground state is a mixed state of the singlet and the S$_z$ = 0 triplet for $H \perp c$. This suggests the presence of an intra-dimer Dzyaloshinsky-Moriya (DM) interaction with a DM vector perpendicular to the c-axis.
03/2009;
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ABSTRACT: Using single crystal inelastic neutron scattering with and without the application of an external magnetic field and powder neutron diffraction, we have characterized magnetic interactions in Ba3Cr2O8. Even without a field, we found that there exist three singlet-to-triplet excitation modes in the (h, h, l) scattering plane. Our complete analysis shows that the three modes are due to spatially anisotropic interdimer interactions that are induced by lattice distortions of the tetrahedron of oxygens surrounding the Jahn-Teller active Cr5+(3d1). The strong intradimer coupling of J0=2.38(2) meV and weak interdimer interactions (|Jinter|< or =0.52(2) meV) makes Ba3Cr2O8 a good model system for weakly coupled s=1/2 quantum spin dimers.
Physical Review Letters 02/2009; 102(3):037206. · 7.37 Impact Factor
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ABSTRACT: Low-energy spin excitations were investigated in the static stripe phase of La2-xSrxCuO4 using elastic and inelastic neutron scattering on single crystals. For x=1/8 in which long-range static stripe order exists, an energy gap of E(g)=4 meV exists in the excitation spectrum in addition to strong quasielastic, incommensurate spin fluctuations associated with the static stripes. When x increases, the spectral weight of the spin fluctuations shifts from the quasielastic continuum to the excitation spectrum above E(g). The dynamic correlation length as a function of energy and the temperature evolution of the energy spectrum suggest a phase separation of two distinct magnetic phases in real space.
Physical Review Letters 02/2009; 102(4):047001. · 7.37 Impact Factor
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ABSTRACT: We report bulk magnetization, and elastic and inelastic neutron scattering measurements under an external magnetic field H on the weakly coupled distorted kagome system, Cu2(OD)3Cl. Our results show that the ordered state below 6.7 K is a canted antiferromagnet and consists of large antiferromagnetic ac components and smaller ferromagnetic b components. By first-principles calculations and linear spin wave analysis, we present a simple spin Hamiltonian with nonuniform nearest neighbor exchange interactions resulting in a system of coupled spin trimers with a single-ion anisotropy that can qualitatively reproduce the spin dynamics of Cu2(OD)3Cl.
Physical Review Letters 10/2008; 101(10):107201. · 7.37 Impact Factor
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ABSTRACT: Using single crystal inelastic neutron scattering with and without application of an external magnetic field and powder neutron diffraction, we have characterized magnetic interactions in Ba$_3$Cr$_2$O$_8$. Even without field, we found that there exist three singlet-to-triplet excitation modes in $(h,h,l)$ scattering plane. Our complete analysis shows that the three modes are due to spatially anisotropic interdimer interactions that are induced by local distortions of the tetrahedron of oxygens surrounding the Jahn-Teller active Cr$^{5+} (3d^1)$. The strong intradimer coupling of $J_0 = 2.38(2)$ meV and weak interdimer interactions ($|J_{\rm inter}| \leq 0.52(2)$ meV) makes Ba$_3$Cr$_2$O$_8$ a good model system for weakly-coupled $s = 1/2$ quantum spin dimers.
09/2008;
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ABSTRACT: Low energy spin excitations were investigated in the static stripe phase of La_{2-x}Sr_xCuO_4 using elastic and inelastic neutron scattering on single crystals. For x = 1/8 in which long-range static stripe order exists, an energy gap of E_g = 4 meV exists in the excitation spectrum in addition to strong quasi-elastic, incommensurate spin fluctuations associated with the static stripes. When x increases, the spectral weight of the spin fluctuations shifts from the quasi-elastic continuum to the excitation spectrum above E_g. The dynamic correlation length as a function of energy and the temperature evolution of the energy spectrum suggest a phase separation of two distinct magnetic phases in real space.
05/2008;
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ABSTRACT: A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S=1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating-valence-bond state, in which every pair of neighbouring quantum spins forms an entangled spin singlet (valence bonds) and these singlets are quantum mechanically resonating among themselves. Here we provide an experimental indication for such quantum paramagnetic states existing in frustrated antiferromagnets, Zn(x)Cu(4-x)(OD)(6)Cl(2), where the S=1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu(4)(OD)(6)Cl(2), where distorted kagome planes are weakly coupled, a dispersionless excitation mode appears in the magnetic excitation spectrum below approximately 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence-bond solid, that breaks translational symmetry. Doping with non-magnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The valence-bond-solid state is suppressed, and for ZnCu(3)(OD)(6)Cl(2), where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low-energy spin fluctuations become featureless.
Nature Material 12/2007; 6(11):853-7. · 32.84 Impact Factor
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S-H Lee,
G Gasparovic,
C Broholm,
M Matsuda,
J-H Chung,
Y J Kim,
H Ueda,
G Xu,
P Zschack,
K Kakurai,
H Takagi,
W Ratcliff,
T H Kim,
S-W Cheong
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ABSTRACT: In this paper, we discuss neutron and synchrotron x-ray diffraction data obtained from single crystals of ACr2O4 (A = Zn, Cd). The Cr spinels undergo three-dimensional spin-Peierls transitions at low temperatures that involve cubic-to-tetragonal lattice distortions and magnetic long-range ordering. Our results show that the magnetic structures selected by these systems are closely related to the lattice distortions that are undertaken.
Journal of Physics Condensed Matter 03/2007; 19(14):145259. · 2.55 Impact Factor
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S. -H Lee,
G. Gasparovic,
C. Broholm,
M. Matsuda,
J.-H. Chung,
Y. J. Kim,
H. Ueda,
G. Xu,
P. Zschack,
K. Kakurai,
H. Takagi,
W. Ratcliff,
T. H. Kim,
S.-W. Cheong
Journal of Physics-Condensed Matter. 01/2007; 19.
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ABSTRACT: Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr2O4, undergoes an elongation along the c axis (c > a = b) at its spin-Peierls-like phase transition at T(N) = 7.8 K. The Néel phase (T < T(N)) has an incommensurate spin structure with a characteristic wave vector Q(M) = (0, delta,1) with delta approximately 0.09 and with spins lying on the ac plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr2O4, that undergoes a c-axis contraction and a commensurate spin order. The magnetic excitation of the incommensurate Néel state has a weak anisotropy gap of 0.6 meV and it consists of at least three bands extending up to 5 meV.
Physical Review Letters 12/2005; 95(24):247204. · 7.37 Impact Factor
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ABSTRACT: Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr$_2$O$_4$, undergoes an elongation along the c-axis ($c > a = b$) at its spin-Peierls-like phase transition at $T_N$ = 7.8 K. The N\'{e}el phase ($T < T_N$) has an incommesurate spin structure with a characteristic wave vector \textbf{Q}$_M$ = (0,$\delta$,1) with $\delta \sim$ 0.09 and with spins lying on the $ac$-plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr$_2$O$_4$, that undergoes a c-axis contraction and a commensurate spin order. The magnetic excitations of the incommensurate N\'{e}el state has a weak anisotropy gap of 0.6 meV and it consists of at least three bands extending up to 5 meV. Comment: PRL in press
10/2005;
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S-H Lee,
D Louca,
H Ueda,
S Park,
T J Sato,
M Isobe,
Y Ueda,
S Rosenkranz,
P Zschack,
J Iñiguez,
Y Qiu,
R Osborn
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ABSTRACT: Our powder inelastic neutron scattering data indicate that ZnV2O4 is a system of spin chains that are three-dimensionally tangled in the cubic phase above 50 K due to randomly occupied t(2g) orbitals of V3+ (3d(2)) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition.
Physical Review Letters 11/2004; 93(15):156407. · 7.37 Impact Factor
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ABSTRACT: A three dimensional system of interacting spins typically develops static long range order when it is cooled. If the spins are quantum S 1 2 , however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating valence bond state1, 2, in which every pair of neighbouring quantum spins forms an entangled spin singlet valence bonds and these singlets are quantum mechanically resonating among themselves. Here we provide an experimental indication for such quantum paramagnetic states existing in frustrated antiferromagnets, ZnxCu4 x OD 6Cl2, where the S 1 2 magnetic Cu2 moments form layers of a two dimensional kagome lattice. We find that in Cu4 OD 6Cl2, where distorted kagome planes are weakly coupled, a dispersionless excitation mode appears in the magnetic excitation spectrum below approx20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence bond solid, that breaks translational symmetry. Doping with non magnetic Zn2 ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The valence bond solid state is suppressed, and for ZnCu3 OD 6Cl2, where the kagome planes are undistorted and 90 occupied by the Cu2 ions, the low energy spin fluctuations become featureless
