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김세훈,이철의,N. S. Dalal,한덕영 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.5
H2C4O4 (Squaric acid, henceforth abbreviated by SQA) is one of the most studied hydrogenbonded anti-ferroelectrics in the KH2PO4 (KDP) family. Here, we report the static hydrogen lineshape which shows a mixed state behavior of order-disorder and a displacive feature in the antiferroelctric (AFE)/paraelectric (PE) phase transition of SQA. We obtained the dipolar splitting data from the nonlinear fitting curve of the static hydrogen lineshape. The calculated proton-proton distance showed an increase at temperatures above the transition temperature Tc. As temperature increases through Tc, the inter-atomic distance shows a peculiar S-shape variation. This S-shape variation around Tc is also observed in other NMR reports for SQA, where the chemical shift variation is used to characterize the deformation of the C4O4 squarate group. Here, we used the dipolar interaction, instead of the chemical shift information, to detect the geometrical change of the hydrogen. Below the transition temperature Tc, the proton interactions showed long-range order whereas it showed short range order at temperatures above Tc. The peculiar S-shape variation from the hydrogen lineshape analysis can be understood as another piece of experimental evidence for a mixed state for the phase transition in SQA.
Phase Transitions in KTiOPO₄ Studied by ³¹P Nuclear Magnetic Relaxation
K. S. Kim,C. H. Lee,Cheol Eui Lee,N. S. Dalal,R. Fu,S. Y. Jeong,J. N. Kim,S. C. Kim 한국자기학회 2000 Journal of Magnetics Vol.5 No.3
Undoped and Cr-doped samples of electrooptic material KTiOPO₄ were studied by ³¹P nuclear magnetic resonance (NMR). Spin-lattice relaxation time (T₁) measurements manifested phase transition behaviors that are attributed to changes in the dominant charge carriers in different temperature ranges.
High-resolution 31P nuclear magnetic resonance studyof phase transitions in TlH2PO4
CheolEuiLee,N.S.Dalal,R.Fu 한국물리학회 2003 Current Applied Physics Vol.3 No.5
High-resolution 31P nuclear magnetic resonance (NMR) techniques were employed to study a KH2PO4-type ferroelectric system, TlH2PO4. A marked temperature dependence of the isotropic chemical shift below the ferroelastic phase transition temperature is indicative of an electronic instability. The NMR linewidth showed a discontinuity at the ferroelastic phase transition, and the anisotropy was measured to increase rapidly below the antiferroelectric phase transition. Thus, the changes in the microscopic environments associated with the phase transitions were sensitively reflected in a characteristic manner. 2003 Elsevier B.V. All rights reserved.
High resolution 31P nuclear magnetic resonance study of Cr3þ-doping effects on KTiOPO4
CheolEuiLee,N.S.Dalal,R.Fu 한국물리학회 2003 Current Applied Physics Vol.3 No.4
31P nuclear magnetic resonance was employed to investigate Cr3þ -doping eects on KTiOPO4. The high resolution31P isotropicchemical shift and linewidth measurements sensitively revealed changes in the microscopic environments caused by the doping.. 2002 Elsevier Science B.V. All rights reserved.
Magnetic field-temperature phase diagram of multiferroic [(CH3)2NH2]Mn(HCOO)3
Clune, A. J.,Hughey, K. D.,Lee, C.,Abhyankar, N.,Ding, X.,Dalal, N. S.,Whangbo, M.-H.,Singleton, J.,Musfeldt, J. L. American Physical Society 2017 Physical Review B Vol.96 No.10
<P>We combined pulsed field magnetization and first-principles spin-density calculations to reveal the magnetic field-temperature phase diagram and spin state character in multiferroic [(CH3)(2)NH2]Mn(HCOO)(3). Despite similarities with the rare earth manganites, the phase diagram is analogous to other Mn-based quantum magnets with a 0.31 T spin flop, a 15.3 T transition to the fully polarized state, and short-range correlations that persist above the ordering temperature. The experimentally accessible saturation field opens the door to exploration of the high-field phase.</P>
Kim, Se-Hun,Han, J.H.,Lee, Cheol-Eui,Lee, Kwang-Sei,Kim, Chang-Sam,Dalal, N.S.,Han, Doug-Young Korean Magnetic Resonance Society 2010 Journal of the Korean Magnetic Resonance Society Vol.14 No.2
We studied the hydrogen-bonded $TlH_2PO_4$ (TDP) ferroelectrics treated with the proton-beam bombardment. The TDP material was irradiated with 1-MeV proton beam at a dose of $10^{15}/cm^2$. In order to analyze the hydrogen environment in TDP, we carried out the $^1H$ high resolution nuclear magnetic resonance (NMR) - i.e., Combined Rotation And Multiple Pulse Spectroscopy (CRAMPS) measurement. The isotropic chemical shift of hydrogen indicates its displacive property is related to the $PO_4$ lattice deformation which occurs throughout the antiferroelectric-, the ferroelastic- and the paraelastic-phase transitions. The temperature dependence of $\sigma_{iso}$ reveals the electronic charge redistribution is induced by the proton-beam irradiation and the elastic property.