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Nuclear quadrupole resonance studies of an organic quantum magnet NiCl2 - [SC(NH2)2]4 dichloro-tetrakis-thiourea-nickel Allen R. Majewski Department of Physics University of Florida February 2015 Overview • The value of nuclear quadrupole resonance spectroscopy • DTN - motivation (spin ordering) • NQR technique • The superhet and results • Theory of EFG and quadrupole coupling constant • Search for DTN NQR • DFT analysis - GIPAW NQR is a chemical fingerprint • The NQR frequency of a given nucleus is compound specific => NQR is a chemical fingerprint • The NQR frequency has EXTREME temperature dependence. It is an excellent thermometer. Motivation - industrial/civil use of NQR Explosives (14N) detection: As of 29 July 2014, there were 587.2 square kilometers (226.7 square miles) of Croatian territory suspected to contain land mines. These areas are located in 11 counties and 82 cities and municipalities. Flight security : Detection of explosives, 14N containing narcotics … Oil industry : Oil well logging by precise measurement of P, T … Motivation – Bose Glass in NiCl2-[SC(NH2)2]4 (DTN) Phase diagram – DTN @ T < 1.2K BEC magnet Superheterodyne Spectrometer Hetero means “other” Example: A radio station is transmitting on 300 kHz and the oscillating receiver was set to 400 kHz, the station would be heard not only at the original 300 kHz, but also at 100 kHz and 700 kHz. Pulse Generator – overview Pulse generator - details Probe section Impedance matching Impedance matching – circuit A isolation – hybrid tee Isolation – quarter waves RF amplifier stage – miteq fast recovery Receiver section Generator and receiver – side by side IF (audio) amplifier stage – LM386 Labview controller - YB Coil – large L Probe section DAQ – by YB NaClO3 response to a transient at 29.963 MHz, pulse width=50us, signal averaging N=1000; Fourier transform (left) of discrete FID (above). res =7204 pixels; horizontal_axis=100us/div Cap and coil selection Theory of the nuclear quadrupole resonance The electrostatic energy of a charge in fields written in terms of multipole moments. The last term in the energy is the quadrupole moment’s interaction with the electric field gradient . The traceless quadrupole moment tensor is the familiar expression. Electric field gradient (EFG) ij V Quadrupole interaction energy with field. Laplace’s equation -> EFG is traceless. Nucleus has cylindrical symmetry –> only one indep.endent nonvanishing quadrupole moment Choice of axes permit ordering eigenvalues of EFG by convention Only two independent parameters Assymetry parameter Cq or quadrupole coupling constant ~ eq NQR transition frequencies Hamiltonian for quadrupole interaction of nucleus with external field …matrix elements… energy levels NQR transition frequencies NQR frequencies Use of density functional theory in NQR NiCl2 - [SC(NH2)2]4 dichloro-tetrakis-thiourea-nickel Allen R. Majewski Department of Physics University of Florida February 2015 Original Plan – 35Cl in DTN • Chlorine NQR typically at 25 – 42 MHz • But with cubic symmetry, Cq = 0 -> no NQR • With some symmetry, NQR much lower Only two independent parameters Assymetry parameter Cq or quadrupole coupling constant ~ eq Can we calculate the quadrupole coupling constant? • Not really • Density functional theory is possibility • WEIN2k, CASTEP, Quantum Espresso • Frozen core, pseudopotentials Timeline • In summer 2013, Tim Green (Oxford) used CASTEP to calculate EFG in DTN • In fall 2013, I traveled to ETH to use Quantum Espresso to replicate calc • Both predicted chlorine resonance below 10 MHz due to symmetry See Handout for data summary • Evidence for accuracy of DFT simulation: 1) CASTEP correctly predicted NQR in Sodium Chlorate as a control 2) CASTEP and QE predicted 14N NQR in DTN similar to that of known values for thiourea!! 3) CASTEP and QE widely agree, despite being completely independent software packages External • http://www.phys.ufl.edu/~majewski/nqr/equation.pdf • http://www.phys.ufl.edu/~majewski/nqr/nqr-combined.pdf • http://www.phys.ufl.edu/~majewski/nqr/Calculations-of-the-EFGtensor-in-DTN-using-GIPAW-with-CASTEP-and-QE-software.pdf • http://www.phys.ufl.edu/~majewski/nqr/tek • http://www.phys.ufl.edu/~majewski/nqr/dft.pdf • http://www.phys/ufl.edu/~majewski • https://apps.ufl.edu/Citrix/XenAppEXT/auth/login.aspx