arXiv subjects

N. S. Dalal

Publications and source records attributed to N. S. Dalal.

At least 19 recordsLinked to original sources

arXiv2017

Spin-Orbit Coupling Fluctuations as a Mechanism of Spin Decoherence

We discuss a general framework to address spin decoherence resulting from fluctuations in a spin Hamiltonian. We performed a systematic study on spin decoherence in the compound K$_6$[V$_{15}$As$_6$O$_{42}$(D$_2$O)] $\cdot$ 8D$_2$O, using high-field Electron Spin Resonance (ESR). By analyzing the anisotropy of resonance linewidths as a function of orientation, temperature and field, we find that the spin-orbit term is a major decoherence source. The demonstrated mechanism can alter the lifetime of any spin qubit and we discuss how to mitigate it by sample design and field orientation.

cond-mat.mes-hall↗

arXiv2015

NMR detection of dynamical processes in antiferroelectric nanoclusters during the order-disorder transition in NH4H2AsO4

We study the dynamics of inorganic antiferroelectric nanoclusters formed during an order-disorder transition and demonstrate the coexistence of the two phases in a region of 2-3 K around the transition temperature TN~215 K. Single crystals of NH4H2AsO4, a model hydrogen-bonded compound, show an antiferroelectric-paraelectric transition studied by means of highly sensitive magic angle spinning 15N NMR at 21.1 T. The phase co-existence is demonstrated by a double-peak structure of the chemical shift. Two-dimensional chemical exchange spectroscopy and spin-lattice relaxation time (T1) measurements show that the clusters are dynamic with sizes ~50 nm and lifetimes approaching seconds as T->TN. Their occupancy increases rapidly to fill the crystal volume below $T_N$. This study provides evidence for the commonality of the phase transitions in systems with electric properties and provides an improved spectroscopic method for such studies.

cond-mat.mes-hall↗

arXiv2014

Anisotropy of the molecular magnet V$_{15}$ spin Hamiltonian detected by high-field electron spin resonance

The molecular compound K$_6$[V$^{IV}_{15}$As$^{III}_6$O$_{42}$(H$_2$O)] $\cdot$ 8H$_2$O, in short V$_{15}$, has shown important quantum effects such as coherent spin oscillations. The details of the spin quantum dynamics depend on the exact form of the spin Hamiltonian. In this study, we present a precise analysis of the intramolecular interactions in V$_{15}$. To that purpose, we performed high-field electron spin resonance measurements at 120 GHz and extracted the resonance fields as a function of crystal orientation and temperature. The data are compared against simulations using exact diagonalization to obtain the parameters of the molecular spin Hamiltonian.

cond-mat.mes-hall↗

arXiv2011

AC Susceptibility and Electron Spin Resonance Studies of Spin Dynamics in n Ba$_3$NbFe$_3$Si$_2$O$_{14}$: A Geometrically Frustrated Lattice

We report ac susceptibility and high-frequency electron spin resonance (ESR) measurements on the geometrically frustrated compound Ba$_3$NbFe$_3$Si$_2$O$_{14}$ with the Néel temperature $T_N=27 K$. An unusually large frequency-dependence of ac susceptibility in the temperature range of 20 - 100 K reveals a spin-glass-like behavior, signalling the presence of frustration related slow magnetic fluctuations. ESR experiments show a multi-step magnetic and spin chirality ordering process. For temperatures above 30 K, the weak temperature dependence of the ESR linewidth $ΔH_{pp}\propto T^{-p}$ with $p=0.8$ evidences the development of short-range correlated spin clusters. The critical broadening with $p =1.8$, persisting down to 14 K, indicates the coexistence of the short-range ordered spin clusters within a helically ordered state. Below 9.5 K, the anomalously large decrease of the linewidth reveals the stabilization of a long-range ordered state with one chirality.

cond-mat.str-el↗

arXiv2009

Magnetic field induced quantum phase transition of the $S = 1/2$ antiferromagnet $K_2NaCrO_8$

The magnetic properties of alkali-metal peroxychromate K$_2$NaCrO$_8$ are governed by the $S = 1/2$ pentavalent chromium cation, Cr$^{5+}$. Specific heat, magnetocalorimetry, ac magnetic susceptibility, torque magnetometry, and inelastic neutron scattering data have been acquired over a wide range of temperature, down to 60 mK, and magnetic field, up to 18 T. The magnetic interactions are quasi-two-dimensional prior to long-range ordering, where $T_N = 1.66$ K in $H = 0$. In the $T \to 0$ limit, the magnetic field tuned antiferromagnetic-ferromagnetic phase transition suggests a critical field $H_c = 7.270$ T and a critical exponent $α= 0.481 \pm 0.004$. The neutron data indicate the magnetic interactions may extend over intra-planar nearest-neighbors and inter-planar next-nearest-neighbor spins.

cond-mat.str-el↗

arXiv2009

Fluctuation-Induced Heat Release from Temperature-Quenched Nuclear Spins near a Quantum Critical Point

At a quantum critical point (QCP) -- a zero-temperature singularity in which a line of continuous phase transition terminates -- quantum fluctuations diverge in space and time, leading to exotic phenomena that can be observed at non-zero temperatures. Using a quantum antiferromagnet, we present calorimetric evidence that nuclear spins frozen in a high-temperature metastable state by temperature quenching are annealed by quantum fluctuations near the QCP. This phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, serves as an excellent marker of a quantum critical region around the QCP and provides a probe of the dynamics of the divergent quantum fluctuations.

cond-mat.str-el↗

arXiv2009

Entrapment of magnetic micro-crystals for on-chip electron spin resonance studies

On-chip Electron Spin Resonance (ESR) of magnetic molecules requires the ability to precisely position nanosized samples in antinodes of the electro-magnetic field for maximal magnetic interaction. A method is developed to entrap micro-crystals containing spins in a well defined location on a substrate's surface. Traditional cavity ESR measurements are then performed on a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr$^{5+}$ spins were entrapped as well and measured while approaching the lower limit of the ESR sensitivity. This method suggests the feasibility of on-chip ESR measurements at dilution refrigerator temperatures by enabling the positioning of samples atop an on-chip superconducting cavity.

cond-mat.mes-hall↗

arXiv2009

Multiphoton coherent manipulation in large-spin qubits

Large spin Mn2+ ions (S=5/2) diluted in a non-magnetic MgO matrix of high crystalline symmetry are used to realize a six level system that can be operated by means of multi-photon coherent Rabi oscillations. This spin system has a very small anisotropy which can be tuned in-situ to reversibly transform the system between harmonic and non-harmonic level configurations. Decoherence effects are strongly suppressed as a result of the quasi-isotropic electron interaction with the crystal field and with the 55Mn nuclear spins. These results suggest new ways of manipulating, reading and resetting spin quantum states which can be applied to encode a qubit across several quantum levels.

cond-mat.mes-hall↗

arXiv2008

Dynamical Behavior of Spins in the Rare-Earth Kagomé Pr$_3$Ga$_5$SiO$_{14}$

We report on the use of $^{69,71}$Ga nuclear magnetic resonance to probe spin dynamics in the rare-earth kagomé system Pr$_3$Ga$_5$SiO$_{14}$. We find that the spin-lattice relaxation rate $^{69}1/T_1$ exhibits a maximum around 30 K, below which the Pr$^{3+}$ spin correlation time $τ$ shows novel field-dependent behavior consistent with a field-dependent gap in the excitation spectrum. The spin-spin relaxation rate $^{69}1/T_{2}$ exhibits a peak at a lower temperature (10 K) below which field-dependent power-law behavior close to $T^{2}$ is observed. These results point to field-induced formation of nanoscale magnetic clusters consistent with recent neutron scattering measurements.

cond-mat.str-el↗

arXiv2008

Electron Spin Relaxation and 39K Pulsed ENDOR Studies on Cr5+ doped K3NbO8 at 9.7 and 240 GHz

Cr^5+ doped K_3NbO_8, considered to be useful as a electron spin qubit, has been investigated by pulsed X-band (~9.7 GHz) and 240 GHz electron paramagnetic resonance and electron nuclear double resonance (ENDOR). Comparison of the low temperature electronic spin-lattice relaxation rate 1/T_1 at 9.7 and 240 GHz shows that it is 250 times faster at 240 GHz than at X-band. On the other hand, the spin-spin relaxation rate 1/T_2 appears largely frequency independent and is very likely related to the superhyperfine (SHF) coupling of the Cr^5+ electron with the surrounding potassium and niobium nuclei. This coupling was investigated by HYSCORE at 9.7 GHz and pulsed Mims ENDOR at 240 GHz. The high frequency and field enabled us to unambiguously measure the hyperfine and quadrupole couplings of the 39K in spite of its small magnetic moment. We find that the largest 39K SHF coupling is positive, with 0.522 MHz and 0.20 MHz as its isotropic and dipolar parts respectively. 93Nb ENDOR was dominantly due to its quadrapolar interaction, with a coupling of about 0.8 MHz, and a SHF coupling of about 0.08 MHz. The significance of these data to spin qubit studies is pointed out.

physics.chem-ph↗

arXiv2007

Coherent manipulation of electron spins up to ambient temperatures in Cr$^{5+}$(S=1/2) doped K$_3$NbO$_8$

We report coherent spin manipulation on Cr$^{5+}$ (\emph{S} = 1/2, \emph{I} = 0) doped K$_3$NbO$_8$, which constitutes a dilute two-level model relevant for use as a spin qubit. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At liquid helium temperature the phase coherence relaxation time \emph{$T_2$} reaches $\sim 10$ $μ$s and, with a Rabi frequency of 20 MHz, yields a single qubit figure of merit \emph{$Q_M$} of about 500. This shows that a diluted ensemble of Cr$^{5+}$ (\emph{S} = 1/2) doped K$_3$NbO$_8$ is a potential candidate for solid-state quantum information processing.

cond-mat.mes-hall↗

arXiv2007

AC susceptibility and $^{51}$V NMR study of MnV$_2$O$_4$

We report $^{51}$V zero-field NMR of manganese vanadate spinel of MnV$_2$O$_4$, together with both ac and dc magnetization measurements. The field and temperature dependence of ac susceptibilities show a reentrant-spin-glass-like behavior below the ferrimagnetic(FEM) ordering temperature. The zero-field NMR spectrum consists of multiple lines ranging from 240 MHz to 320 MHz. Its temperature dependence reveals that the ground state is given by the simultaneous formation of a long-range FEM order and a short-range order component. We attribute the spin-glass-like anomalies to freezing and fluctuations of the short-range ordered state caused by the competition between spin and orbital ordering of the V site.

cond-mat.str-el↗

arXiv2006

Phonon-bottleneck enhanced magnetic hysteresis in a molecular paddle wheel complex of Ru$_2^{5+}$

The ruthenium based molecular magnet [Ru$_2$(D(3,5-Cl$_2$Ph)F)$_4$Cl(0.5H$_2$O)$\cdotp$C$_6$H$_{14}$] (hereafter Ru$_2$) behaves as a two-level system at sufficiently low temperatures. The authors performed spin detection by means of single-crystal measurements and obtained magnetic hysteresis loops around zero bias as a function of field sweeping rate. Compared to other molecular systems, Ru$_2$ presents an enhanced irreversibility as shown by ``valleys'' of negative differential susceptibility in the hysteresis curves. Simulations based on phonon bottleneck model are in good qualitative agreement and suggest an abrupt spin reversal combined with insufficient thermal coupling between sample and cryostat phonon bath.

cond-mat.mes-hall↗

arXiv2006

Excitation spectrum of a model antiferromagnetic spin-trimer

We present an inelastic neutron scattering (INS) study of the excitation spectrum of a model S = 1/2 equilateral Heisenberg trimer, Cu_3(O_2C_16H_23)_6.1.2C_6H_12. Earlier measurements were interpreted on the basis of a spin ground state consisting of a pair of degenerate S = 1/2 doublets, with an S = 3/2 quartet excited state. Given that the simplest model of magnetism for this compound includes a degenerate ground state, using thermodynamic probes to determine the excitation energy between the ground and excited states is challenging. An estimate of the excitation energy between the ground and excited states based on magnetic susceptibility measurements is roughly ~28 meV. INS measurements of this compound are likewise challenging since inter- and intramolecular vibrational modes associated with the organic ligands are at frequencies similar to the magnetic excitations. However, by measuring a non-magnetic analog, i.e. ligands only, as well as the temperature dependence of the excitations in the magnetic system, we are able to determine the excitation energy as being approximately 36.5 meV at T = 10 K, with a gradual softening to 32 meV with increasing temperature. We emphasize the consideration of ligand/lattice based excitations while interpreting magnetic excitations of such nanomagnetic systems containing organic molecules while underscoring the importance of INS in determining the magnetic excitation spectrum in systems with degenerate ground states.

cond-mat.str-el↗

arXiv2005

Symmetry of Magnetic Quantum Tunneling in the Single-Molecule Magnet Mn12-Acetate

The symmetry of magnetic quantum tunneling (MQT) in the single molecule magnet Mn12-acetate has been determined by sensitive low-temperature magnetic measurements in the pure quantum tunneling regime and high frequency EPR spectroscopy in the presence of large transverse magnetic fields. The combined data set definitely establishes the transverse anisotropy terms responsible for the low temperature quantum dynamics. MQT is due to a disorder induced locally varying quadratic transverse anisotropy associated with rhombic distortions in the molecular environment (2nd order in the spin-operators). This is superimposed on a 4th order transverse magnetic anisotropy consistent with the global (average) S4 molecule site symmetry. The hard axes associated with these forms of the transverse anisotropy are not collinear, leading to a complex interplay between local and global symmetries, the consequences of which are analyzed in detail.

cond-mat.mes-hall↗

arXiv2004

Discrete easy-axis tilting in Mn12-acetate, as determined by EPR: implications for the magnetic quantum tunneling mechanism

The variation with microwave frequency and temperature of previously reported anomalous peaks in the EPR spectra of Mn12-acetate, under large transverse fields, reveals that the molecular easy magnetization axes are tilted with respect to the global symmetry direction. More importantly, on the basis of the angle-dependence of fine structures observed in the EPR spectra we infer that the tilt distribution must be discrete, as was previously suspected from studies which demonstrated the presence of a locally varying rhombic anisotropy [S. Hill et al., Phys. Rev. Lett. 90, 217204 (2003)]. The tilts are confined to two orthogonal planes, and the distribution extends up to ~1.7 degrees away from the the global easy (z-) axis. We ascribe the tilting to the hydrogen-bonding effect associated with the disordered acetic acid solvent molecules. The effect is considerably larger than deduced from x-ray diffraction analyses. These data constitute the sought-after evidence for the presence of transverse fields in Mn12-acetate, and provide a possible explanation for the lack of selection rules in the resonant quantum tunneling behavior seen in low-temperature hysteresis experiments for this S = 10 system.

cond-mat.mes-hall↗

arXiv2003

Characterization of the S = 9 excited state in Fe8Br8 by Electron Paramagnetic Resonance

High Frequency electron paramagnetic resonance has been used to observe the magnetic dipole, $Δ$ M$_s$ = $\pm$ 1, transitions in the $S = 9$ excited state of the single molecule magnet Fe$_8$Br$_8$. A Boltzmann analysis of the measured intensities locates it at 24 $\pm$ 2 K above the $S = 10$ ground state, while the line positions yield its magnetic parameters D = -0.27 K, E = $\pm$0.05 K, and B$_4^0$ = -1.3$\times$ 10$^{-6}$ K. D is thus smaller by 8% and E larger by 7% than for $S = 10$. The anisotropy barrier for $S = 9$ is estimated as 22 K,which is 25% smaller than that for $S = 10$ (29 K). These data also help assign the spin exchange constants(J's) and thus provide a basis for improved electronic structure calculations of Fe$_8$Br$_8$.

cond-mat.str-el↗

arXiv2003

Definitive spectroscopic determination of the transverse interactions responsible for the magnetic quantum tunneling in Mn12-acetate

We present detailed angle-dependent single crystal electron paramagnetic resonance (EPR) data for field rotations in the hard plane of the S=10 single molecule magnet Mn12-acetate. A clear four-fold variation in the resonance positions may be attributed to an intrinsic fourth order transverse anisotropy (O44). Meanwhile, a four-fold variation of the EPR lineshapes confirms a recently proposed model wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy (O22). These findings explain most aspects of the magnetic quantum tunneling observed in Mn12-acetate.

cond-mat.mtrl-sci↗