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Broadening the knowledge and understanding on the magnetic correlations in van der Waals layered magnets is critical in realizing their potential next-generation applications. In this study, we employ high frequency (240 GHz) electron spin resonance (ESR) spectroscopy on plate-like CrBr$_{3}$ to gain insight into the magnetic interactions as a function of temperature (200 - 4 K) and the angle of rotation ${\theta}$. We find that the temperature dependence of the ESR linewidth is well described by the Ginzberg-Landau critical model as well as Berezinskii-Kosterlitz-Thouless (BKT) transition model, indicative of the presence of two-dimensional (2D) correlations. This suggests that the three-dimensional ferromagnet CrBr$_{3}$, which has been described as an Ising or Heisenberg ferromagnet, could present 2D magnetic correlations and BKT-like behavior even in its bulk form; an observation that, to the best of our knowledge, has not been reported in the literature. Furthermore, our findings show that the resonance field follows a $(3cos^2{\theta} - 1)$-like angular dependence, while the linewidth follows a $(3cos^2{\theta} - 1)^2$-like angular dependence. This observed angular dependence of the resonance field and linewidth further confirm an unanticipated 2D magnetic behavior in CrBr$_{3}$. This behavior is likely due to the interaction of the external magnetic field applied during the ESR experiment that allows for the mediation of long-range vortex-like correlations between the spin clusters that may have formed due to magnetic phase separation. This study demonstrates the significance of employing spin sensitive techniques such as ESR to better understand the magnetic correlations in similar van der Waals magnets., Comment: 6 pages, 5 figures, 1 table

We investigate the low-temperature magnetic properties of the molecule-based chiral spin chain [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O (pym = pyrimidine). Electron-spin resonance, magnetometry and heat capacity measurements reveal the presence of staggered $g$ tensors, a rich low-temperature excitation spectrum, a staggered susceptibility and a spin gap that opens on the application of a magnetic field. These phenomena are reminiscent of those previously observed in non-chiral staggered chains, which are explicable within the sine-Gordon quantum-field theory. In the present case, however, although the sine-Gordon model accounts well for the form of the temperature-dependence of the heat capacity, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon theory as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure of [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O, particularly a uniform Dzyaloshinskii-Moriya coupling and a four-fold periodic staggered field., Comment: 6 pages, 3 figures and supplemental information

The magneto-electronic field effects in organic semiconductors at high magnetic fields are described by field-dependent mixing between singlet and triplet states of weakly bound charge carrier pairs due to small differences in their Land\'e g-factors that arise from the weak spin-orbit coupling in the material. In this work, we corroborate theoretical models for the high-field magnetoresistance of organic semiconductors, in particular of diodes made of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) at low temperatures, by conducting magnetoresistance measurements along with multi-frequency continuous-wave electrically detected magnetic resonance experiments. The measurements were performed on identical devices under similar conditions in order to independently assess the magnetic field-dependent spin-mixing mechanism, the so-called {\Delta}g mechanism, which originates from differences in the charge-carrier g-factors induced by spin-orbit coupling., Comment: 12 pages, 3 figures

We measure electrically detected magnetic resonance (EDMR) on organic light-emitting diodes (OLEDs) made of the polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at room temperature and high magnetic fields, where spectral broadening of the resonance due to spin-orbit coupling (SOC) exceeds that due to the local hyperfine fields. Density-functional-theory calculations on an open-shell model of the material reveal g-tensors of charge-carrier spins in the lowest unoccupied (electron) and highest occupied (hole) molecular orbitals. These tensors are used for simulations of magnetic resonance line-shapes. Besides providing the first quantification and direct observation of SOC effects on charge-carrier states in these weakly SO-coupled hydrocarbons, this procedure demonstrates that spin-related phenomena in these materials are fundamentally monomolecular in nature., Comment: 6 pages, 4 figures

Employing complementary torque magnetometry and electron spin resonance on single crystals of herbertsmithite, the closest realization to date of a quantum kagome antiferromagnet featuring a spin-liquid ground state, we provide novel insight into different contributions to its magnetism. At low temperatures, two distinct types of defects with different magnetic couplings to the kagome spins are found. Surprisingly, their magnetic response contradicts the three-fold symmetry of the ideal kagome lattice, suggesting the presence of a global structural distortion that may be related to the establishment of the spin-liquid ground state., Comment: Published version with minor text corrections

We report $^{35}$Cl NMR, ESR, $\mu$SR and specific heat measurements on the $S=1/2$ frustrated kagom\'e magnet kapellasite, $\alpha-$Cu$_3$Zn(OH)$_6$Cl$_2$, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction $J_1$ and give an energy scale for the competing interactions $|J| \sim 10$ K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term $D$, with $|D/J|\sim 3$%. These findings validate a posteriori the use of the $J_1 - J_2 - J_d$ Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagom\'e lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by $^{35}$Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and $\mu$SR in the low-$T$, correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low-energy modes., Comment: 15 pages, 11 figures. To appear in Phys. Rev. B

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.

We report an electron spin resonance investigation of the geometrically frustrated spin-1/2 kagome antiferromagnet vesignieite, BaCu$_3$V$_2$O$_8$(OH)$_2$. Analysis of the line widths and line shifts indicates the dominance of in-plane Dzyaloshinsky-Moriya anisotropy that is proposed to suppress strongly quantum spin fluctuations and thus to promote long-range ordering rather than a spin-liquid state. We also evidence an enhanced spin-phonon contribution that might originate from a lattice instability and discuss the origin of a low-temperature mismatch between intrinsic and bulk susceptibility in terms of local inhomogeneity.

Ground state and low-energy excitations of the quasi-one-dimensional antiferromagnet CuSe$_2$O$_5$ were experimentally studied using bulk magnetization, neutron diffraction, muon spin relaxation and antiferromagnetic resonance measurements. Finite interchain interactions promote long-range antiferromagnetic order below $T_N=17$ K. The derived spin canted structure is characterized by the magnetic propagation vector $\mathbf{k}=(1, 0, 0)$ and the reduced magnetic moment $\bm{m}=[0.13(7), 0.50(1), 0.00(8)]\mu_B$. The values of the magnetic anisotropies determined from the field and angular dependencies of the antiferromagnetic resonance comply well with a previous electron paramagnetic resonance study and correctly account for the observed magnetic ground state and spin-flop transition., Comment: 13 pages, 12 figures

Electron paramagnetic resonance (EPR) spectroscopy interrogates unpaired electron spins in solids and liquids to reveal local structure and dynamics; for example, EPR has elucidated parts of the structure of protein complexes that have resisted all other techniques in structural biology. EPR can also probe the interplay of light and electricity in organic solar cells and light-emitting diodes, and the origin of decoherence in condensed matter, which is of fundamental importance to the development of quantum information processors. Like nuclear magnetic resonance (NMR), EPR spectroscopy becomes more powerful at high magnetic fields and frequencies, and with excitation by coherent pulses rather than continuous waves. However, the difficulty of generating sequences of powerful pulses at frequencies above 100 GHz has, until now, confined high-power pulsed EPR to magnetic fields of 3.5 T and below. Here we demonstrate that ~1 kW pulses from a free-electron laser (FEL) can power a pulsed EPR spectrometer at 240 GHz (8.5 T), providing transformative enhancements over the alternative, a state-of-the-art ~30 mW solid state source. Using the UC Santa Barbara FEL as a source, our 240 GHz spectrometer can rotate spin-1/2 electrons through pi/2 in only 6 ns (vs. 300 ns with the solid state source). Fourier transform EPR on nitrogen impurities in diamond demonstrates excitation and detection of EPR lines separated by ~200 MHz. Decoherence times for spin-1/2 systems as short as 63 ns are measured, enabling measurement of the decoherence time in a frozen solution of nitroxide free-radicals at temperatures as high as 190 K. Both FELs and the quasi-optical technology developed for the spectrometer are scalable to frequencies well in excess of 1 THz, opening the possibility of high-power pulsed EPR spectroscopy up to the highest static magnetic fields on earth., Comment: Submitted version including 12 pages, 4 figures and online supporting materials

Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While the low temperature ferromagnet-like feature is observed in both types of ribbons, such room temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a 'positive exchange bias'. Electron spin resonance measurements infer that the carbon related defects may responsible for the observed magnetic behaviour in both types of ribbons. Furthermore, proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance provides no indications for the presence of potassium (cluster) related signals, emphasizing the intrinsic magnetic nature of the ribbons. Our combined experimental results may infer the coexistence of ferromagnetic clusters with anti-ferromagnetic regions leading to disordered magnetic phase. We discuss the origin of the observed contrast in the magnetic behaviours of these two types of ribbons.

We present an electron spin resonance (ESR) study of single-crystalline spin chain-system CuSe$_2$O$_5$ in the frequency range between 9 GHz and 450 GHz. In a wide temperature range above the N\'{e}el temperature $T_N=17$ K we observe strong and anisotropic frequency dependence of a resonance linewidth. Although sizeable interchain interaction $J_{IC}\approx 0.1 J$ ($J$ is the intrachain interaction) is present in this system, the ESR results agree well with the Oshikawa-Affleck theory for one-dimensional $S=1/2$ Heisenberg antiferromagnet. This theory is used to extract the anisotropies present in CuSe$_2$O$_5$. We find that the symmetric anisotropic exchange $J_c=(0.04 \pm 0.01) \:J$ and the antisymmetric Dzyaloshinskii-Moriya (DM) interaction $D=(0.05\pm 0.01)\:J$ are very similar in size in this system. Staggered-field susceptibility induced by the presence of the DM interaction is witnessed in the macroscopic susceptibility anisotropy., Comment: 8 pages, 7 figures, 2 tables, published in Phys. Rev. B

The ability to probe the spin properties of solid state systems electrically underlies a wide variety of emerging technology. Here, we extend electrical readout of the nuclear spin states of phosphorus donors in silicon to the coherent regime with modified Hahn echo sequences. We find that, whilst the nuclear spins have electrically detected phase coherence times exceeding 2 ms, they are nonetheless limited by the artificially shortened lifetime of the probing donor electron., Comment: 10 pages, 3 figures

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 N\'{e}el 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 $\Delta H_{pp}\propto T^{-p}$ 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., Comment: one-columb 10 pages, 3 figures

We present electron spin resonance (ESR) investigation of the acentric \FeLang, featuring a unique single-domain double-chiral magnetic ground state. Combining simulations of the ESR line-width anisotropy and the antiferromagnetic-resonance modes allows us to single out the Dzyaloshinsky-Moriya (DM) interaction as the leading magnetic anisotropy term. We demonstrate that the rather minute out-of-plane DM component $d_c=45$ mK is responsible for selecting a unique ground state, which endures thermal fluctuations up to astonishingly high temperatures., Comment: 4+ pages, 4 figures, 3+ pages of supplementary information with 2 additional figures, accepted for publication in PRL

Decoherence in Nature has become one of the most pressing problems in physics. Many applications, including quantum information processing, depend on understanding it; and fundamental theories going beyond quantum mechanics have been suggested [1-3], where the breakdown of quantum theory appears as an 'intrinsic decoherence', mimicking environmental decoherence [4]. Such theories cannot be tested until we have a handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate predictions for environmental decoherence in molecular-based quantum magnets [5]. Experimental understanding of decoherence in molecular magnets has been limited by short decoherence times, which make coherent spin manipulation extremely difficult [6-9]. Here we reduce the decoherence by applying a strong magnetic field. The theory predicts the contributions to the decoherence from phonons, nuclear spins, and intermolecular dipolar interactions, for a single crystal of the Fe8 molecular magnet. In experiments we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally - there are no other visible decoherence sources. Our investigation suggests that the decoherence time is ultimately limited by nuclear spins, and can be extended up to about 500 microseconds, by optimizing the temperature, magnetic field, and nuclear isotopic concentrations., Comment: Submitted version including 11 pages, 3 figures and online supporting materials. Appeared on Nature Advance Online Publication (AOP) on July 20th, 2011. (http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10314.html)

Electron and nuclear spins are very promising candidates to serve as quantum bits (qubits) for proposed quantum computers, as the spin degrees of freedom are relatively isolated from their surroundings, and can be coherently manipulated e.g. through pulsed EPR and NMR. For solid state spin systems, impurities in crystals based on carbon and silicon in various forms have been suggested as qubits, and very long relaxation rates have been observed in such systems. We have investigated a variety of these systems at high magnetic fields in our multi-frequency pulsed EPR/ENDOR spectrometer. A high magnetic field leads to large electron spin polarizations at helium temperatures giving rise to various phenomena that are of interest with respect to quantum computing. For example, it allows the initialization of the both the electron spin as well as hyperfine-coupled nuclear spins in a well defined state by combining millimeter and RF radiation; it can increase the T2 relaxation times by eliminating decoherence due to dipolar interaction; and it can lead to new mechanisms for the coherent electrical readout of electron spins. We will show some examples of these and other effects in Si:P, SiC:N, and nitrogen-related centers in diamond., Comment: 13 pages, 5 figures. Confererence Proceedings of the 6th Asia-Pacific ESR/EPR Society Conference in Cairns, Australia (2008)

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., Comment: Published version

The electron spin g- and hyperfine tensors of the endohedral metallofullerene Sc@C82 are anisotropic. Using electron spin resonance (ESR) and density functional theory (DFT), we can relate their principal axes to the coordinate frame of the molecule, finding that the g-tensor is not axially symmetric. The Sc bond with the cage is partly covalent and partly ionic. Most of the electron spin density is distributed around the carbon cage, but 5% is associated with the scandium d_yz orbital, and this drives the observed anisotropy.

Pulsed electrically-detected magnetic resonance of phosphorous (31P) in bulk crystalline silicon at very high magnetic fields (B > 8.5 T) and low temperatures (T = 2.8 K) is presented. We find that the spin-dependent capture and reemission of highly polarized (>95%) conduction electrons by equally highly polarized 31P donor electrons introduces less decoherence than other mechanisms for spin-to-charge conversion. This allows the electrical detection of spin coherence times in excess of 100 microseconds: 50 times longer than the previous maximum for electrically-detected spin readout experiments., Comment: 13 pages, 3 figures. References updated, figure removed to reduce length of paper

We experimentally demonstrate a method for obtaining nuclear spin hyperpolarization, that is, polarization significantly in excess of that expected for a thermal equilibrium. By exploiting a modified Overhauser process, we obtain more than 68% nuclear anti-polarization of phosphorus donors in silicon. This polarization is reached with a time constant of ~150 seconds, at a temperature of 1.37 K and a magnetic field of 8.5 T. The ability to obtain such large polarizations is discussed with regards to its significance for quantum information processing and magnetic resonance imaging., Comment: 4 pages, 3 figures

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.

Dynamic nuclear polarization transfers spin polarization from electrons to nuclei. We have achieved this by a new method, simultaneously exciting transitions of electronic and nuclear spins. The efficiency of this technique improves with increasing magnetic field. Experimental results are shown for N@C60 with continuous-wave microwaves, which can be expected to produce even higher polarization than the corresponding pulsed techniques for electron spins greater than 1/2. The degree of nuclear polarization in this case can be easily monitored through the intensities of the well resolved hyperfine components in the EPR spectrum. The nuclear spin-lattice relaxation time is orders of magnitude longer than that of the electrons., Comment: 7 pages, 3 figures

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the \kagome spin-1/2 compound {\herbert}. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component $|D_z|=15(1)$ K to be perpendicular to the \kagome planes. Through the temperature dependent ESR line-width we observe a building up of nearest-neighbor spin-spin correlations below $\sim$150 K., Comment: 4 pages, 3 figures, minor modifications

An experimental study on the nature of spin-dependent excess charge carrier transitions at the interface between (111) oriented phosphorous doped ([P] ~ 10^15 cm^3) crystalline silicon and silicon dioxide at high magnetic field (B_0 ~ 8.5 T) is presented. Electrically detected magnetic resonance (EDMR) spectra of the hyperfine split 31P donor electron transitions and paramagnetic interface defects were conducted at temperatures in the range 3 K < T < 12 K. The results at these previously unattained (for EDMR) magnetic field strengths reveal the dominance of spin-dependent processes that differ from the previously well investigated recombination between the 31P donor and the P_b state, which dominates at low magnetic fields. While magnetic resonant current responses due to 31P and P_b states are still present, they do not correlate and only the P_b contribution can be associated with an interface process due to spin-dependent tunneling between energetically and physically adjacent P_b states. This work provides an experimental demonstration of spin-dependent tunneling between physically adjacent and identical electronic states as proposed by Kane for readout of donor qubits., Comment: 10 pages, 4 figures

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 $\sim 10$ $\mu$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., Comment: 4 pages

We report all-optical switching due to state-filling in quantum dots (QDs) within a Mach-Zehnder Interferometric (MZI) switch. The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530-1570 nm probe beam is switched by optical excitation of one MZI-arm from the top. By exciting below the InGaAsP bandgap, we prove that the refractive index nonlinearity is only due to the QDs. The switching efficiency is 2 rad/(microW absorbed power). Probe wavelength insensitivity was obtained using a broad distribution of QDs., Comment: 12 pages

The thermodynamic and magnetic properties of MnCl3(C12H8N2), have been studied down to 2 K. The material is an S = 2 antiferromagnetic linear chain that undergoes long-range ordering to a canted antiferromagnetic state at 23 K. The comparison of magnetic data from polycrystalline samples with the results of specific heat measurements using a pressed pellet of powder reveals the importance of the length of the chains in the sample in relation to the magnetic correlation length.