arXiv subjects

C. Berthier

Publications and source records attributed to C. Berthier.

At least 37 records · Page 2Linked to original sources

arXiv2012

Quantum-critical spin dynamics in quasi-one-dimensional antiferromagnets

By means of nuclear spin-lattice relaxation rate 1/T1, we follow the spin dynamics as a function of the applied magnetic field in two gapped one-dimensional quantum antiferromagnets: the anisotropic spin-chain system NiCl2-4SC(NH2)2 and the spin-ladder system (C5H12N)2CuBr4. In both systems, spin excitations are confirmed to evolve from magnons in the gapped state to spinons in the gapples Tomonaga-Luttinger-liquid state. In between, 1/T1 exhibits a pronounced, continuous variation, which is shown to scale in accordance with quantum criticality. We extract the critical exponent for 1/T1, compare it to the theory, and show that this behavior is identical in both studied systems, thus demonstrating the universality of quantum critical behavior.

cond-mat.str-el↗

arXiv2012

Spin-chain system as a tunable simulator of frustrated planar magnetism

At low temperatures, weakly coupled spin chains develop a magnetic order that reflects the character of gapless spin fluctuations along the chains. Using nuclear magnetic resonance, we identify and characterize two ordered states in the gapless region of the antiferromagnetic, Ising-like spin-chain system BaCo2V2O8, both arising from the incommensurate fluctuations along the chains. They correspond to the columnar and ferromagnetic ordered states of the frustrated J1-J2 spin model on a square lattice, where the spins are encoded in original spin chains. As a result of field-dependent incommensurate fluctuations and frustrated interchain interaction, J1 can be tuned continuously with the magnetic field, and its value with respect to a fixed J2 selects the ordered state. Spin-chain systems can thus be used as tunable simulators of frustrated planar magnetism.

cond-mat.str-el↗

arXiv2011

Magnetic-field-induced charge-stripe order in the high temperature superconductor YBa2Cu3Oy

Electronic charges introduced in copper-oxide planes generate high-transition temperature superconductivity but, under special circumstances, they can also order into filaments called stripes. Whether an underlying tendency of charges to order is present in all cuprates and whether this has any relationship with superconductivity are, however, two highly controversial issues. In order to uncover underlying electronic orders, magnetic fields strong enough to destabilise superconductivity can be used. Such experiments, including quantum oscillations in YBa2Cu3Oy (a notoriously clean cuprate where charge order is not observed) have suggested that superconductivity competes with spin, rather than charge, order. Here, using nuclear magnetic resonance, we demonstrate that high magnetic fields actually induce charge order, without spin order, in the CuO2 planes of YBa2Cu3Oy. The observed static, unidirectional, modulation of the charge density breaks translational symmetry, thus explaining quantum oscillation results, and we argue that it is most likely the same 4a-periodic modulation as in stripe-ordered cuprates. The discovery that it develops only when superconductivity fades away and near the same 1/8th hole doping as in La2-xBaxCuO4 suggests that charge order, although visibly pinned by CuO chains in YBa2Cu3Oy, is an intrinsic propensity of the superconducting planes of high Tc cuprates.

cond-mat.supr-con↗

arXiv2011

Magnetic structure of azurite above the 1/3 magnetization plateau

The transition from the 1/3 magnetization plateau towards the saturation magnetization in azurite has been studied by low-temperature, high-magnetic-field, high-frequency proton nuclear magnetic resonance (NMR). The observed symmetrical splitting of the NMR spectra is incompatible with the longitudinal incommensurate order appearing when the longitudinal correlation function becomes dominant over the transverse one, which is the expected framework for the existence of the 2/3 magnetization plateau. The spectra are rather interpreted in terms of a more standard transverse antiferromagnetic (canted) order.

cond-mat.str-el↗

arXiv2010

Anisotropy of Magnetic Interactions in the Spin-Ladder Compound (C$_5$H$_{12}$N)$_2$CuBr$_4$

Magnetic excitations in the spin-ladder material (C$_5$H$_{12}$N)$_2$CuBr$_4$ [BPCB] are probed by high-resolution multi-frequency electron spin resonance (ESR) spectroscopy. Our experiments provide a direct evidence for a biaxial anisotropy ($\sim 5\%$ of the dominant exchange interaction), that is in contrast to a fully isotropic spin-ladder model employed for this system previously. It is argued that this anisotropy in BPCB is caused by spin-orbit coupling, which appears to be important for describing magnetic properties of this compound. The zero-field zone-center gap in the excitation spectrum of BPCB, $Δ_0/k_{B}=16.5$ K, is detected directly. Furthermore, an ESR signature of the inter-ladder exchange interactions is obtained. The detailed characterization of the anisotropy in BPCB completes the determination of the full spin hamiltonian of this exceptional spin-ladder material and shows ways to study anisotropy effects in spin ladders.

cond-mat.str-el↗

arXiv2009

Field Evolution of Coexisting Superconducting and Magnetic Orders in CeCoIn$_5$

We present nuclear magnetic resonance (NMR) measurements on the three distinct In sites of CeCoIn$_5$ with magnetic field applied in the [100] direction. We identify the microscopic nature of the long range magnetic order (LRO) stabilized at low temperatures in fields above 10.2 T while still in the superconducting (SC) state. We infer that the ordered moment is oriented along the $\hat c$-axis and map its field evolution. The study of the field dependence of the NMR shift for the different In sites indicates that the LRO likely coexists with a modulated SC phase, possibly that predicted by Fulde, Ferrell, Larkin, and Ovchinnikov. Furthermore, we discern a field region dominated by strong spin fluctuations where static LRO is absent and propose a revised phase diagram.

cond-mat.str-el↗

arXiv2009

Homogeneous vs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co and K doped iron pnictides

In Ba(Fe0.95Co0.05)2As2 all of the 75As NMR intensity at the paramagnetic resonance position vanishes abruptly below Tonset(SDW)=56 K, indicating that magnetic (spin density wave) order is present in all of the sample volume, despite bulk superconductivity below Tc=15 K. The two phases thus coexist homogeneously at the microscopic scale. In Ba0.6K0.4Fe2As2, on the other hand, the signal loss below Tonset(SDW)~75 K is not complete, revealing that magnetic order is bound to finite-size areas of the sample, while the remaining NMR signal shows a clear superconducting response below Tc=37 K. Thus, the two phases are not homogeneously mixed, at least for this potassium concentration. For both samples, spatial electronic and/or magnetic inhomogeneity is shown to characterize the NMR properties in the normal state.

cond-mat.supr-con↗

arXiv2009

Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR

High magnetic field $^{63,65}$Cu NMR spectra were used to determine the local spin polarization in the 1/3 magnetization plateau of azurite, Cu$_3$(CO$_3$)$_2$(OH)$_2$, which is a model system for the distorted diamond antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the Cu2 (dimer) sites is found to be field independent, negative and strongly anisotropic, corresponding to $\approx$10 % of fully polarized spin in a $d$-orbital. This is close to the expected configuration of the "quantum" plateau, where a singlet state is stabilized on the dimer. However, the observed non-zero spin polarization points to some triplet admixture, induced by strong asymmetry of the diamond bonds $J_1$ and $J_3$.

cond-mat.str-el↗

arXiv2008

Controlling Luttinger liquid physics in spin ladders under a magnetic field

We present a 14N nuclear magnetic resonance study of a single crystal of CuBr4(C5H12N)2 (BPCB) consisting of weakly coupled spin-1/2 Heisenberg antiferromagnetic ladders. Treating ladders in the gapless phase as Luttinger liquids, we are able to fully account for (i) the magnetic field dependence of the nuclear spin-lattice relaxation rate 1/T_1 at 250 mK and for (ii) the phase transition to a 3D ordered phase occuring below 110 mK due to weak interladder exchange coupling. BPCB is thus an excellent model system where the possibility to control Luttinger liquid parameters in a continuous manner is demonstrated and Luttinger liquid model tested in detail over the whole fermion band.

cond-mat.str-el↗

arXiv2008

NMR evidence for the persistence of spin-superlattice above the 1/8 magnetization plateau in SrCu2(BO3)2

We present 11B NMR studies of the 2D frustrated dimer spin system SrCu2(BO3)2 in the field range 27-31 T covering the upper phase boundary of the 1/8 magnetization plateau, identified at 28.4 T. Our data provide a clear evidence that above 28.4 T the spin-superlattice of the 1/8 plateau is modified but does not melt even though the magnetization increases. Although this is precisely what is expected for a supersolid phase, the microscopic nature of this new phase is much more complex. We discuss the field-temperature phase diagram on the basis of our NMR data.

cond-mat.str-el↗

arXiv2008

Comment on ``Texture in the Superconducting Order Parameter of CeCoIn$_5$ Revealed by Nuclear Magnetic Resonance''

The study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been of considerable recent interest. Below the temperature $T^*$ which is believed to be the transition temperature ($T$) to the FFLO phase in CeCoIn$_5$, K. Kakuyanagi et al. (Phys. Rev. Lett. 94, 047602 (2005)) reported a composite NMR spectrum with a tiny component observed at frequencies corresponding to the normal state signal. The results were interpreted as evidence for the emergence of an FFLO state. This result is inconsistent with two other NMR studies of V. F. Mitrovi{ć} et al. (Phys. Rev. Lett. 97, 117002 (2006)) and B.-L. Young et al. (Phys. Rev. Lett. 98, 036402 (2007)). In this comment we show that the findings of K. Kakuyanagi et al. do not reflect the true nature of the FFLO state but result from excess RF excitation power used in that experiment.

cond-mat.str-el↗

arXiv2008

Field Dependence of the Ground State in the Exotic Superconductor CeCoIn$_5$: a Nuclear Magnetic Resonance Investigation

We report $^{115}$In nuclear magnetic resonance (NMR) measurements in CeCoIn$_5$ at low temperature ($T \approx 70$ mK) as a function of magnetic field ($H_0$) from 2 T to 13.5 T applied perpendicular to the $\hat c$-axis. NMR line shift reveals that below 10 T the spin susceptibility increases as $\sqrt{H_0}$. We associate this with an increase of the density of states due to the Zeeman and Doppler-shifted quasiparticles extended outside the vortex cores in a d-wave superconductor. Above 10 T a new superconducting state is stabilized, possibly the modulated phase predicted by Fulde, Ferrell, Larkin and Ovchinnikov (FFLO). This phase is clearly identified by a strong and linear increase of the NMR shift with the field, before a jump at the first order transition to the normal state.

cond-mat.str-el↗

arXiv2008

Similar glassy features in the NMR response of pure and disordered La1.88Sr0.12CuO4

High Tc superconductivity in La2-xSrxCuO4 coexists with (striped and glassy) magnetic order. Here, we report NMR measurements of the 139La spin-lattice relaxation, which displays a stretched-exponential time dependence, in both pure and disordered x=0.12 single crystals. An analysis in terms of a distribution of relaxation rates T1^-1 indicates that i) the spin-freezing temperature is spatially inhomogeneous with an onset at Tg(onset)=20 K for the pristine samples, and ii) the width of the T1^-1 distribution in the vicinity of Tg(onset) is insensitive to an ~1% level of atomic disorder in CuO2 planes. This suggests that the stretched-exponential 139La relaxation, considered as a manifestation of the systems glassiness, may not arise from quenched disorder.

cond-mat.supr-con↗

arXiv2008

Electronic texture of the thermoelectric oxide Na0.75CoO2

From 59Co and 23Na NMR, we demonstrate the impact of the Na+ vacancy ordering on the cobalt electronic states in Na0.75CoO2: at long time scales, there is neither a disproportionation into 75 % Co3+ and 25 % Co4+ states, nor a mixed-valence metal with a uniform Co3.25+ state. Instead, the system adopts an intermediate configuration in which 30 % of the lattice sites form an ordered pattern of localized Co3+ states. Above 180 K, an anomalous mobility of specific Na+ sites is found to coexist with this electronic texture, suggesting that the formation of the latter may contribute to stabilizing the Na+ ordering. Control of the ion doping in these materials thus appears to be crucial for fine-tuning of their thermoelectric properties.

cond-mat.str-el↗

arXiv2007

Field dependence of the quantum ground state in the Shastry-Sutherland system SrCu$_2$(BO$_3$)$_2$

We present magnetic torque measurements on the Shastry-Sutherland quantum spin system SrCu$_2$(BO$_3$)$_2$ in fields up to 31 T and temperatures down to 50 mK. A new quantum phase is observed in a 1 T field range above the 1/8 plateau, in agreement with recent NMR results. Since the presence of the DM coupling precludes the existence of a true Bose-Einstein condensation and the formation of a supersolid phase in SrCu$_2$(BO$_3$)$_2$, the exact nature of the new phase in the vicinity of the plateau remains to be explained. Comparison between magnetization and torque data reveals a huge contribution of the Dzyaloshinskii-Moriya interaction to the torque response. Finally, our measurements demonstrate the existence of a supercooling due to adiabatic magnetocaloric effects in pulsed field experiments.

cond-mat.str-el↗

arXiv2007

NMR Study of Large Skyrmions in Al_{0.13}Ga_{0.87}As Quantum Wells

A nuclear magnetic resonance (NMR) study is reported of multiple (30) Al$_{0.13}$Ga$_{0.87}$As quantum well (QW) sample near the Landau level filling factor $ν=1$. In these Al$_{0.13}$Ga$_{0.87}$As QWs the effective $g$ factor is nearly zero. This can lead to two effects: vanishing electronic polarization $(P)$ and skyrmionic excitations composed of a huge number of spins. As small $P$ values cause an overlap of the NMR signals from the QW and barriers, a special technique was employed to allow these two signals to be distinguished. The QW signal corresponds to a small, negative, and very broad distribution of spin polarization that exhibits thermally induced depolarization. Such a distribution can be attributed to sample inhomogeneities and/or to large skyrmions, the latter possibility being favored by observation of a very fast $T_{2}^{-1}$ rate.

cond-mat.mes-hall↗

arXiv2007

$^{77}$Se NMR evidence for the Jaccarino-Peter mechanism in the field induced superconductor, $λ$(BETS)$_2$FeCl$_4$}

We have performed $^{77}$Se NMR on a single crystal sample of the field induced superconductor $λ$-(BETS)$_{2}$FeCl$_{4}$. Our results obtained in the paramagnetic state provide a microscopic insight on the exchange interaction $J$ between the spins \textbf{s} of the BETS $π$ conduction electrons and the Fe localized $d$ spins \textbf{S}. The absolute value of the Knight shift \textbf{K} decreases when the polarization of the Fe spins increases. This reflects the ``negative'' spin polarization of the $π$ electrons through the exchange interaction $J$. The value of $J$ has been estimated from the temperature and the magnetic field dependence of \textbf{K} and found in good agreement with that deduced from transport measurements (L. Balicas \textit{et al}. Phys. Rev. Lett. \textbf{87}, 067002 (2001)). This provides a direct microscopic evidence that the field induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter (Phys. Rev. Lett. \textbf{9}, 290 (1962)). Furthermore, an anomalous broadening of the NMR line has been observed at low temperature, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.

cond-mat.str-el↗

arXiv2007

Phase Diagram of CeCoIn_5 in the Vicinity of H_{c2} as Determined by NMR

We report ^{115}In nuclear magnetic resonance (NMR) measurements in the heavy-fermion superconductor CeCoIn_5 as a function of temperature in different magnetic fields applied parallel to the $(\hat a, \hat b)$ plane. The measurements probe a part of the phase diagram in the vicinity of the superconducting critical field H_{c2} where a possible inhomogeneous superconducting state, Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), is stabilized. We have identified clear NMR signatures of two phase transitions occurring in this part of the phase diagram. The first order phase transitions are characterized by the sizable discontinuity of the shift. We find that a continuous second order phase transition from the superconducting to the FFLO state occurs at temperature below which the shift becomes temperature independent. We have compiled the first phase diagram of CeCoIn_5 in the vicinity of H_{c2} from NMR data and found that it is in agreement with the one determined by thermodynamic measurements.

cond-mat.str-el↗