arXiv subjects

J. P. Carlo

Publications and source records attributed to J. P. Carlo.

At least 19 recordsLinked to original sources

arXiv2022

Spin-orbit coupling controlled ground states in the double perovskite iridates A2BIrO6 (A = Ba, Sr; B = Lu, Sc)

Iridates with the 5$d^4$ electronic configuration have attracted recent interest due to reports of magnetically-ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $J = 0$ electronic ground state for each Ir$^{5+}$ ion. The major focus of prior research has been on the double perovskite iridates Ba$_2$YIrO$_6$ and Sr$_2$YIrO$_6$, where the nature of the ground states (i.e. ordered vs non-magnetic) is still controversial. Here we present neutron powder diffraction, high energy resolution fluorescence detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates Ba$_2$LuIrO$_6$, Sr$_2$LuIrO$_6$, Ba$_2$ScIrO$_6$, and Sr$_2$ScIrO$_6$ that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $J = 0$ electronic ground states for the Ir$^{5+}$ ions in all cases, with similar values for Hund's coupling $J_{\rm H}$ and the spin-orbit coupling constant $λ_{\rm SOC}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do reveal weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large $λ_{\rm SOC}$ is the key driving force behind the electronic and magnetic ground states realized in the 5$d^4$ double perovskite iridates, which agrees well with conventional wisdom.

cond-mat.str-el↗

arXiv2015

Neutron Scattering Studies of Spin-Phonon Hybridization and Superconducting Spin-Gaps in High Temperature Superconductor $La_{2-x}(Sr,Ba)_{x}CuO_{4}$

We present time-of-fight neutron-scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2-x}Sr_{x}CuO_{4}$ (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional (3D) commensurate long range antiferromagnetic order, for x $\leq$ 0.02, to two dimensional (2D) incommensurate antiferromagnetism co-existing with superconductivity for x $\geq$ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clear resonant enhancement of the inelastic scattering coincident with the low energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore we show that the low temperature, low energy magnetic spectral weight is substantially larger for samples with non-superconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO.

cond-mat.supr-con↗

arXiv2014

(Ca,Na)(Zn,Mn)2As2: a new spin & charge doping decoupled diluted ferromagnetic semiconductor with hexagonal CaAl2Si2 structure

Here we report the successful synthesis of a spin- & charge-decoupled diluted magnetic semiconductor (Ca,Na)(Zn,Mn)2As2, crystallizing into the hexagonal CaAl2Si2 structure. The compound shows a ferromagnetic transition with a Curie temperature up to 33 K with 10% Na doping, which gives rise to carrier density of np~10^20 cm^-3. The new DMS is a soft magnetic material with HC<400 Oe. The anomalous Hall effect is observed below the ferromagnetic ordering temperature. With increasing Mn doping, ferromagnetic order is accompanied by an interaction between the local spin and mobile charge, giving rise to a minimum in resistivity at low temperatures and localizing the conduction electrons. The system provides an ideal platform for studying the interaction of the local spins and conduction electrons.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic and Superconducting Properties of Underdoped Ba(Fe1-xCox)2As2 Measured with Muon Spin Relaxation/Rotation

We report muon spin relaxation/rotation (uSR) measurements of single crystal Ba(Fe1-xCox)2As2 with x=0.038 and 0.047. Zero field (ZF)-uSR and Transverse field (TF)-iSR measurements of these underdoped samples find the presence of magnetism and superconductivity. We find internal fields along the c-axis whose magnitude decreases with increasing doping. We find evidence for a low-temperature volume fraction that is only weakly magnetic, where that volume fraction increases with increasing Co doping of the sample. TF-uSR measurements show slight changes in the spectra that indicate magnetic inhomogeneities due to the loss of Fe moments in the system, the effect of which is larger in the higher Co doping. We discuss the existence of superconductivity in these samples in close proximity to strong magnetic order.

cond-mat.supr-con↗

arXiv2014

Long-Range Magnetic Order in the 5d^2 Double Perovskite Ba2CaOsO6: Comparison with Spin-Disordered Ba2YReO6

The B-site ordered double perovskite Ba2CaOsO6 was studied by d.c. magnetic susceptibility, powder neutron diffraction and muon spin relaxation methods. The lattice parameter is a = 8.3619(6) A at 280K and cubic symmetry (Fm-3m) is retained to 3.5K with a = 8.3426(5) A. Curie-Weiss susceptibility behavior is observed for T > 100K and the derived constants are C = 0.3361(1)emu-K/mole and T_CW = -156.2(3) K, in excellent agreement with literature values. This Curie constant is much smaller than the spin-only value of 1.00 emu-K/mole for a 5d^2 Os6+ configuration, indicating a major influence of spin-orbit coupling. Previous studies had detected both susceptibility and heat capacity anomalies near 50K but no definitive conclusion was drawn concerning the nature of the ground state. While no ordered Os moment could be detected by powder neutron diffraction, muon spin relaxation (muSR) data show clear long-lived oscillations indicative of long-range magnetic order below T_C = 50K. An estimate of the ordered moment on Os6+ is ~0.2 mu_B, based upon a comparison with muSR data for Ba2YRuO6 with a known ordered moment of 2.2 mu_B. These results are compared with those for isostructural Ba2YReO6 which contains Re5+, also 5d^2, and has a nearly identical unit cell constant, a = 8.3628(2) A. In contrast, Ba2YReO6 shows T_CW = -616 K, and a complex spin-disordered and, ultimately, spin-frozen ground state below 50 K, indicating a much higher level of geometric frustration than in Ba2CaOsO6. A comparison is made to recent theory on d^2 double perovskites.

cond-mat.str-el↗

arXiv2013

Spin Gap and the Nature of the 4d3 Magnetic Ground State in the Frustrated FCC Antiferromagnet Ba2YRuO6

The geometrically frustrated double perovskite Ba2YRuO6 has magnetic 4d3 Ru5+ ions decorating an undistorted face-centered cubic (FCC) lattice. This material has been previously reported to exhibit commensurate long-range antiferromagnetic order below T_N = 36K, a factor f = 15 times lower than its Curie-Weiss temperature Theta_CW = -522 K, and purported short-range order to T* = 47K. We report new time-of-flight neutron spectroscopy of Ba2YRuO6 which shows the development of a 5 meV spin gap in the vicinity of the [100] magnetic ordering wavevector below T_N = 36K, with the transition to long-range order occurring at T* = 47K. We also report spin waves extending to 14 meV, a surprisingly small bandwidth in light of the large Theta_CW. We compare the spin gap and bandwidth to relevant neutron studies of the isostructural 4d1 material Ba2YMoO6,and discuss the results in the framework of relatively strong spin-orbit coupling expected in 4d magnetic systems.

cond-mat.str-el↗

arXiv2013

Two Dimensional Incommensurate and Three Dimensional Commensurate Magnetic Order and Fluctuations in $La_{2-x}Ba_{x}CuO_{4}$

We present neutron scattering measurements on single crystals of lightly doped $La_{2-x}Ba_{x}CuO_{4}$, with $0 \leq x \leq? 0.035$. These reveal the evolution of the magnetism in this prototypical doped Mott insulator from a three dimensional (3D) commensurate (C) antiferromagnetic ground state, which orders at a relatively high TN, to a two dimensional (2D) incommensurate (IC) ground state with finite ranged static correlations, which appear below a relatively low effective TN. At low temperatures, the 2D IC magnetism co-exists with the 3D C magnetism for doping concentrations as low as ? 0.0125. We find no signal of a 3D C magnetic ground state by x $\sim$? 0.025, consistent with the upper limit of x $\sim$? 0.02 observed in the sister family of doped Mott insulators, $La_{2-x}Sr_{x}CuO_{4}$. The 2D IC ground states observed for $0.0125 \leq x \leq 0.035$ are diagonal, and are rotated by 45 degrees within the orthorhombic basal plane compared with those previously reported for samples with superconducting ground states: $La_{2-x}Ba_{x}CuO_{4}$, with $0.05 \leq? x \leq? 0.095. We construct a phase diagram based solely on magnetic order parameter measurements, which displays much of the complexity of standard high temperature superconductivity phase diagrams discussed in the literature. Analysis of high energy-resolution inelastic neutron scattering at moderately low temperatures shows a progressive depletion of the very low energy dynamic magnetic susceptibility as x increases from 0.0125 to 0.035. This low energy, dynamic susceptibility falls off? with increasing temperature on a scale much higher than the effective 2D IC TN appropriate to these materials. Appreciable dynamic 2D IC magnetic fluctuations inhabit much of the "pseudogap" regime of the phase diagram.

cond-mat.supr-con↗

arXiv2011

Triplet and in-gap magnetic states in the ground state of the quantum frustrated FCC antiferromagnet Ba2YMoO6

The geometrically frustrated double perovskite Ba2YMoO6 is characterized by quantum s=1/2 spins at the Mo5+ sites of an undistorted face-centered cubic (FCC) lattice. Previous low-temperature characterization revealed an absence of static long-range magnetic order and suggested a non-magnetic spin-singlet ground state. We report new time-of-flight and triple-axis neutron spectroscopy of Ba2YMoO6 that shows a 28 meV spin excitation with a bandwidth of ~4 meV, which vanishes above ~125 K. We identify this as the singlet-triplet excitation out of a singlet ground state, and further identify a weaker continuum of magnetic states within the gap, reminiscent of spin-polaron states arising due to weak disorder.

cond-mat.str-el↗

arXiv2010

Static magnetic order of Sr$_{4}$A$_{2}$O$_{6}$Fe$_{2}$As$_{2}$ (A = Sc and V) revealed by local probes

Static magnetic order of quasi two-dimensional FeAs compounds Sr4A2O6-xFe2As2, with A = Sc and V, has been detected by 57Fe Moessbauer and muon spin relaxation (μSR) spectroscopies. The non-superconducting stoichiometric (x = 0) A = Sc system exhibits a static internal/hyperfine magnetic field both at the 57Fe and μ+ sites, indicating antiferromagnetic order of Fe moments below TN = 35 K with ~ 0.1 Bohr magneton per Fe at T = 2 K. The superconducting and oxygen deficient (x = 0.4) A = V system exhibits a static internal field only at the μ+ site below TN ~ 40 K, indicating static magnetic order of V moments co-existing with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments.

cond-mat.supr-con↗

arXiv2010

Superfluid Density and Field-Induced Magnetism in Ba(Fe1-xCox)2As2 and Sr(Fe1-xCox)2As2 Measured with Muon Spin Relaxation

We report muon spin rotation ($μ$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $λ$ we find that for optimally- and over-doped samples, $1/λ(T\to 0)^2$ varies monotonically with the superconducting transition temperature T$_{\rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.

cond-mat.supr-con↗

arXiv2010

Magnetic phase transition in V2O3 nanocrystals

V2O3 nanocrystals can be synthesized through hydrothermal reduction of VO(OH)2 using hydrazine as a reducing agent. Addition of different ligands to the reaction produces nanoparticles, nanorods and nanoplatelets of different sizes. Small nanoparticles synthesized in this manner show suppression of the magnetic phase transition to lower temperatures. Using muon spin relaxation spectroscopy and synchrotron x-ray diffraction, it is determined that the volume fraction of the high-temperature phase, characterized by a rhombohedral structure and paramagnetism, gradually declines with decreasing temperature, in contrast to the sharp transition observed in bulk V2O3.

cond-mat.str-el↗

arXiv2010

Muon spin rotation/relaxation measurements of the non-centrosymmetric superconductor Mg10Ir19B16

We have searched for time-reversal symmetry breaking fields in the non-centrosymmetric superconductor Mg$_{10}$Ir$_{19}$B$_{16}$ via muon spin relaxation in zero applied field, and we measured the temperature dependence of the superfluid density by muon spin rotation in transverse field to investigate the superconducting pairing symmetry in two polycrystalline samples of signficantly different purities. In the high purity sample, we detected no time-reversal symmetry breaking fields greater than 0.05 G. The superfluid density was also found to be exponentially-flat as T$\to $0, and so can be fit to a single-gap BCS model. In contrast, the lower purity sample showed an increase in the zero-field $μ$SR relaxation rate below T$_c$ corresponding to a characteristic field strength of 0.6 G. While the temperature-dependence of the superfluid density was also found to be consistent with a single-gap BCS model, the magnitude as T$\to $0 was found to be much lower for a given applied field than in the case of the high purity sample. These findings suggest that the dominant pairing symmetry in high quality Mg$_{10}$Ir$_{19}$B$_{16}$ samples corresponds to the spin-singlet channel, while sample quality drastically affects the superconducting properties of this system.

cond-mat.supr-con↗

arXiv2009

Study of the ground state properties of $LiHo_xY_{1-x}F_4$ using $μ$SR

$LiHo_xY_{1-x}F_4$ is an insulating system where the magnetic Ho$^{3+}$ ions have an Ising character, and interact mainly through magnetic dipolar fields. We used the muon spin relaxation technique to study the nature of the ground state for samples with x=0.25, 0.12, 0.08, 0.045 and 0.018. In contrast with some previous works, we have not found any signature of canonical spin glass behavior down to $\approx$15mK. Instead, below $\approx$300mK we observed dynamic magnetism characterized by a single correlation time with a temperature independent fluctuation rate. We observed that this low temperature fluctuation rate increases with x up to 0.08, above which it levels off. The 300mK energy scale corresponds to the Ho3+ hyperfine interaction strength, suggesting that the hyperfine interaction may be intimately involved with the spin dynamics in this system.

cond-mat.str-el↗

arXiv2009

Muon Spin Rotation Measurement of the Magnetic Field Penetration Depth in Ba(Fe0.93 Co0.07)2 As2 : Evidence for Multiple Superconducting Gaps

We have performed transverse field muon spin rotation measurements of single crystals of Ba(Fe$_{0.93}$Co$_{0.07})_2$As$_2$ with the applied magnetic field along the $\hat{c}$ direction. Fourier transforms of the measured spectra reveal an anisotropic lineshape characteristic of an Abrikosov vortex lattice. We have fit the $μ$SRSR spectra to a microscopic model in terms of the penetration depth $λ$ and the Ginzburg-Landau parameter $κ$. We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 200G to 1000G. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of $2Δ_1=3.7k_BT_c$ and $2Δ_2=1.6k_BT_c$, corresponding to roughly 6 meV and 3 meV respectively.

cond-mat.supr-con↗

arXiv2009

$μ$SR studies of the frustrated quasi-2d square-lattice spin system Cu(Cl,Br)La(Nb,Ta)$_{2}$O$_{7}$: evolution from spin-gap to antiferromagnetic state

We report muon spin relaxation ($μ$SR) and magnetic susceptibility measurements on Cu(Cl,Br)La(Nb,Ta)$_{2}$O$_{7}$, which demonstrate: (a) the absence of static magnetism in (CuCl)LaNb$_{2}$O$_{7}$ down to 15 mK confirming a spin-gapped ground state; (b) phase separation between partial volumes with a spin-gap and static magnetism in (CuCl)La(Nb,Ta)$_{2}$O$_{7}$; (c) history-dependent magnetization in the (Nb,Ta) and (Cl,Br) substitution systems; (d) a uniform long-range collinear antiferromagnetic state in (CuBr)LaNb$_{2}$O$_{7}$; and (e) a decrease of Néel temperature with decreasing Br concentration $x$ in Cu(Cl$_{1-x}$Br$_{x}$)LaNb$_{2}$O$_{7}$ with no change in the ordered Cu moment size for $0.33 \leq x \leq 1$. Together with several other $μ$SR studies of quantum phase transitions in geometrically-frustrated spin systems, the present results reveal that the evolution from a spin-gap to a magnetically ordered state is often associated with phase separation and/or a first order phase transition.

cond-mat.str-el↗

arXiv2008

Superconductivity coexisting with phase-separated static magnetic order in (Ba,K)Fe$_{2}$As$_{2}$, (Sr,Na)Fe$_{2}$As$_{2}$ and CaFe$_{2}$As$_{2}$

The recent discovery and subsequent developments of FeAs-based superconductors have presented novel challenges and opportunities in the quest for superconducting mechanisms in correlated-electron systems. Central issues of ongoing studies include interplay between superconductivity and magnetism as well as the nature of the pairing symmetry reflected in the superconducting energy gap. In the cuprate and RE(O,F)FeAs (RE = rare earth) systems, the superconducting phase appears without being accompanied by static magnetic order, except for narrow phase-separated regions at the border of phase boundaries. By muon spin relaxation measurements on single crystal specimens, here we show that superconductivity in the AFe$_{2}$As$_{2}$ (A = Ca,Ba,Sr) systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of (Ba$_{0.5}$K$_{0.5}$)Fe$_{2}$As$_{2}$ exhibits a nearly linear variation in T at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multi-gap effects.

cond-mat.supr-con↗

arXiv2008

Muon spin relaxation studies of magnetic order and superfluid density in antiferromagnetic NdOFeAs, BaFe2As2 and superconducting (Ba,K)Fe2As2

Zero-field (ZF) muon spin relaxation ($μ$SR) measurements have revealed static commensurate magnetic order of Fe moments in NdOFeAs below $T_{N} \sim 135$ K, with the ordered moment size nearly equal to that in LaOFeAs, and confirmed similar behavior in BaFe$_{2}$As$_{2}$. In single crystals of superconducting (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$, $μ$SR spectra indicate static magnetism with incommensurate or short-ranged spin structure in $\sim$ 70 % of volume below $T_{N} \sim$ 80 K, coexisting with remaining volume which exhibits superfluid-response consistent with nodeless gap below $T_{c}\sim 30$ K.

cond-mat.supr-con↗

arXiv2008

Absence of time reversal symmetry breaking in La_{2-x}Sr_{x}CuO_{4}

We have performed zero-field muon spin rotation measurements on single crystals of La_{2-x}Sr_{x}CuO_{4} to search for spontaneous currents in the pseudo-gap state. By comparing measurements on materials across the phase diagram, we put strict upper limits on any possible time-reversal symmetry breaking fields that could be associated with the pseudo-gap. Comparison between experimental limits and proposed circulating current states effectively eliminates the possibility that such states exist in this family of materials.

cond-mat.supr-con↗