Adiabatic physics of an exchange-coupled spin-dimer system: magnetocaloric effect, zero-point fluctuations, and possible two-dimensional universal behavior (1610.08661)
J. Brambleby, P. A. Goddard, J. Singleton, M. Jaime, T. Lancaster, L. Huang, J. Wosnitza, C. V. Topping, K. E. Carreiro, H. E. Tran, Z. E. Manson, J. L. Manson
Jan. 6, 2017 cond-mat.str-el
We present the magnetic and thermal properties of the bosonic-superfluid phase in a spin-dimer network using both quasistatic and rapidly-changing pulsed magnetic fields. The entropy derived from a heat-capacity study reveals that the pulsed-field measurements are strongly adiabatic in nature and are responsible for the onset of a significant magnetocaloric effect (MCE). In contrast to previous predictions we show that the MCE is not just confined to the critical regions, but occurs for all fields greater than zero at sufficiently low temperatures. We explain the MCE using a model of the thermal occupation of exchange-coupled dimer spin-states and highlight that failure to take this effect into account inevitably leads to incorrect interpretations of experimental results. In addition, the heat capacity in our material is suggestive of an extraordinary contribution from zero-point fluctuations and appears to indicate universal behavior with different critical exponents at the two field-induced critical points. The data are consistent with a two-dimensional nature of spin excitations in the system.
The magnetic phase diagram of underdoped YBa2Cu3Oy inferred from torque magnetization and thermal conductivity (1402.7371)
Fan Yu, Max Hirschberger, Toshinao Loew, Gang Li, Benjamin J. Lawson, Tomoya Asaba, J. B. Kemper, Tian Liang, Juan Porras, G. S. Boebinger, J. Singleton, B. Keimer, Lu Li, N. P. Ong
Sept. 19, 2016 cond-mat.supr-con
Strong evidence for charge-density correlation in the underdoped phase of the cuprate YBa2Cu3Oy was obtained by nuclear magnetic resonance (NMR) and resonant x-ray scatter- ing. The fluctuations were found to be enhanced in strong magnetic fields. Recently, 3D (three dimensional) charge-density wave (CDW) formation with long-range order (LRO) was observed by x-ray diffraction in H >15 T. To elucidate how the CDW transition impacts the pair condensate, we have used torque magnetization to 45 T and thermal conductivity $\kappa_{xx}$ to construct the magnetic phase diagram in untwinned crystals with hole density p = 0.11. We show that the 3D CDW transitions appear as sharp features in the susceptibility and $\kappa_{xx}$ at the fields HK and Hp, which define phase boundaries in agreement with spectroscopic techniques. From measurements of the melting field Hm(T) of the vortex solid, we obtain evidence for two vortex solid states below 8 K. At 0.5 K, the pair condensate appears to adjust to the 3D CDW by a sharp transition at 24 T between two vortex solids with very different shear moduli. At even higher H (42 T) the second vortex solid melts to a vortex liquid which survives to fields well above 45 T. de Haas-van Alphen oscillations appear at fields 24-28 T, below the lower bound for the upper critical field Hc2.
Electric polarization observed in single crystals of multiferroic Lu2MnCoO6 (1604.07802)
S. Chikara, J. Singleton, J. Bowlan, D. A. Yarotski, N. Lee, H. Y. Choi, Y. J. Choi, V. S. Zapf
April 26, 2016 cond-mat.mtrl-sci
We report electric polarization and magnetization measurements in single crystals of double perovskite Lu2MnCoO6 using pulsed magnetic fields and optical second harmonic generation (SHG) in DC magnetic fields. we observe well-resolved magnetic field-induced changes in the electric polarization in single crystals and thereby resolve the question about whether multiferroic behavior is intrinsic to these materials or an extrinsic feature of polycrystals. We find electric polarization along the crystalline b-axis, that is suppressed by applying a magnetic fields along c-axis and advance a model for the origin of magnetoelectric coupling. We furthermore map the phase diagram using both capacitance and electric polarization to identify regions of ordering and regions of magnetoelectric hysteresis. This compound is a rare example of coupled hysteretic behavior in the magnetic and electric properties. The ferromagnetic-like magnetic hysteresis loop that couples to hysteretic polarization can be attributed not to ordinary ferromagnetic domains, but to the rich physics of magnetic frustration of Ising-like spins in the axial next-nearest neighbor interaction model.
The monoclinic crystal structure of $\alpha$-RuCl$_3$ and the zigzag antiferromagnetic ground state (1509.02670)
R. D. Johnson, S. Williams, A. A. Haghighirad, J. Singleton, V. Zapf, P. Manuel, I. I. Mazin, Y. Li, H. O. Jeschke, R. Valenti, R. Coldea
Dec. 15, 2015 cond-mat.str-el
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.
The magnetic ground state of two isostructual polymeric quantum magnets, [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ and [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, investigated with neutron powder diffraction (1505.08092)
J. Brambleby, P. A. Goddard, R. D. Johnson, J. Liu, D. Kaminski, A. Ardavan, A. J. Steele, T. Lancaster, P. Manuel, P. J. Baker, J. Singleton, S. G. Schwalbe, P. M. Spurgeon, H. E. Tran, P. K. Peterson, J. F. Corbey, J. L. Manson
Sept. 4, 2015 cond-mat.str-el
The magnetic ground state of two isostructural coordination polymers (i) the quasi two-dimensional S = 1/2 square-lattice antiferromagnet [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$; and (ii) a new compound [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, were examined with neutron powder diffraction measurements. We find the ordered moments of the Heisenberg S = 1/2 Cu(II) ions in [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 0.6(1)$\mu_{B}$, whilst the ordered moments for the Co(II) ions in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 3.02(6)$\mu_{B}$. For Cu(II), this reduced moment indicates the presence of quantum fluctuations below the ordering temperature. We show from heat capacity and electron spin resonance measurements, that due to the crystal electric field splitting of the S = 3/2 Co(II) ions in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, this isostructual polymer also behaves as an effective spin-half magnet at low temperatures. The Co moments in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ show strong easy-axis anisotropy, neutron diffraction data which do not support the presence of quantum fluctuations in the ground state and heat capacity data which are consistent with 2D or close to 3D spatial exchange anisotropy.
Magnetism in a family of $S = 1$ square lattice antiferromagnets Ni$X_2$(pyz)$_2$ ($X = $ Cl, Br, I, NCS; pyz = pyrazine) (1410.3077)
J. Liu, P. A. Goddard, J. Singleton, J. Brambleby, F. Foronda, J. S. Möller, Y. Kohama, S. Ghannadzadeh, A. Ardavan, S. J. Blundell, T. Lancaster, F. Xiao, R. C. Williams, F. L. Pratt, P. J. Baker, K. Wierschem, S. H. Lapidus, K. H. Stone, P. W. Stephens, J. Bendix, M. R. Lees, T. J. Woods, K. E. Carreiro, H. E. Tran, C. J. Villa, J. L. Manson
June 23, 2015 cond-mat.str-el
The crystal structures of Ni$X_2$(pyz)$_2$ ($X$ = Cl (\textbf{1}), Br (\textbf{2}), I (\textbf{3}) and NCS (\textbf{4})) were determined at 298~K by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN$_4$$X_2$ units that are bridged by pyz ligands. The 2D layered motifs displayed by \textbf{1}-\textbf{4} are relevant to bifluoride-bridged [Ni(HF$_2$)(pyz)$_2$]$Z$F$_6$ ($Z$ = P, Sb) which also possess the same 2D layers. In contrast, terminal $X$ ligands occupy axial positions in \textbf{1}-\textbf{4} and cause a staggering of adjacent layers. Long-range antiferromagnetic order occurs below 1.5 (Cl), 1.9 (Br and NCS) and 2.5~K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and $g$ factor of \textbf{2}, \textbf{3} and \textbf{4} are measured by electron spin resonance where no zero--field splitting was found. The magnetism of \textbf{1}-\textbf{4} crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsed-field magnetization, magnetic susceptibility and $T_\textrm{N}$ of \textbf{2} and \textbf{4}. Magnetization curves for \textbf{2} and \textbf{4} calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. \textbf{3} is characterized as a three-dimensional antiferromagnet with the interlayer interaction ($J_\perp$) slightly stronger than the interaction within the two-dimensional [Ni(pyz)$_2$]$^{2+}$ square planes ($J_\textrm{pyz}$).
Fermi surface reconstruction and multiple quantum phase transitions in the antiferromagnet CeRhIn$_5$ (1501.01890)
L. Jiao, Y. Chen, Y. Kohama, D. Graf, E. D. Bauer, J. Singleton, J.-X. Zhu, Z. F. Weng, G. M. Pang, T. Shang, J. L. Zhang, H. O. Lee, T. Park, M. Jaime, J. D. Thompson, F. Steglich, Q. Si, H. Q. Yuan
Jan. 8, 2015 cond-mat.supr-con, cond-mat.str-el
Conventional, thermally-driven continuous phase transitions are described by universal critical behaviour that is independent of the specific microscopic details of a material. However, many current studies focus on materials that exhibit quantum-driven continuous phase transitions (quantum critical points, or QCPs) at absolute zero temperature. The classification of such QCPs and the question of whether they show universal behaviour remain open issues. Here we report measurements of heat capacity and de Haas-van Alphen (dHvA) oscillations at low temperatures across a field-induced antiferromagnetic QCP (B$_{c0}\simeq$ 50 T) in the heavy-fermion metal CeRhIn$_5$. A sharp, magnetic-field-induced change in Fermi surface is detected both in the dHvA effect and Hall resistivity at B$_0^*\simeq$ 30 T, well inside the antiferromagnetic phase. Comparisons with band-structure calculations and properties of isostructural CeCoIn$_5$ suggest that the Fermi-surface change at B$_0^*$ is associated with a localized to itinerant transition of the Ce-4f electrons in CeRhIn$_5$. Taken in conjunction with pressure data, our results demonstrate that at least two distinct classes of QCP are observable in CeRhIn$_5$, a significant step towards the derivation of a universal phase diagram for QCPs.
Controlling magnetic order and quantum disorder in one- and zero-dimensional molecule-based magnets (1311.7614)
T. Lancaster, P.A. Goddard, S.J. Blundell, F.R. Foronda, S. Ghannadzadeh, J.S. Möller, P.J. Baker, F.L. Pratt, C. Baines, L. Huang, J. Wosnitza, R.D. McDonald, K.A. Modic, J. Singleton, C.V. Topping, T.A.W. Beale, F. Xiao, J.A. Schlueter, R.D. Cabrera, K.E. Carreiro, H.E. Tran, J.L. Manson
Nov. 29, 2013 cond-mat.str-el
We investigate the structural and magnetic properties of two molecule-based magnets synthesized from the same starting components. Their different structural motifs promote contrasting exchange pathways and consequently lead to markedly different magnetic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H$_{2}$O)(gly)$_{2}$](ClO$_{4}$)$_{2}$ may be considered a quasi-one-dimensional quantum Heisenberg antiferromagnet while the related compound [Cu(pyz)(gly)](ClO$_{4}$), which is formed from dimers of antiferromagnetically interacting Cu$^{2+}$ spins, remains disordered down to at least 0.03 K in zero field, but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons.
Multiple quantum phase transitions in a heavy fermion antiferromagnet (1308.0294)
L. Jiao, H. Q. Yuan, Y. Kohama, E. D. Bauer, J.-X. Zhu, J. Singleton, T. Shang, J. L. Zhang, Y. Chen, H. O. Lee, T. Park, M. Jaime, J. D. Thompson, F. Steglich, Q. Si
Aug. 1, 2013 cond-mat.str-el
We report measurements of magnetic quantum oscillations and specific heat at low temperatures across a field-induced antiferromagnetic quantum critical point (QCP)(B_{c0}\approx50T) of the heavy-fermion metal CeRhIn_5. A sharp magnetic-field induced Fermi surface reconstruction is observed inside the antiferromagnetic phase. Our results demonstrate multiple classes of QCPs in the field-pressure phase diagram of this heavy-fermion metal, pointing to a universal description of QCPs. They also suggest that robust superconductivity is promoted by unconventional quantum criticality of a fluctuating Fermi surface.
Multiferroic behavior in the new double-perovskite Lu$_2$MnCoO$_6$ (1105.2058)
S. Yanez-Vilar, E. D. Mun, V. S. Zapf, B. G. Ueland, J. Gardner, J. D. Thompson, J. Singleton, M. Sanchez-Andujar, J. Mira, N. Biskup, M. A. Senaris-Rodriguez, C. D. Batista
May 10, 2011 cond-mat.mtrl-sci
We present a new member of the multiferroic oxides, Lu$_2$MnCoO$_6$, which we have investigated using X-ray diffraction, neutron diffraction, specific heat, magnetization, electric polarization, and dielectric constant measurements. This material possesses an electric polarization strongly coupled to a net magnetization below 35 K, despite the antiferromagnetic ordering of the $S = 3/2$ Mn$^{4+}$ and Co$^{2+}$ spins in an $\uparrow \uparrow \downarrow \downarrow$ configuration along the c-direction. We discuss the magnetic order in terms of a condensation of domain boundaries between $\uparrow \uparrow$ and $\downarrow \downarrow$ ferromagnetic domains, with each domain boundary producing a net electric polarization due to spatial inversion symmetry breaking. In an applied magnetic field the domain boundaries slide, controlling the size of the net magnetization, electric polarization, and magnetoelectric coupling.
Localized and itinerant dichotomy of electrons in Ba(Fe,Co)2As2 (1102.5476)
H. Q. Yuan, L. Jiao, F. F. Balakirev, J. Singleton, C. Setty, J. P. Hu, T. Shang, L. J. Li, G. H. Cao, Z. A. Xu, B. Shen, H. H. Wen
Feb. 27, 2011 cond-mat.supr-con
Variant approaches, either based on the Fermi surface nesting or started from the proximity to a Mott-insulator, were proposed to elucidate the physics in iron pnictides, but no consensus has been reached. A fundamental problem concerns the nature of their 3d electrons. Here we report the magnetoresistivity (\rho_xx) and the Hall resistivity (\rho_xy) of Ba(Fe1-xCox)2As2 (x=0 and 0.05) in a magnetic field of up to 55T. The magnetic transition is extremely robust against magnetic field, giving strong evidence that the magnetic ordering is formed by local moments. The magnetic state is featured with a huge magnetoresistance and a distinguished Hall resistivity, \rho_xy(H), which shows a pronounced parabolic field dependence, while the paramagnetic state shows little magnetoresistance and follows a simple linear magnetic field dependence on the Hall resistivity. Analyses of our data, based on a two-carrier model, demonstrate that the electron carriers in the magnetic state rapidly increase upon applying a magnetic field, partially compensating the loss of electron carriers at T_M. We argue that the 3d-electrons in Ba(Fe1-xCox)2As2 are divided into those who are close to forming localized moments controlling the magnetic transition and the others giving rise to complex transport properties through their interaction with the former.
Evidence of nodal gap structure in the non-centrosymmetric superconductor Y2C3 (1102.0622)
J. Chen, M. B. Salamon, S. Akutagawa, J. Akimitsu, J. Singleton, H. Q. Yuan
Feb. 3, 2011 cond-mat.supr-con
The magnetic penetration depth $\lambda (T)$ and the upper critical field $% \mu_{0}H_{c2}(T_{c})$ of the non-centrosymmetric (NCS) superconductor Y$_{2} $C$_{3}$ have been measured using a tunnel-diode (TDO) based resonant oscillation technique. We found that the penetration depth $\lambda (T)$ and its corresponding superfluid density $\rho_{s}(T)$ show linear temperature dependence at very low temperatures ($T\ll T_{c}$), indicating the existence of line nodes in the superconducting energy gap. Moreover, the upper critical field $\mu_{0}H_{c2}(T_{c})$ presents an upturn at low temperatures with a rather high value of $\mu_{0}H_{c2}(0)$ $\simeq 29$T, which slightly exceeds the weak-coupling Pauli limit. We discuss the possible origins for these nontrivial superconducting properties, and argue that the nodal gap structure in Y$_{2}$C$_{3}$ is likely attributed to the absence of inversion symmetry, which allows the admixture of spin-singlet and spin-triplet pairing states.
Large upper critical field in non-centrosymmetric superconductor Y2C3 (1101.3145)
H.Q. Yuan, J. Chen, J. Singleton, S. Akutagawa, J. Akimitsu
Jan. 17, 2011 cond-mat.supr-con
We determine the upper critical field $\mu_0 H_{c2}(T_c)$ of non-centrosymmetric superconductor $Y_2 C_3$ using two distinct methods: the bulk magnetization M(T) and the tunnel-diode oscillator (TDO) based impedance measurements. It is found that the upper critical field reaches a value of 30T at zero temperature which is above the weak-coupling Pauli paramagnetic limit. We argue that the observation of such a large $\mu_0 H_{c2}(0)$ in $Y_2 C_3$ could be attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of broken inversion symmetry.
The magnetoresistance and Hall effect in CeFeAsO: a high magnetic field study (1101.3170)
H.Q. Yuan, L. Jiao, J. Singleton, F.F. Balakirev, G.F. Chen, J.L. Luo, N.L. Wang
The longitudinal electrical resistivity and the transverse Hall resistivity of CeFeAsO are simultaneously measured up to a magnetic field of 45T using the facilities of pulsed magnetic field at Los Alamos. Distinct behaviour is observed in both the magnetoresistance Rxx({\mu}0H) and the Hall resistance Rxy({\mu}0H) while crossing the structural phase transition at Ts \approx 150K. At temperatures above Ts, little magnetoresistance is observed and the Hall resistivity follows linear field dependence. Upon cooling down the system below Ts, large magnetoresistance develops and the Hall resistivity deviates from the linear field dependence. Furthermore, we found that the transition at Ts is extremely robust against the external magnetic field. We argue that the magnetic state in CeFeAsO is unlikely a conventional type of spin-density-wave (SDW).
Weak anisotropy of the superconducting upper critical field in Fe1.11Te0.6Se0.4 single crystals (0909.5328)
M. H. Fang, J. H. Yang, F. F. Balakirev, Y. Kohama, J. Singleton, B. Qian, Z. Q. Mao, H. D. Wang, H. Q. Yuan
Sept. 29, 2009 cond-mat.supr-con, cond-mat.str-el
We have determined the resistive upper critical field Hc2 for single crystals of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to 60T. A rather high zero-temperature upper critical field of mu0Hc2(0) approx 47T is obtained, in spite of the relatively low superconducting transition temperature (Tc approx 14K). Moreover, Hc2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T=0. We suggest that the isotropic superconductivity in Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for (Ba,K)Fe2As2, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the high-Tc cuprates.
Comparison of multiwavelength observations of 9 broad-band pulsars with the spectrum of the emission from an extended current with a superluminally rotating distribution pattern (0908.1349)
H. Ardavan, A. Ardavan, J. Singleton, J. Fasel, W. Junor, J. Middleditch, M. R. Perez, A. Schmidt, P. Sengupta, P. Volegov
Aug. 10, 2009 astro-ph.GA, astro-ph.SR
The observed spectra of 9 pulsars for which multiwavelength data are available from radio to $X$- or $\gamma$-ray bands (Crab, Vela, Geminga, B0656+14, B1055-52, B1509-58, B1706-44, B1929+10, and B1951+32) are compared with the spectrum of the radiation generated by an extended polarization current whose distribution pattern rotates faster than light {\it in vacuo}. It is shown that by inferring the values of two free parameters from observational data (values that are consistent with those of plasma frequency and electron cyclotron frequency in a conventional pulsar magnetosphere), and by adjusting the spectral indices of the power laws describing the source spectrum in various frequency bands, one can account {\em quantitatively} for the entire spectrum of each pulsar in terms of a single emission process. This emission process (a generalization of the synchrotron-\'Cerenkov process to a volume-distributed source in vacuum) gives rise to an oscillatory radiation spectrum. Thus, the bell-shaped peaks of pulsar spectra in the ultraviolet or $X$-ray bands (the features that are normally interpreted as manifestations of thermal radiation) appear in the present model as higher-frequency maxima of the same oscillations that constitute the emission bands observed in the radio spectrum of the Crab pulsar. Likewise, the sudden steepening of the gradient of the spectrum by -1, which occurs around $10^{18}-10^{21}$ Hz, appears as a universal feature of the pulsar emission: a feature that reflects the transit of the position of the observer across the frequency-dependent Rayleigh distance. Inferred values of the free parameters of the present model suggest, moreover, that the lower the rotation frequency of a pulsar, the more weighted towards higher frequencies will be its observed spectral intensity.
A new collective phenomenon arising from spin anisotropic perturbations to a Heisenberg square lattice manifested in paramagnetic resonance experiments (0907.3514)
S. Cox, R.D. McDonald, J. Singleton, S. Miller, P.A. Goddard, S. El Shawish, J. Bonca, J.A. Schlueter, J.L. Manson
We report unexpected behaviour in a family of Cu spin- 1/2 systems, in which an apparent gap in the low energy magneto-optical absorption spectrum opens at low temperature. This previously unreported collective phenomenon arises at temperatures where the energy of the dominant exchange interaction exceeds the thermal energy. Simulations of the observed shifts in electron paramagnetic resonance spectral weight, which include spin anisotropy, reproduce this behavior yielding the magnitude of the spin anisotropy in these compounds.
Nearly Isotropic superconductivity in (Ba,K)Fe2As2 (0807.3137)
H. Q. Yuan, J. Singleton, F. F. Balakirev, S. A. Baily, G. F. Chen, J. L. Luo, N. L. Wang
Dec. 3, 2008 cond-mat.supr-con
Superconductivity was recently observed in the iron-arsenic-based compounds with a superconducting transition temperature (Tc) as high as 56K [1-7], naturally raising comparisons with the high Tc copper oxides. The copper oxides have layered crystal structures with quasi-two-dimensional electronic properties, which led to speculations that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity at temperatures above 40 K [8,9]. Early work on the iron-arsenic compounds seemed to support this view [7,10]. Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe2As2 in a magnetic field up to 60 T. We find that the superconducting properties are in fact quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. Such behaviour is strikingly different from all previously-known layered superconductors [9,11], and indicates that reduced dimensionality in these compounds is not a prerequisite for high-temperature superconductivity. We suggest that this situation arises because of the underlying electronic structure of the iron-arsenide compounds, which appears to be much more three dimensional than that of the copper oxides. Extrapolations of low-field single-crystal data incorrectly suggest a high anisotropy and a greatly exaggerated zero-temperature upper critical field.
Isotope effect in quasi-two-dimensional metal-organic antiferromagnets (0807.1506)
P. A. Goddard, J. Singleton, C. Maitland, S. J. Blundell, T. Lancaster, P. J. Baker, R. D. McDonald, S. Cox, P. Sengupta, J. L. Manson, K. A. Funk, J. A. Schlueter
July 11, 2008 cond-mat.str-el
Although the isotope effect in superconducting materials is well-documented, changes in the magnetic properties of antiferromagnets due to isotopic substitution are seldom discussed and remain poorly understood. This is perhaps surprising given the possible link between the quasi-two-dimensional (Q2D) antiferromagnetic and superconducting phases of the layered cuprates. Here we report the experimental observation of shifts in the N\'{e}el temperature and critical magnetic fields ($\Delta T_{\rm N}/T_{\rm N}\approx 4%$; $\Delta B_{\rm c}/B_{\rm c}\approx 4%$) in a Q2D organic molecular antiferromagnets on substitution of hydrogen for deuterium. These compounds are characterized by strong hydrogen bonds through which the dominant superexchange is mediated. We evaluate how the in-plane and inter-plane exchange energies evolve as the hydrogens on different ligands are substituted, and suggest a possible mechanism for this effect in terms of the relative exchange efficiency of hydrogen and deuterium bonds.
Superconducting properties of Fe-based layered superconductor LaO$_{0.9}$F$_{0.1-\delta}$FeAs (0803.0128)
G. F. Chen, Z. Li, G. Li, J. Zhou, D. Wu, J. Dong, W. Z. Hu, P. Zheng, Z. J. Chen, H. Q. Yuan, J. Singleton, J. L. Luo, N. L. Wang
June 28, 2008 cond-mat.supr-con
We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of about 54 T. A clear signature of superconducting gap opening below T$_c$ was observed in the far-infrared reflectance spectra, with 2$\Delta/\textit{k}T_c\approx$3.5-4.2. Furthermore, we show that the new superconductor has electron-type conducting carrier with a rather low carrier density.
Mechanism of generation of the emission bands in the dynamic spectrum of the Crab pulsar (0804.3123)
H. Ardavan, A. Ardavan, J. Singleton, M. Perez
We show that the proportionately spaced emission bands in the dynamic spectrum of the Crab pulsar (Hankins T. H. & Eilek J. A., 2007, ApJ, 670, 693) fit the oscillations of the square of a Bessel function whose argument exceeds its order. This function has already been encountered in the analysis of the emission from a polarization current with a superluminal distribution pattern: a current whose distribution pattern rotates (with an angular frequency $\omega$) and oscillates (with a frequency $\Omega>\omega$ differing from an integral multiple of $\omega$) at the same time (Ardavan H., Ardavan A. & Singleton J., 2003, J Opt Soc Am A, 20, 2137). Using the results of our earlier analysis, we find that the dependence on frequency of the spacing and width of the observed emission bands can be quantitatively accounted for by an appropriate choice of the value of the single free parameter $\Omega/\omega$. In addition, the value of this parameter, thus implied by Hankins & Eilek's data, places the last peak in the amplitude of the oscillating Bessel function in question at a frequency ($\sim\Omega^3/\omega^2$) that agrees with the position of the observed ultraviolet peak in the spectrum of the Crab pulsar. We also show how the suppression of the emission bands by the interference of the contributions from differring polarizations can account for the differences in the time and frequency signatures of the interpulse and the main pulse in the Crab pulsar. Finally, we put the emission bands in the context of the observed continuum spectrum of the Crab pulsar by fitting this broadband spectrum (over 16 orders of magnitude of frequency) with that generated by an electric current with a superluminally rotating distribution pattern.
Exchange parameters of copper-based quasi-two-dimensional Heisenberg magnets measured using high magnetic fields and muon-spin rotation (0803.4169)
P.A. Goddard, J. Singleton, P. Sengupta, R.D. McDonald, T. Lancaster, S.J. Blundell, F.L. Pratt, S. Cox, N. Harrison, J.L. Manson, H.I. Southerland, J.A. Schlueter
March 28, 2008 cond-mat.supr-con, cond-mat.str-el
Pulsed-field magnetization experiments (fields $B$ of up to 85 T and temperatures $T$ down to 0.4 K) are reported on nine organic Cu-based two-dimensional (2D) Heisenberg magnets. All compounds show a low-$T$ magnetization that is concave as a function of $B$, with a sharp ``elbow'' transition to a constant value at a field $B_{\rm c}$. Monte-Carlo simulations including a finite interlayer exchange energy $J_{\perp}$ quantitatively reproduce the data; the concavity indicates the effective dimensionality and $B_{\rm c}$ is an accurate measure of the in-plane exchange energy $J$. Using these values and Ne\'el temperatures measured by muon-spin rotation, it is also possible to obtain a quantitative estimate of $|J_{\perp}/J|$. In the light of these results, it is suggested that in magnets of the form [Cu(HF$_2$)(pyz)$_2$]X, where X is an anion, the sizes of $J$ and $J_{\perp}$ are controlled by the tilting of the pyrazine (pyz) molecule with respect to the 2D planes.
Quantum Oscillations in the Underdoped Cuprate YBa2Cu4O8 (0707.0057)
E. A. Yelland, J. Singleton, C. H. Mielke, N. Harrison, F. F. Balakirev, B. Dabrowski, J. R. Cooper
Feb. 4, 2008 cond-mat.supr-con, cond-mat.str-el
We report the observation of quantum oscillations in the underdoped cuprate superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed magnetic fields up to 85T. There is a clear signal, periodic in inverse field, with frequency 660+/-15T and possible evidence for the presence of two components of slightly different frequency. The quasiparticle mass is m*=3.0+/-0.3m_e. In conjunction with the results of Doiron-Leyraud et al. for YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a general feature of underdoped copper oxide planes and provide information about the doping dependence of the Fermi surface.
Angle-dependent magnetoresistance oscillations due to magnetic breakdown orbits (cond-mat/0702570)
A. F. Bangura, P. A. Goddard, J. Singleton, S. W. Tozer, A. I. Coldea, A. Ardavan, R. D. McDonald, S. J. Blundell, J. A. Schlueter
Dec. 17, 2007 cond-mat.supr-con, cond-mat.str-el
We present experimental evidence for a hitherto unconfirmed type of angle-dependent magnetoresistance oscillation caused by magnetic breakdown. The effect was observed in the organic superconductor kappa-(BEDT-TTF)$_2$Cu(NCS)$_2$ using hydrostatic pressures of up to 9.8 kbar and magnetic fields of up to 33 T. In addition, we show that similar oscillations are revealed in ambient pressure measurements, provided that the Shubnikov-de Haas oscillations are suppressed either by elevated temperatures or filtering of the data. These results provide a compelling validation of Pippard's semiclassical picture of magnetic breakdown.
Very weak electron-phonon coupling and strong strain coupling in manganites (0704.2598)
S. Cox, J.C. Loudon, A.J. Williams, J.P. Attfield, J. Singleton, P.A. Midgley, N.D. Mathur
Oct. 19, 2007 cond-mat.str-el
The coupling of the manganite stripe phase to the lattice and to strain has been investigated via transmission electron microscopy studies of polycrystalline and thin film manganites. In polycrystalline \PCMOfiftwo a lockin to $q/a^*=0.5$ in a sample with $x>0.5$ has been observed for the first time. Such a lockin has been predicted as a key part of the Landau CDW theory of the stripe phase. Thus it is possible to constrain the size of the electron-phonon coupling in the CDW Landau theory to between 0.04% and 0.05% of the electron-electron coupling term. In the thin film samples, films of the same thickness grown on two different substrates exhibited different wavevectors. The different strains present in the films on the two substrates can be related to the wavevector observed via Landau theory. It is demonstrated that the the elastic term which favours an incommensurate modulation has a similar size to the coupling between the strain and the wavevector, meaning that the coupling of strain to the superlattice is unexpectedly strong.