Showing 1–50 of 76 results for author: Reyes, A P

$^{51}$V NMR evidence for interlayer-modulated charge order and a first-order low-temperature transition in CsV$_3$Sb$_5$

Authors: Xiaoling Wang, Arneil P. Reyes, Hrishit Banerjee, Andrea N. Capa Salinas, Stephen Wilson, Brenden R. Ortiz

Abstract: Charge order in the kagome superconductor CsV$_3$Sb$_5$ exhibits a complex three-dimensional organization and intermediate-temperature anomalies whose bulk character has remained unsettled. We use orientation-dependent $^{51}$V NMR as a site-selective probe to determine the stacking of the charge density wave (CDW) state and its thermal evolution. Below $T_{\mathrm{CDW}}\!\approx\!94$~K, the field… ▽ More Charge order in the kagome superconductor CsV$_3$Sb$_5$ exhibits a complex three-dimensional organization and intermediate-temperature anomalies whose bulk character has remained unsettled. We use orientation-dependent $^{51}$V NMR as a site-selective probe to determine the stacking of the charge density wave (CDW) state and its thermal evolution. Below $T_{\mathrm{CDW}}\!\approx\!94$~K, the field-linear splitting of the $^{51}$V central transition together with the anisotropy of the Knight-shift tensor identify an interlayer-modulated $3\mathbf{q}$ CDW whose local environments are consistent with a four-layer $2\times2\times4$ stacking with mixed trihexagonal/Star-of-David distortions, in agreement with synchrotron x-ray determinations. For comparison, RbV$_3$Sb$_5$ serves as a reference exhibiting a uniform trihexagonal $2\times2\times2$ stacking, allowing us to isolate features unique to the $2\times2\times4$ state in CsV$_3$Sb$_5$. With $\mathbf{H}_0\!\parallel\!c$, the $^{51}$V quadrupolar satellites through the intermediate temperature scale near $T_{\mathrm{CO}}\!\approx\!65$ K reorganize into two well-resolved electric-field-gradient manifolds that coexist over a finite interval; their relative spectral weights interchange on cooling while the total integrated satellite intensity remains conserved and $ν_Q$ within each manifold is nearly temperature independent. The coexistence without critical broadening, together with conserved intensity, provides bulk evidence consistent with a first-order charge-order transition near $T_{\mathrm{CO}}$. Our measurements do not resolve whether this lower-temperature transition corresponds to a distinct in-plane order or a reorganization of the $3\mathbf{q}$ state; rather, they delimit this window and provide bulk, site-resolved constraints that connect prior reported anomalies to a thermodynamic first-order transition. △ Less

Submitted 4 September, 2025; originally announced September 2025.

Comments: 7 pages, 4 figures

From Bulk to Nanowire: A Dimensional Journey with CeIn$_3$

Authors: M. H. Carvalho, D. Zau, A. P. Reyes, R. Cong, S. D. House, H. P. Pizzi, A. M. Caffer, D. S. Passos, R. C. Santos, G. S. Freitas, K. R. Pirota, R. R. Urbano, P. J. G. Pagliuso

Abstract: In this work, we have explored the Metallic-Flux Nanonucleation method to synthesize single crystals and nanowires (diameter $\approx$ 170 nm) of CeIn$_{3}$ and compare their properties. The effects of reduced dimensionality were systematically investigated using Energy Dispersive Spectroscopy (EDS), Selected area electron diffraction (SAED), magnetic susceptibility, heat capacity, and Nuclear Mag… ▽ More In this work, we have explored the Metallic-Flux Nanonucleation method to synthesize single crystals and nanowires (diameter $\approx$ 170 nm) of CeIn$_{3}$ and compare their properties. The effects of reduced dimensionality were systematically investigated using Energy Dispersive Spectroscopy (EDS), Selected area electron diffraction (SAED), magnetic susceptibility, heat capacity, and Nuclear Magnetic Resonance (NMR). Semi-quantitative EDS analysis revealed a Ce:In ratio of 1:3.1(1), and the SAED results confirmed that the nanowires are polycrystalline with a cubic unit cell. Magnetic susceptibility, specific heat, and NMR data indicated a suppression of the antiferromagnetic transition to $T_N$ $\approx$ 2.4 K compared to the bulk value ($\approx$ 10 K). Furthermore, NMR analysis at temperatures below 2.8 K showed a reduced quadrupole frequency, $ν_Q$ $\approx$ 1.77(2) MHz, and provided evidence of polycrystalline nanowires formed within the nanoporous alumina template, in agreement with SAED results. We attribute these findings to an increasing magnetic order frustration induced by dimensionality in CeIn$_{3}$ nanowires. △ Less

Submitted 4 August, 2025; v1 submitted 6 July, 2025; originally announced July 2025.

Nuclear spin coherence in superconducting Nb$_3$Sn

Authors: Gan Zhai, William P. Halperin, Arneil P. Reyes, Sam Posen, Chiara Tarantini, Manish Mandal, David C. Larbalestier

Abstract: We have investigated the normal and superconducting states of the technologically important compound Nb$_3$Sn using $^{93}$Nb nuclear magnetic resonance. From spin-lattice relaxation we find strong suppression of the zero-temperature superconducting order parameter by magnetic field. Additionally we have identified an anomalously large electron-nuclear exchange interaction from spin-spin relaxatio… ▽ More We have investigated the normal and superconducting states of the technologically important compound Nb$_3$Sn using $^{93}$Nb nuclear magnetic resonance. From spin-lattice relaxation we find strong suppression of the zero-temperature superconducting order parameter by magnetic field. Additionally we have identified an anomalously large electron-nuclear exchange interaction from spin-spin relaxation measurements, an order of magnitude beyond that of the nuclear dipolar coupling. This RKKY interaction evolves from normal to superconducting states, becoming essentially Lorentzian in the low temperature limit. △ Less

Submitted 17 June, 2025; v1 submitted 1 February, 2025; originally announced February 2025.

Hidden magnetic phases in i-MAX compounds

Authors: Dror Yahav, Ariel Maniv, Daniel Potashnikov, Asaf Pesach, El'ad N. Caspi, Arneil P. Reyes, Quanzheng Tao, Johanna Rosen, Eran Maniv

Abstract: We uncover a high-field magnetic phase in i-MAX compounds exhibiting a canted antiferromagnetic (AFM) order with unprecedented properties, revealed through NMR and AC susceptibility. Intriguingly, as the atomic number of Rare Earth increases, the transition field of this canted AFM phase grows at the expense of the lower-field AFM state. Our findings point to the complexity of the magnetic structu… ▽ More We uncover a high-field magnetic phase in i-MAX compounds exhibiting a canted antiferromagnetic (AFM) order with unprecedented properties, revealed through NMR and AC susceptibility. Intriguingly, as the atomic number of Rare Earth increases, the transition field of this canted AFM phase grows at the expense of the lower-field AFM state. Our findings point to the complexity of the magnetic structure in i-MAX compounds, demonstrating a non-trivial evolution of their phase diagram while increasing both the atomic number of the Rare Earth element and the external field. △ Less

Submitted 3 December, 2024; originally announced December 2024.

Comments: 8 pages, 5 figures

Anisotropy stabilized magnetic phases of the triangular antiferromagnet RbFe(MoO$_4$)$_2$

Authors: Yu. A. Sakhratov, L. E. Svistov, A. P. Reyes

Abstract: The magnetic H - T phase diagram of a quasi-two-dimensional easy plane antiferromagnet RbFe(MoO$_4$)$_2$ (S = 5/2) with an equilateral triangular lattice structure is studied with $^{87}$Rb NMR technique for field directed along hard axis C3. The studies confirm the two step transition from the low field umbrella-like incommensurate magnetic phase to the paramagnetic state observed recently (Mitam… ▽ More The magnetic H - T phase diagram of a quasi-two-dimensional easy plane antiferromagnet RbFe(MoO$_4$)$_2$ (S = 5/2) with an equilateral triangular lattice structure is studied with $^{87}$Rb NMR technique for field directed along hard axis C3. The studies confirm the two step transition from the low field umbrella-like incommensurate magnetic phase to the paramagnetic state observed recently (Mitamura et al. 2016). The transitions were identified as a lambda anomaly in the spin-lattice relaxation rate and a step increase of magnetic susceptibility at intermediate transition. The $^{87}$Rb NMR study precludes the possibility of either V or fan spin textures in the X phase. The additional transition is presumably associated with loss of inter plane magnetic order before the transition to paramagnetic state of individual triangular planes. △ Less

Submitted 13 December, 2023; originally announced December 2023.

Comments: 7 pages, 7 figures. arXiv admin note: substantial text overlap with arXiv:2112.06182

Journal ref: JETP,137, 526-532, (2023)

Nuclear Magnetic Resonance Investigation of Superconducting and Normal State Nb$_3$Sn

Authors: Gan Zhai, William P. Halperin, Arneil P. Reyes, Sam Posen, Zuhawn Sung, Chiara Tarantini, Michael D. Brown, David C. Larbalestier

Abstract: The superconductor Nb$_3$Sn has a high critical temperature and high critical field, widely used for high-field superconducting magnets. In this work we investigate its microscopic electronic structure with $^{93}$Nb nuclear magnetic resonance (NMR). The high-quality Nb$_3$Sn powder sample was studied in both 3.2T and 7T magnetic fields in the temperature range from 1.5K to 300K. From measurement… ▽ More The superconductor Nb$_3$Sn has a high critical temperature and high critical field, widely used for high-field superconducting magnets. In this work we investigate its microscopic electronic structure with $^{93}$Nb nuclear magnetic resonance (NMR). The high-quality Nb$_3$Sn powder sample was studied in both 3.2T and 7T magnetic fields in the temperature range from 1.5K to 300K. From measurement of the spectrum and its theoretical analysis, we find evidence for anisotropy despite its cubic crystal structure. This anisotropy is manifest in alignment of powder grains under certain temperature and field cycling conditions. The Knight shift and spin-lattice relaxation rate, $T_1^{-1}$, were measured in the normal state. Additionally, $T_1^{-1}$ was measured in the superconducting state and compared with BCS theory revealing a weak field dependence, with an energy gap $Δ(0)=2.0\pm0.08k_B T_c$ at 3.2T and $Δ(0)=1.73\pm0.08k_B T_c$ at 7T, indicating suppression of the order parameter by magnetic field. △ Less

Submitted 1 November, 2023; originally announced November 2023.

Effects of charge doping on Mott insulator with strong spin-orbit coupling, Ba$_2$NaOsO$_6$

Authors: E. Garcia, R. Cong, P. C. Forino, A. Tassetti, G. Allodi, A. P. Reyes, P. M. Tran, P. M. Woodward, C. Franchini, S. Sanna, V. F. Mitrović

Abstract: The effects of doping on the electronic evolution of the Mott insulating state have been extensively studied in efforts to understand mechanisms of emergent quantum phases of materials. The study of these effects becomes ever more intriguing in the presence of entanglement between spin and orbital degrees of freedom. Here, we present a comprehensive investigation of charge doping in the double per… ▽ More The effects of doping on the electronic evolution of the Mott insulating state have been extensively studied in efforts to understand mechanisms of emergent quantum phases of materials. The study of these effects becomes ever more intriguing in the presence of entanglement between spin and orbital degrees of freedom. Here, we present a comprehensive investigation of charge doping in the double perovskite Ba$_2$NaOsO$_6$, a a complex Mott insulator where such entanglement plays an important role. We establish that the insulating magnetic ground state evolves from canted antiferromagnet (cAF)to Néel order for dopant levels exceeding ~ 10 %. Furthermore, we determine that a broken local point symmetry (BLPS) phase, precursor to the magnetically ordered state, occupies an extended portion of the (H-T) phase diagram with increased doping. This finding reveals that the breaking of the local cubic symmetry is driven by a multipolar order, most-likely of the antiferro-quadrupolar type. △ Less

Submitted 23 November, 2022; v1 submitted 10 October, 2022; originally announced October 2022.

Competition between spin ordering and superconductivity near the pseudogap boundary in La2-xSrxCuO4: insights from NMR

Authors: I. Vinograd, R. Zhou, H. Mayaffre, S. Krämer, S. K. Ramakrishna, A. P. Reyes, T. Kurosawa, N. Momono, M. Oda, S. Komiya, S. Ono, M. Horio, J. Chang, M. -H. Julien

Abstract: When superconductivity is suppressed by high magnetic fields in La2-xSrxCuO4, striped antiferromagnetic (AFM) order becomes the magnetic ground state of the entire pseudogap regime, up to its end at the doping p* [M. Frachet, I. Vinograd et al., Nat. Phys. 16, 1064 (2020)]. Glass-like freezing of this state is detected in 139La NMR measurements of the spin-lattice relaxation rate 1/T1. Here, we pr… ▽ More When superconductivity is suppressed by high magnetic fields in La2-xSrxCuO4, striped antiferromagnetic (AFM) order becomes the magnetic ground state of the entire pseudogap regime, up to its end at the doping p* [M. Frachet, I. Vinograd et al., Nat. Phys. 16, 1064 (2020)]. Glass-like freezing of this state is detected in 139La NMR measurements of the spin-lattice relaxation rate 1/T1. Here, we present a quantitative analysis of 1/T1 data in the hole-doping range p=x=0.12-0.171, based on the Bloembergen-Purcell-Pound (BPP) theory, modified to include statistical distribution of parameters arising from strong spatial inhomogeneity. We observe spin fluctuations to slow down at temperatures T near the onset of static charge order and, overall, the effect of the field B may be seen as equivalent to strengthening stripe order by approaching p=0.12 doping. In details however, our analysis reveals significant departure from usual field-induced magnetic transitions. The continuous growth of the amplitude of the fluctuating moment with increasing B suggests a nearly-critical state in the B->0 limit, with very weak quasi-static moments possibly confined in small areas like vortex cores. Further, the nucleation of spin order in the vortex cores is shown to account quantitatively for both the value and the p dependence of a field scale characterizing bulk spin freezing. The correlation time of the fluctuating moment appears to depend exponentially on B/T (over the investigated range). This explains the timescale dependence of various experimental manifestations, including why, for transport measurements, the AFM moments may be considered static over a considerable range of B and T. These results make the high-field magnetic ground state up to p* an integral part of the discussion on putative quantum criticality. △ Less

Submitted 10 July, 2022; originally announced July 2022.

Journal ref: Phys. Rev. B 106, 054522 (2022)

Martensitic transformation in V_3Si single crystal: ^51V NMR evidence for coexistence of cubic and tetragonal phases

Authors: A. A. Gapud, S. K. Ramakrishnan, E. L. Green, A. P. Reyes

Abstract: The Martensitic transformation (MT) in A15 binary-alloy superconductor V_3Si, though studied extensively, has not yet been conclusively linked with a transition to superconductivity. Previous NMR studies have mainly been on powder samples and with little emphasis on temperature dependence during the transformation. Here we study a high-quality single crystal, where quadrupolar splitting of NMR spe… ▽ More The Martensitic transformation (MT) in A15 binary-alloy superconductor V_3Si, though studied extensively, has not yet been conclusively linked with a transition to superconductivity. Previous NMR studies have mainly been on powder samples and with little emphasis on temperature dependence during the transformation. Here we study a high-quality single crystal, where quadrupolar splitting of NMR spectra for ^51V allowed us to distinguish between spectra from transverse chains of V as a function of temperature. Our data revealed that (1) the MT is not abrupt, but rather there is a microscopic coexistence of pre-transformed cubic phase and transformed tetragonal phase over a few K below and above Tm, while (2) no pre-transformed phase can be found at Tc, and (3) the Martensitic lengthening of one axis occurs predominantly in a plane perpendicular to the crystal growth axis, as twinned domains. △ Less

Submitted 7 June, 2022; v1 submitted 7 April, 2022; originally announced April 2022.

Comments: Revised manuscript submitted 3 June 2022 to Physica C

High-field magnetic structure of the triangular antiferromagnet RbFe(MoO4)2

Authors: Yu. A. Sakhratov, O. Prokhnenko, A. Ya. Shapiro, H. D. Zhou, L. E. Svistov, A. P. Reyes, O. A. Petrenko

Abstract: The magnetic H - T phase diagram of a quasi-two-dimensional antiferromagnet RbFe(MoO4)2 with an equilateral triangular lattice structure is studied with 87Rb NMR and neutron diffraction techniques. This combination of experimental techniques allows us to determine the ordered components of the magnetic moments on the Fe3+ ions within various high-field phases - the Y, UUD, V, and fan structures, s… ▽ More The magnetic H - T phase diagram of a quasi-two-dimensional antiferromagnet RbFe(MoO4)2 with an equilateral triangular lattice structure is studied with 87Rb NMR and neutron diffraction techniques. This combination of experimental techniques allows us to determine the ordered components of the magnetic moments on the Fe3+ ions within various high-field phases - the Y, UUD, V, and fan structures, stabilized in the compound by the in-plane magnetic field. It is also established that the transition from the V to the fan phase is of first-order, whereas the transition from the fan phase to the polarized paramagnetic phase is continuous. An analysis of the NMR spectra shows that the high-field fan phase of RbFe(MoO4)2 can be successfully described by a periodic commensurate oscillation of the magnetic moments around the field direction in each Fe layer combined with an incommensurate modulation of the magnetic structure perpendicular to the layers. △ Less

Submitted 12 December, 2021; originally announced December 2021.

Comments: 13 pages, 15 figures

Antiferromagnetic Switching Driven by the Collective Dynamics of a Coexisting Spin Glass

Authors: Eran Maniv, Nityan Nair, Shannon C. Haley, Spencer Doyle, Caolan John, Stefano Cabrini, Ariel Maniv, Sanath K. Ramakrishna, Yun-Long Tang, Peter Ercius, Ramamoorthy Ramesh, Yaroslav Tserkovnyak, Arneil P. Reyes, James G. Analytis

Abstract: The theory behind the electrical switching of antiferromagnets is premised on the existence of a well defined broken symmetry state that can be rotated to encode information. A spin glass is in many ways the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bere… ▽ More The theory behind the electrical switching of antiferromagnets is premised on the existence of a well defined broken symmetry state that can be rotated to encode information. A spin glass is in many ways the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bereft of information. In this study, we show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the antiferromagnet Fe$_{1/3+δ}$NbS$_2$, which is rooted in the electrically-stimulated collective winding of the spin glass. The local texture of the spin glass opens an anisotropic channel of interaction that can be used to rotate the equilibrium orientation of the antiferromagnetic state. The use of a spin glass' collective dynamics to electrically manipulate antiferromagnetic spin textures has never been applied before, opening the field of antiferromagnetic spintronics to many more material platforms with complex magnetic textures. △ Less

Submitted 6 August, 2020; originally announced August 2020.

Comments: 7 pages, 4 Figures, supplement available on reasonable request

Journal ref: Science Advances 08 Jan 2021: Vol. 7, no. 2, eabd8452

A Charge Density Wave-like Transition in High Temperature Quenched $Bi_2Se_3$

Authors: Yanan Li, Christian Parsons, Sanath Kumar Ramakrishna, Anand Prashant Dwivedi, Marvin A. Schofield, Arneil P. Reyes, Prasenjit Guptasarma

Abstract: Hexagonally deformed Fermi surfaces and strong nesting, found in topological insulators (TIs) such as $Bi_2Se_3$ and $Bi_2Te_3$, have led to several predictions of the existence of Density Wave order in these systems. Recent evidence for strong Fermi surface nesting in superconducting $Cu-Bi_2Se_3$ and $Nb-Bi_2Se_3$ has further led to speculation about the importance of charge order in the context… ▽ More Hexagonally deformed Fermi surfaces and strong nesting, found in topological insulators (TIs) such as $Bi_2Se_3$ and $Bi_2Te_3$, have led to several predictions of the existence of Density Wave order in these systems. Recent evidence for strong Fermi surface nesting in superconducting $Cu-Bi_2Se_3$ and $Nb-Bi_2Se_3$ has further led to speculation about the importance of charge order in the context of unconventional superconductivity. Here, we report the first observation of a novel anomaly in $Bi_2Se_3$ at 140K, which may be associated with a Charge Density Wave (CDW)-like transition. This transition was identified from both structural and electronic measurements, where: a) a periodic lattice distortion at above room temperature was characterized as a diffuse charge order in $Bi_2Se_3$ between \vec{k} and \vec{k}\pm\mathrmΔ\vec{k} from electron diffraction; and b) an opening of energy gap signatured with metal-to-insulator like transition at 140K was identified from resistivity vs temperature measurement. This is further corroborated by nuclear magnetic resonance (NMR) studies of the spin-lattice relaxation ( $\frac{1}{T_1}$) rate of the $^{209}Bi$ nucleus, which also displays a transition at 140K associated with an opening of an energy gap of ~8meV. Additionally, we also observe another anomaly in $\frac{1}{T_1}$ near 200K, which appears to display anisotropy with the direction of the applied magnetic field. △ Less

Submitted 6 February, 2023; v1 submitted 28 February, 2020; originally announced February 2020.

Comments: 16 pages, 7 figures

Two Coupled Chains are Simpler than One: Field-induced Chirality in a Frustrated Quantum Spin Ladder

Authors: M. Pikulski, T. Shiroka, F. Casola, A. P. Reyes, P. L. Kuhns, S. Wang, H. -R. Ott, J. Mesot

Abstract: Although the frustrated spin chain (zigzag chain) is a Drosophila of frustrated magnetism, the understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is incomplete. We address this problem through nuclear magnetic resonance (NMR) experiments on $\text{BiCu}_2\text{PO}_6$ in magnetic fields up to 45 T, revealing a field-induced spiral magnetic structure. Conjo… ▽ More Although the frustrated spin chain (zigzag chain) is a Drosophila of frustrated magnetism, the understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is incomplete. We address this problem through nuclear magnetic resonance (NMR) experiments on $\text{BiCu}_2\text{PO}_6$ in magnetic fields up to 45 T, revealing a field-induced spiral magnetic structure. Conjointly, we present advanced numerical calculations showing that even moderate rung coupling dramatically simplifies the phase diagram below half-saturation magnetization by stabilizing a field-induced chiral phase. Surprisingly for a one-dimensional model, this phase and its response to Dzyaloshinskii-Moriya (DM) interactions adhere to classical expectations. While explaining the behavior at the highest accessible magnetic fields, our results imply a different origin for the solitonic phases occurring at lower fields in $\text{BiCu}_2\text{PO}_6$. An exciting possibility is that the known, DM-mediated coupling between chirality and crystal lattice gives rise to a new kind of spin-Peierls instability. △ Less

Submitted 12 November, 2019; originally announced November 2019.

Comments: Revised manuscript, 7 pages, 6 figures

Journal ref: Sci Rep 10, 15862 (2020)

Orbitally defined field-induced electronic state in a Kondo lattice

Authors: G. G. Lesseux, H. Sakai, T. Hattori, Y. Tokunaga, S. Kambe, P. L. Kuhns, A. P. Reyes, J. D. Thompson, P. G. Pagliuso, R. R. Urbano

Abstract: CeRhIn$_{5}$ is a Kondo-lattice prototype in which a magnetic field B$\bf{^{\ast}\simeq}$ 30 T induces an abrupt Fermi-surface (FS) reconstruction and pronounced in-plane electrical transport anisotropy all within its antiferromagnetic state. Though the antiferromagnetic order at zero field is well-understood, the origin of an emergent state at B$^{\ast}$ remains unknown due to challenges inherent… ▽ More CeRhIn$_{5}$ is a Kondo-lattice prototype in which a magnetic field B$\bf{^{\ast}\simeq}$ 30 T induces an abrupt Fermi-surface (FS) reconstruction and pronounced in-plane electrical transport anisotropy all within its antiferromagnetic state. Though the antiferromagnetic order at zero field is well-understood, the origin of an emergent state at B$^{\ast}$ remains unknown due to challenges inherent to probing states microscopically at high fields. Here, we report low-temperature Nuclear Magnetic Resonance (NMR) measurements revealing a discontinuous decrease in the $^{115}$In formal Knight shift, without changes in crystal or magnetic structures, of CeRhIn$_{5}$ at fields spanning B$^{\ast}$. We show that the emergent state above B$^{\ast}$ results from a change in Ce's 4f orbitals that arises from field-induced evolution of crystal-electric field (CEF) energy levels. This change in orbital character enhances hybridisation between the 4f and the conduction electrons (c.e.) that leads ultimately to an itinerant quantum-critical point at B$\bf{_{c0} \simeq}$ 50 T. △ Less

Submitted 8 June, 2019; v1 submitted 7 May, 2019; originally announced May 2019.

Journal ref: Phys. Rev. B 101, 165111 (2020)

Tunable Giant Exchange Bias in an Intercalated Transition Metal Dichalcogenide

Authors: Spencer Doyle, Caolan John, Eran Maniv, Ryan A. Murphy, Ariel Maniv, Sanath K. Ramakrishna, Yun-Long Tang, Ramamoorthy Ramesh, Jeffrey R. Long, Arneil P. Reyes, James G. Analytis

Abstract: The interplay of symmetry and quenched disorder leads to some of the most fundamentally interesting and technologically important properties of correlated materials. It also poses the most vexing of theoretical challenges. Nowhere is this more apparent than in the study of spin glasses. A spin glass is characterized by an ergodic landscape of states - an innumerable number of possibilities that ar… ▽ More The interplay of symmetry and quenched disorder leads to some of the most fundamentally interesting and technologically important properties of correlated materials. It also poses the most vexing of theoretical challenges. Nowhere is this more apparent than in the study of spin glasses. A spin glass is characterized by an ergodic landscape of states - an innumerable number of possibilities that are only weakly distinguished energetically, if at all. We show in the material Fe$_x$NbS$_2$, this landscape of states can be biased by coexisitng antiferromagnetic order. This process leads to a phenomenon of broad technological importance: giant, tunable exchange bias. We observe exchange biases that exceed those of conventional materials by more than two orders of magnitude. This work illustrates a novel route to giant exchange bias by leveraging the interplay of frustration and disorder in exotic materials. △ Less

Submitted 11 April, 2019; originally announced April 2019.

Comments: 18 pages, 4 figures

Journal ref: Nature Physics, volume 17, pages 525-530 (2021)

Search for a nematic phase in quasi-2D antiferromagnet CuCrO2 by NMR in electric field

Authors: Yu. A. Sakhratov, J. J. Kweon, E. S. Choi, H. D. Zhou, L. E. Svistov, A. P. Reyes

Abstract: The magnetic phase diagram of CuCrO2 was studied with a novel method of simultaneous Cu NMR and electric polarization techniques with the primary goal of demonstrating that regardless of cooling history of the sample the magnetic phase with specific helmet-shaped NMR spectra associated with interplanar disorder possesses electric polarization. Our result unequivocally confirms the assumption of Sa… ▽ More The magnetic phase diagram of CuCrO2 was studied with a novel method of simultaneous Cu NMR and electric polarization techniques with the primary goal of demonstrating that regardless of cooling history of the sample the magnetic phase with specific helmet-shaped NMR spectra associated with interplanar disorder possesses electric polarization. Our result unequivocally confirms the assumption of Sakhratov et al. Phys. Rev. B \bf{94}, 094410 (2016) that the high-field low-temperature phase is in fact a 3D-polar phase characterised by a 3D magnetic order with tensor order parameter. In comparison with the results obtained in pulsed fields, a modified phase diagram is introduced defining the upper boundary of the first-order transition from the 3D-spiral to the 3D-polar phase. △ Less

Submitted 9 January, 2018; v1 submitted 1 January, 2018; originally announced January 2018.

Comments: 4pages,5 figures

Journal ref: Phys. Rev. B 97, 094409 (2018)

Spin susceptibility of charge ordered YBa2Cu3Oy across the upper critical field

Authors: R. Zhou, M. Hirata, T. Wu, I. Vinograd, H. Mayaffre, S. Krämer, A. P. Reyes, P. L. Kuhns, R. Liang, W. N. Hardy, D. A. Bonn, M. -H. Julien

Abstract: The value of the upper critical field Hc2, a fundamental characteristic of the superconducting state, has been subject to strong controversy in high-Tc copper-oxides. Since the issue has been tackled almost exclusively by macroscopic techniques so far, there is a clear need for local-probe measurements. Here, we use 17O NMR to measure the spin susceptibility $χ_{spin}$ of the CuO2 planes at low te… ▽ More The value of the upper critical field Hc2, a fundamental characteristic of the superconducting state, has been subject to strong controversy in high-Tc copper-oxides. Since the issue has been tackled almost exclusively by macroscopic techniques so far, there is a clear need for local-probe measurements. Here, we use 17O NMR to measure the spin susceptibility $χ_{spin}$ of the CuO2 planes at low temperature in charge ordered YBa2Cu3Oy. We find that $χ_{spin}$ increases (most likely linearly) with magnetic field H and saturates above field values ranging from 20 to 40 T. This result is consistent with Hc2 values claimed by G. Grissonnanche et al. [Nat. Commun. 5, 3280 (2014)] and with the interpretation that the charge-density-wave (CDW) reduces Hc2 in underdoped YBa2Cu3Oy. Furthermore, the absence of marked deviation in $χ_{spin}(H)$ at the onset of long-range CDW order indicates that this Hc2 reduction and the Fermi-surface reconstruction are primarily rooted in the short-range CDW order already present in zero field, not in the field-induced long-range CDWorder. Above Hc2, the relatively low values of $χ_{spin}$ at T=2 K show that the pseudogap is a ground-state property, independent of the superconducting gap. △ Less

Submitted 31 October, 2017; originally announced November 2017.

Comments: To appear

Journal ref: PNAS 114, 13148 (2017)

Phase Diagram of Ba$_{2}$NaOsO$_{6}$, a Mott insulator with strong spin orbit interactions

Authors: W. Liu, R. Cong, E. Garcia, A. P. Reyes, H. O. Lee, I. R. Fisher, V. F. Mitrović

Abstract: We report $^{23}$Na nuclear magnetic resonance (NMR) measurements of the Mott insulator with strong spin-orbit interaction Ba$_{2}$NaOsO$_{6}$ as a function of temperature in different magnetic fields ranging from 7 T to 29 T. The measurements, intended to concurrently probe spin and orbital/lattice degrees of freedom, are an extension of our work at lower fields reported in Nat. Commun., v 8, 144… ▽ More We report $^{23}$Na nuclear magnetic resonance (NMR) measurements of the Mott insulator with strong spin-orbit interaction Ba$_{2}$NaOsO$_{6}$ as a function of temperature in different magnetic fields ranging from 7 T to 29 T. The measurements, intended to concurrently probe spin and orbital/lattice degrees of freedom, are an extension of our work at lower fields reported in Nat. Commun., v 8, 14407 (2017). We have identified clear quantitative NMR signatures that display the appearance of a canted ferromagnetic phase, which is preceded by local point symmetry breaking. We have compiled the field temperature phase diagram extending up to 29 T. We find that the broken local point symmetry phase extends over a wider temperature range as magnetic field increases. △ Less

Submitted 16 October, 2017; originally announced October 2017.

Comments: 6 pages, 5 figures.arXiv admin note: substantial text overlap with arXiv:1701.06117

Journal ref: Physics B, 2017

Nature of Lattice Distortions in Cubic Double-Perovskite Ba$_2$NaOsO$_6$

Authors: W. Liu, A. P. Reyes, I. R. Fisher, V. F. Mitrović

Abstract: We present detailed calculations of the electric field gradient (EFG) using a point charge approximation in Ba$_2$NaOsO$_6$, a Mott insulator with strong spin-orbit interaction. Recent $^{23}$Na nuclear magnetic resonance (NMR) measurements found that the onset of local point symmetry breaking, likely caused by the formation of quadrupolar order, precedes the formation of long range magnetic order… ▽ More We present detailed calculations of the electric field gradient (EFG) using a point charge approximation in Ba$_2$NaOsO$_6$, a Mott insulator with strong spin-orbit interaction. Recent $^{23}$Na nuclear magnetic resonance (NMR) measurements found that the onset of local point symmetry breaking, likely caused by the formation of quadrupolar order, precedes the formation of long range magnetic order in this compound. An extension of the static $^{23}$Na NMR measurements as a function of the orientation of a 15 T applied magnetic field at 8 K in the magnetically ordered phase is reported. Broken local cubic symmetry induces a non-spherical electronic charge distribution around the Na site and thus finite EFG, affecting the NMR spectral shape. We combine the spectral analysis as a function of the orientation of the magnetic field with calculations of the EFG to determine the exact microscopic nature of the lattice distortions present in low temperature phases of this material. We establish that orthorhombic distortions, constrained along the cubic axes of the perovskite reference unit cell, of oxygen octahedra surrounding Na nuclei are present in the magnetic phase. Other common types of distortions often observed in oxide structures are considered as well. △ Less

Submitted 16 October, 2017; originally announced October 2017.

Comments: 14 pages, 13 figures

Journal ref: Phys. Rev. B 97, 224103 (2018)

Charge-density-wave order takes over antiferromagnetism in Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6}$ superconductors

Authors: S. Kawasaki, Z. Li, M. Kitahashi, C. T. Lin, P. L. Kuhns, A. P. Reyes, Guo-qing Zheng

Abstract: Superconductivity appears in the cuprates when a spin order is destroyed, while the role of charge is less known. Recently, charge density wave (CDW) was found below the superconducting dome in YBa$_2$Cu$_3$O$_y$ when a high magnetic field is applied perpendicular to the CuO$_2$ plane, which was suggested to arise from incipient CDW in the vortex cores that becomes overlapped. Here, by $^{63}$Cu-n… ▽ More Superconductivity appears in the cuprates when a spin order is destroyed, while the role of charge is less known. Recently, charge density wave (CDW) was found below the superconducting dome in YBa$_2$Cu$_3$O$_y$ when a high magnetic field is applied perpendicular to the CuO$_2$ plane, which was suggested to arise from incipient CDW in the vortex cores that becomes overlapped. Here, by $^{63}$Cu-nuclear magnetic resonance, we report the discovery of CDW induced by an in-plane field that does not create vortex cores in the plane, setting in above the dome in single-layered Bi$_2$Sr$_{2-x}$La$_x$CuO$_6$.The onset temperature $T_{\rm CDW}$ takes over the antiferromagnetic order temperature $T_{\rm N}$ beyond a critical doping level at which superconductivity starts to emerge, and scales with the psudogap temperature $T^{*}$. These results provide important insights into the relationship between spin order, CDW and the pseudogap, and their connections to high-temperature superconductivity. △ Less

Submitted 7 November, 2017; v1 submitted 20 April, 2017; originally announced April 2017.

Comments: 22 pages, 8 figures

Journal ref: Nature Communications 8, 1267 (2017)

Novel Magnetism and Local Symmetry Breaking in a Mott Insulator with Strong Spin Orbit Interactions

Authors: L. Lu, M. Song, W. Liu, A. P. Reyes, P. Kuhns, H. O. Lee, I. R. Fisher, V. F. Mitrović

Abstract: Study of the combined effects of strong electronic correlations with spin-orbit coupling (SOC) represents a central issue in quantum materials research. Predicting emergent properties represents a huge theoretical problem since the presence of SOC implies that the spin is not a good quantum number. Existing theories propose the emergence of a multitude of exotic quantum phases, distinguishable by… ▽ More Study of the combined effects of strong electronic correlations with spin-orbit coupling (SOC) represents a central issue in quantum materials research. Predicting emergent properties represents a huge theoretical problem since the presence of SOC implies that the spin is not a good quantum number. Existing theories propose the emergence of a multitude of exotic quantum phases, distinguishable by either local point symmetry breaking or local spin expectation values, even in materials with simple cubic crystal structure such as Ba$_2$NaOsO$_6$. Experimental tests of such theories by local probes are highly sought for. Here, we report on local measurements designed to concurrently probe spin and orbital/lattice degrees of freedom of Ba$_2$NaOsO$_6$. We find that a novel canted ferromagnetic phase which is preceded by local point symmetry breaking is stabilized at low temperatures, as predicted by quantum theories involving multipolar spin interactions. △ Less

Submitted 21 January, 2017; originally announced January 2017.

Comments: to appear in Nature Communications

Journal ref: Nat. Commun. v8, 14407 (2017)

Observation of electronic bound states in charge-ordered YBa$_2$Cu$_3$O$_y$

Authors: R. Zhou, M. Hirata, T. Wu, I. Vinograd, H. Mayaffre, S. Krämer, M. Horvatić, C. Berthier, A. P. Reyes, P. L. Kuhns, R. Liang, W. N. Hardy, D. A. Bonn, M. -H. Julien

Abstract: Observing how electronic states in solids react to a local symmetry breaking provides insight into their microscopic nature. A striking example is the formation of bound states when quasiparticles are scattered off defects. This is known to occur, under specific circumstances, in some metals and superconductors but not, in general, in the charge-density-wave (CDW) state. Here, we report the unfore… ▽ More Observing how electronic states in solids react to a local symmetry breaking provides insight into their microscopic nature. A striking example is the formation of bound states when quasiparticles are scattered off defects. This is known to occur, under specific circumstances, in some metals and superconductors but not, in general, in the charge-density-wave (CDW) state. Here, we report the unforeseen observation of bound states when a magnetic field quenches superconductivity and induces long-range CDW order in YBa$_2$Cu$_3$O$_y$. Bound states indeed produce an inhomogeneous pattern of the local density of states $N(E_F)$ that leads to a skewed distribution of Knight shifts which is detected here through an asymmetric profile of $^{17}$O NMR lines. We argue that the effect arises most likely from scattering off defects in the CDW state, which provides a novel case of disorder-induced bound states in a condensed-matter system and an insightful window into charge ordering in the cuprates. △ Less

Submitted 12 September, 2016; originally announced September 2016.

Journal ref: Phys. Rev. Lett. 118, 017001 (2017)

Magnetic phases of quasi-two-dimensional antiferromagnet on triangular lattice CuCrO$_2$

Authors: Yu. A. Sakhratov, L. E. Svistov, P. L. Kuhns, H. D. Zhou, A. P. Reyes

Abstract: We have carried out $^{63,65}$Cu NMR spectra measurements in magnetic field up to about 45~T on single crystal of a multiferroic triangular antiferromagnet CuCrO$_2$. The measurements were performed for magnetic fields aligned along the crystal $c$-axis. Field and temperature evolution of the spectral shape demonstrates a number of phase transitions. It was found that the 3D magnetic ordering take… ▽ More We have carried out $^{63,65}$Cu NMR spectra measurements in magnetic field up to about 45~T on single crystal of a multiferroic triangular antiferromagnet CuCrO$_2$. The measurements were performed for magnetic fields aligned along the crystal $c$-axis. Field and temperature evolution of the spectral shape demonstrates a number of phase transitions. It was found that the 3D magnetic ordering takes place in the low field range ($H\lesssim15$~T). At higher fields magnetic structures form within individual triangular planes whereas the spin directions of the magnetic ions from neighboring planes are not correlated. It is established that the 2D-3D transition is hysteretic in field and temperature. Lineshape analysis reveals several possible magnetic structures existing within individual planes for different phases of CuCrO$_2$. Within certain regions on the magnetic H-T phase diagram of CuCrO$_2$ a 3D magnetic ordering with tensor order parameter is expected. △ Less

Submitted 10 July, 2016; originally announced July 2016.

Coherent Charge and Spin Density Waves in Underdoped HgBa$_{2}$CuO$_{4+δ}$

Authors: Jeongseop A. Lee, Yizhou Xin, W. P. Halperin, A. P. Reyes, P. L. Kuhns, M. K. Chan

Abstract: Various forms of spin and charge ordering have been identified in a wide range of cuprate superconducting materials, but whether these behaviors are ubiquitous phenomena is not established. In this work we focus on one of the simplest compounds, HgBa$_{2}$CuO$_{4+δ}$ (Hg1201), a superconductor with a high transition temperature, 97 K, having only a single layer and tetragonal structure, in contras… ▽ More Various forms of spin and charge ordering have been identified in a wide range of cuprate superconducting materials, but whether these behaviors are ubiquitous phenomena is not established. In this work we focus on one of the simplest compounds, HgBa$_{2}$CuO$_{4+δ}$ (Hg1201), a superconductor with a high transition temperature, 97 K, having only a single layer and tetragonal structure, in contrast to one of the most extensively studied materials, YBa$_{2}$Cu$_{3}$O$_{6+y}$ (Y123). Using nuclear magnetic resonance we have discovered a coherent spatial modulation of both spin and charge that is temperature and magnetic field independent, in competition with superconductivity similar to other cuprates. However, there is no evidence for the magnetic field and temperature induced charge order observed in Y123. Electronic instabilities are a common feature of cuprates as in the present work on Hg1201, but their manifestations are not universal. △ Less

Submitted 29 March, 2016; originally announced March 2016.

Comments: 5 pages, 6 figures, and supplementary materials

Phase Separation and Superparamagnetism in the Martensitic Phase of $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$

Authors: S. Yuan, P. L. Kuhns, A. P. Reyes, J. S. Brooks, M. J. R. Hoch, V. Srivastava, R. D. James, C. Leighton

Abstract: $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ shape memory alloys in the approximate range $5 \le x \le 10$ display desirable properties for applications as well as intriguing magnetism. These off-stoichiometric Heusler alloys undergo a martensitic phase transformation at a temperature $T_{M}… ▽ More $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ shape memory alloys in the approximate range $5 \le x \le 10$ display desirable properties for applications as well as intriguing magnetism. These off-stoichiometric Heusler alloys undergo a martensitic phase transformation at a temperature $T_{M}$ of 300 - 400 K, from ferromagnetic (F) to nonferromagnetic, with unusually low thermal hysteresis and a large change in magnetization. The low temperature magnetic structures in the martensitic phase of such alloys, which are distinctly inhomogeneous, are of great interest but are not well understood. Our present use of spin echo NMR, in the large hyperfine fields at $^{55}Mn$ sites, provides compelling evidence that nanoscale magnetic phase separation into F and antiferromagnetic (AF) regions occurs below $T_{M}$ in alloys with x in the range 0 to 7. At finite Co substitution the F regions are found to be of two distinct types, corresponding to high and low local concentrations of Co on Ni sites. Estimates of the size distributions of both the F and AF nanoregions have been made. At x = 7 the AF component is not long-range ordered, even below 4 K, and is quite different to the AF component found at x = 0; by x = 14 the F phase is completely dominant. Of particular interest, we find, for x = 7, that field cooling leads to dramatic changes in the AF regions. These findings provide insight into the origins of magnetic phase separation and superparamagnetism in these complex alloys, particularly their intrinsic exchange bias, which is of considerable current interest. △ Less

Submitted 22 January, 2016; originally announced January 2016.

Comments: 27 pages,5 figures

NMR evidence for inhomogeneous glassy behavior driven by nematic fluctuations in iron arsenide superconductors

Authors: A. P. Dioguardi, M. M. Lawson, B. T. Bush, J. Crocker, K. R. Shirer, D. M. Nisson, T. Kissikov, S. Ran, S. L. Bud'ko, P. C. Canfield, S. Yuan, P. L. Kuhns, A. P. Reyes, H. -J. Grafe, N. J. Curro

Abstract: We present $^{75}$As nuclear magnetic resonance spin-lattice and spin-spin relaxation rate data in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$ as a function of temperature, doping and magnetic field. The relaxation curves exhibit a broad distribution of relaxation rates, consistent with inhomogeneous glassy behavior up to 100 K. The doping and temperature response of the widt… ▽ More We present $^{75}$As nuclear magnetic resonance spin-lattice and spin-spin relaxation rate data in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$ as a function of temperature, doping and magnetic field. The relaxation curves exhibit a broad distribution of relaxation rates, consistent with inhomogeneous glassy behavior up to 100 K. The doping and temperature response of the width of the dynamical heterogeneity is similar to that of the nematic susceptibility measured by elastoresistance measurements. We argue that quenched random fields which couple to the nematic order give rise to a nematic glass that is reflected in the spin dynamics. △ Less

Submitted 30 September, 2015; v1 submitted 5 March, 2015; originally announced March 2015.

Comments: Accepted to Physical Review B

Journal ref: Phys. Rev. B 92, 165116 (2015)

Suppression of 3D-ordering by defects in the $S=1/2$ frustrated chain magnet LiCuVO$_4$

Authors: L. A. Prozorova, S. S. Sosin, L. E. Svistov, N. Buettgen, J. B. Kemper, A. P. Reyes, S. Riggs, O. A. Petrenko, A. Prokofiev

Abstract: We report on a heat capacity study of high quality single crystal samples of \lcvo\ -- a frustrated spin $S=1/2$ chain system -- in magnetic field amounting to 3/4 of the saturation field. At low fields up to about 7~T, a linear temperature dependence of the specific heat, $C_p\propto T$, resulting from 1D magnetic correlations in the spin chains is followed upon cooling by a sharp lambda anomaly… ▽ More We report on a heat capacity study of high quality single crystal samples of \lcvo\ -- a frustrated spin $S=1/2$ chain system -- in magnetic field amounting to 3/4 of the saturation field. At low fields up to about 7~T, a linear temperature dependence of the specific heat, $C_p\propto T$, resulting from 1D magnetic correlations in the spin chains is followed upon cooling by a sharp lambda anomaly of the transition into a 3D helical phase with $C_p\propto T^3$ behavior at low temperature. The transition from a spin liquid into a spin-modulated (SM) phase at higher fields occurs via a hump-like anomaly which, as the temperature decreases further turns into a $C_p\propto T^2$ law distinctive for a quasi-2D system. We suggest an explanation for how nonmagnetic defects in the Cu$^{2+}$ chains can suppress 3D long-range ordering in the SM phase and leave it undisturbed in a helical phase. △ Less

Submitted 13 January, 2015; originally announced January 2015.

Comments: 4 pages, 4 figures

Internal static electric and magnetic field at the copper cite in a single crystal of the electron-doped high-T$_{c}$ superconductor Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$

Authors: Guoqing Wu, F. Zamborszky, A. P. Reyes, P. L. Kuhns, R. L. Greene, W. G. Clark

Abstract: We report $^{63, 65}$Cu-NMR spectroscopy and Knight shift measurements on a single crystal of the electron-doped high-$T_{c}$ superconductor Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ (PCCO) with an applied magnetic field ($H$) up to 26.42 T. A very small NQR frequency is obtained with the observation of the spectrum, which shows an extremely wide continuous distribution of it that becomes significant narr… ▽ More We report $^{63, 65}$Cu-NMR spectroscopy and Knight shift measurements on a single crystal of the electron-doped high-$T_{c}$ superconductor Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ (PCCO) with an applied magnetic field ($H$) up to 26.42 T. A very small NQR frequency is obtained with the observation of the spectrum, which shows an extremely wide continuous distribution of it that becomes significant narrower below 20 K at $H$ $\parallel$ $c$ where the superconductivity is completely suppressed, indicating a significant change in the charge distribution at the Cu site, while the corresponding changes at $H$ $\perp$ $c$ is negligible when the superconductivity is present or not fully suppressed. The Knight shift and central linewidth are proportional to the applied magnetic field with a high anisotropy. We find that the magnitude of the internal static magnetic field at the copper is dominated by the anisotropic Cu$^{2+}$ 3$d$-orbital contributions, while its weak temperature-dependence is mainly determined by the isotropic contact hyperfine coupling to the paramagnetic Pr$^{3+}$ spins, which also gives rise to the full distribution of the internal static magnetic field at the copper for $H$ $\perp$ $c$. This internal static electric and magnetic field environment at the copper is very different from that in the hole-doped cuprates, and may provide new insight into the understanding of high-$T_{c}$ superconductivity. Other experimental techniques are needed to verify whether the observed significant narrowing of the charge distribution at the Cu site with $H$ $\parallel$ $c$ is caused by the charge ordering (CO) [E. H. da Silva Neto $et ~al.$, to be published in Science] \cite{ehdsn} or a new type of charge modulation. △ Less

Submitted 7 October, 2014; originally announced October 2014.

Comments: 8 pages, 6 figures, submitted to Phys. Rev. B

Search for a spin-nematic phase in the quasi-one-dimensional frustrated magnet LiCuVO$_4$

Authors: N. Büttgen, K. Nawa, T. Fujita, M. Hagiwara, P. Kuhns, A. Prokofiev, A. P. Reyes, L. E. Svistov, K. Yoshimura, M. Takigawa

Abstract: We have performed NMR experiments on the quasi one-dimensional frustrated spin-1/2 system LiCuVO$_4$ in magnetic fields $H$ applied along the c-axis up to field values near the saturation field $H_{\rm sat}$. For the field range $H_{\rm c2}<H<H_{\rm c3}$ ($μ_0H_{\rm c2}\approx 7.5$T and $μ_0H_{\rm c3} = [40.5 \pm 0.2]$T) the $^{51}$V NMR spectra at $T$ = 380mK exhibit a characteristic double-horn… ▽ More We have performed NMR experiments on the quasi one-dimensional frustrated spin-1/2 system LiCuVO$_4$ in magnetic fields $H$ applied along the c-axis up to field values near the saturation field $H_{\rm sat}$. For the field range $H_{\rm c2}<H<H_{\rm c3}$ ($μ_0H_{\rm c2}\approx 7.5$T and $μ_0H_{\rm c3} = [40.5 \pm 0.2]$T) the $^{51}$V NMR spectra at $T$ = 380mK exhibit a characteristic double-horn pattern, as expected for a spin-modulated phase in which the magnetic moments of Cu$^{2+}$ ions are aligned parallel to the applied field $H$ and their magnitudes change sinusoidally along the magnetic chains. For higher fields, the $^{51}$V NMR spectral shape changes from the double-horn pattern into a single Lorentzian line. For this Lorentzian line, the internal field at the $^{51}$V nuclei stays constant for $μ_0 H > 41.4$T, indicating that the majority of magnetic moments in LiCuVO$_4$ are already saturated in this field range. This result is inconsistent with the previously observed linear field dependence of the magnetization $M(H)$ for $H_{\rm c3}<H<H_{\rm sat}$ with $μ_0H_{\rm sat}=45$T [L. E. Svistov {\it et al}., JETP Letters {\bf 93}, 21 (2011)]. We argue that the discrepancy is due to non-magnetic defects in the samples. The results of the spin-lattice relaxation rate of $^7$Li nuclei indicate an energy gap which grows with field twice as fast as the Zeeman energy of a single spin, therefore, suggesting that the two-magnon bound state is the lowest energy excitation. The energy gap tends to close at $μ_0H \approx 41$T. Our results suggest that the theoretically predicted spin-nematic phase, if it exists in LiCuVO$_4$, can be established only within the narrow field range $40.5 < μ_0 H < 41.4$T . △ Less

Submitted 1 October, 2014; originally announced October 2014.

Comments: 8 pages, 9 figures

Journal ref: Phys. Rev B 90, 134401 (2014)

Physical properties and magnetic structure of the intermetallic CeCuBi2 compound

Authors: C. Adriano, P. F. S. Rosa, C. B. R. Jesus, J. R. L. Madergan, T. M. Garitezi, T. Grant, Z. Fisk, D. J. Garcia, A. P. Reyes, P. L. Kuhns, R. R. Urbano, C. Giles, P. G. Pagliuso

Abstract: In this work, we combined magnetization, pressure dependent electrical resistivity, heat-capacity, 63Cu Nuclear Magnetic Resonance (NMR) and X-ray resonant magnetic scattering experiments to investigate the physical properties of the intermetallic CeCuBi2 compound. Our single crystals show an antiferromagnetic ordering at TN ~ 16 K and the magnetic properties indicate that this compound is an Isin… ▽ More In this work, we combined magnetization, pressure dependent electrical resistivity, heat-capacity, 63Cu Nuclear Magnetic Resonance (NMR) and X-ray resonant magnetic scattering experiments to investigate the physical properties of the intermetallic CeCuBi2 compound. Our single crystals show an antiferromagnetic ordering at TN ~ 16 K and the magnetic properties indicate that this compound is an Ising antiferromagnet. In particular, the low temperature magnetization data revealed a spin-flop transition at T = 5 K when magnetic fields about 5.5 T are applied along the c-axis. Moreover, the X-ray magnetic diffraction data below TN revealed a commensurate antiferromagnetic structure with propagation wavevector (0 0 1/2) with the Ce^3+ moments oriented along the c-axis. Furthermore, our heat capacity, pressure dependent resistivity and temperature dependent 63Cu NMR data suggest that CeCuBi2 exhibits a weak heavy fermion behavior with strongly localized Ce^3+ 4f electrons. We thus discuss a scenario taking into account the anisotropic magnetic interaction between the Ce^3+ ions along with the tetragonal crystalline electric field effects in CeCuBi2. △ Less

Submitted 9 February, 2015; v1 submitted 8 July, 2014; originally announced July 2014.

Comments: 8 pages, 7 figures

Journal ref: Phys. Rev. B 90, 235120 (2014)

Magnetic structure and domain conversion of quasi-2D frustrated antiferromagnet CuCrO2 probed by NMR

Authors: Yu. A. Sakhratov, L. E. Svistov, P. L. Kuhns, H. D. Zhou, A. P. Reyes

Abstract: We have carried out 63,65Cu NMR spectra measurements in magnetic field up to about 15.5 T on single crystal of a multiferroic triangular-lattice antiferromagnet CuCrO2. The measurements were performed for perpendicular and parallel orientation of the magnetic field with respect to the c-axis of the crystal, and the detailed angle dependence of the spectra on the magnetic field direction within ab-… ▽ More We have carried out 63,65Cu NMR spectra measurements in magnetic field up to about 15.5 T on single crystal of a multiferroic triangular-lattice antiferromagnet CuCrO2. The measurements were performed for perpendicular and parallel orientation of the magnetic field with respect to the c-axis of the crystal, and the detailed angle dependence of the spectra on the magnetic field direction within ab-plane was studied. The shape of the spectra can be well described in the model of spiral spin structure proposed by recent neutron diffraction experiments. When field is rotated perpendicular to crystal c-axis, we observed, directly for the first time, a remarkable reorientation of spin plane simultaneous with rotation of the incommensurate wavevector by quantitatively deducing the conversion of less energetically favorable domain to a more favorable one. At high enough fields parallel to c-axis, the data are consistent with either a field-induced commensurate spiral magnetic structure or an incommensurate spiral magnetic structure with a disorder in the c direction, suggesting that high fields may have influence on interplanar ordering. △ Less

Submitted 20 June, 2014; v1 submitted 27 May, 2014; originally announced May 2014.

Comments: 8 pages, 10 figures

Journal ref: JETP vol.119, 880 (2014)

Multigap superconductivity in locally non-centrosymmetric SrPtAs: An As nuclear quadrupole resonance investigation

Authors: F. Brückner, R. Sarkar, M. Günther, H. Kühne, H. Luetkens, T. Neupert, A. P. Reyes, P. L. Kuhns, P. K. Biswas, T. Stürzer, D. Johrendt, H. -H. Klauss

Abstract: We report detailed $^{75}$As-NQR investigations of the locally non-centrosymmetric superconductor SrPtAs. The spin-lattice relaxation studies prove weakly coupled multi-gap superconductivity. The Hebel-Slichter peak, a hallmark of conventional superconductivity, is strongly suppressed, which points to an unconventional superconducting state. The observed behavior excludes a superconducting order p… ▽ More We report detailed $^{75}$As-NQR investigations of the locally non-centrosymmetric superconductor SrPtAs. The spin-lattice relaxation studies prove weakly coupled multi-gap superconductivity. The Hebel-Slichter peak, a hallmark of conventional superconductivity, is strongly suppressed, which points to an unconventional superconducting state. The observed behavior excludes a superconducting order parameter with line nodes and is consistent with proposed $f$-wave and chiral $d$-wave order parameters. △ Less

Submitted 20 December, 2013; originally announced December 2013.

Journal ref: Phys. Rev. B 90, 220503 (2014)

NMR investigation of the Knight shift anomaly in CeIrIn5 at high magnetic fields

Authors: A. C. Shockley, N. apRoberts-Warren, D. M. Nisson, P. L. Kuhns, A. P. Reyes, S. Yuan, N. J. Curro

Abstract: We report nuclear magnetic resonance Knight shift data in the heavy fermion material CeIrIn5 at fields up to 30 T. The Knight shift of the In displays a strong anomaly, and we analyze the results using two different interpretations. We find that the Kondo lattice coherence temperature and the effective mass of the heavy electrons remains largely unaffected by the magnetic field, despite the fact t… ▽ More We report nuclear magnetic resonance Knight shift data in the heavy fermion material CeIrIn5 at fields up to 30 T. The Knight shift of the In displays a strong anomaly, and we analyze the results using two different interpretations. We find that the Kondo lattice coherence temperature and the effective mass of the heavy electrons remains largely unaffected by the magnetic field, despite the fact that the Zeeman energy is on the order of the coherence temperature. △ Less

Submitted 26 July, 2013; originally announced July 2013.

Comments: 5 pages, 5 figures; to appear in Phys. Rev. B

Microscopic coexistence of a two-component incommensurate spin density wave with superconductivity in underdoped NaFe$_{0.983}$Co$_{0.017}$As

Authors: Sangwon Oh, A. M. Mounce, Jeongseop A. Lee, W. P. Halperin, C. L. Zhang, S. Carr, Pengcheng Dai, A. P. Reyes, P. L. Kuhns

Abstract: We have performed $^{75}$As and $^{23}$Na nuclear magnetic resonance (NMR) measurements on a single crystal of NaFe$_{0.9835}$Co$_{0.0165}$As and found microscopic coexistence of superconductivity with a two-component spin density wave (SDW). Using $^{23}$Na NMR we measured the spatial distribution of local magnetic fields. The SDW was found to be incommensurate with a major component having magne… ▽ More We have performed $^{75}$As and $^{23}$Na nuclear magnetic resonance (NMR) measurements on a single crystal of NaFe$_{0.9835}$Co$_{0.0165}$As and found microscopic coexistence of superconductivity with a two-component spin density wave (SDW). Using $^{23}$Na NMR we measured the spatial distribution of local magnetic fields. The SDW was found to be incommensurate with a major component having magnetic moment ($\sim0.2\,μ_B$/Fe) and a smaller component with magnetic moment ($\sim0.02\,μ_B$/Fe). Spin lattice relaxation experiments reveal that this coexistence occurs at a microscopic level. △ Less

Submitted 19 July, 2013; originally announced July 2013.

Comments: 5 figures

Journal ref: Phys. Rev. B 88, 134518 (2013)

Progressive slowing down of spin fluctuations in underdoped LaFeAsO$_{1-x}$F$_x$

Authors: F. Hammerath, U. Gräfe, H. Kühne, P. L. Kuhns, A. P. Reyes, G. Lang, S. Wurmehl, B. Büchner, P. Carretta, H. -J. Grafe

Abstract: The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO$_{1-x}$F$_x$ was studied over a broad doping, temperature, and magnetic field range (x = 0 - 0.15, T up to 480K, H up to 30T) by means of As nuclear magnetic resonance (NMR). An enhanced spin-lattice relaxation rate divided by temperature, 1/T1T, in underdoped superconducting samples (x = 0.045, 0.05 and 0.075) sugg… ▽ More The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO$_{1-x}$F$_x$ was studied over a broad doping, temperature, and magnetic field range (x = 0 - 0.15, T up to 480K, H up to 30T) by means of As nuclear magnetic resonance (NMR). An enhanced spin-lattice relaxation rate divided by temperature, 1/T1T, in underdoped superconducting samples (x = 0.045, 0.05 and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally-doped (x = 0.10) and completely absent in overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a BPP-type model is used to describe the occurrence of a peak in 1/T1T clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition. △ Less

Submitted 11 July, 2013; originally announced July 2013.

Comments: 10 pages, 10 figures

Journal ref: Phys. Rev. B 88, 104503 (2013)

Emergence of charge order from the vortex state of a high temperature superconductor

Authors: T. Wu, H. Mayaffre, S. Kramer, M. Horvatic, C. Berthier, P. L. Kuhns, A. P. Reyes, R. Liang, W. N. Hardy, D. A. Bonn, M. -H. Julien

Abstract: Evidence is mounting that charge order competes with superconductivity in high Tc cuprates. Whether this has any relationship to the pairing mechanism is unknown since neither the universality of the competition nor its microscopic nature has been established. Here using nuclear magnetic resonance, we show that, similar to La214, charge order in YBCO has maximum strength inside the superconducting… ▽ More Evidence is mounting that charge order competes with superconductivity in high Tc cuprates. Whether this has any relationship to the pairing mechanism is unknown since neither the universality of the competition nor its microscopic nature has been established. Here using nuclear magnetic resonance, we show that, similar to La214, charge order in YBCO has maximum strength inside the superconducting dome, at doping levels p = 0.11 - 0.12.We further show that the overlap of halos of incipient charge order around vortex cores, similar to those visualised in Bi2212, can explain the threshold magnetic field at which long-range charge order emerges. These results reveal universal features of a competition in which charge order and superconductivity appear as joint instabilities of the same normal state, whose relative balance can be field-tuned in the vortex state. △ Less

Submitted 8 July, 2013; originally announced July 2013.

Comments: For a final version, see http://www.nature.com/ncomms/2013/130703/ncomms3113/full/ncomms3113.html

Journal ref: Nature Communications 4, 2113 (2013)

Superconducting anisotropy in the electron-doped high-T$_{c}$ superconductors Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$

Authors: Guoqing Wu, R. L. Greene, A. P. Reyes, P. L. Kuhns, W. G. Moulton, Bing Wu, Feng Wu, W. G. Clark

Abstract: We report superconducting anisotropy measurements in the electron-doped high-$T_{c}$ superconductors (HTSCs) Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ (PCCO, $x$ = 0.15 and 0.17) with applied magnetic field ($H_{0}$) up to 28 T. Our results show that the upper critical field [$H_{c2}(T)$] is highly anisotropic, and as temperature $T$ $\rightarrow$ 0 the value of it at $H_{0}$ $\parallel$ $c$ [… ▽ More We report superconducting anisotropy measurements in the electron-doped high-$T_{c}$ superconductors (HTSCs) Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ (PCCO, $x$ = 0.15 and 0.17) with applied magnetic field ($H_{0}$) up to 28 T. Our results show that the upper critical field [$H_{c2}(T)$] is highly anisotropic, and as temperature $T$ $\rightarrow$ 0 the value of it at $H_{0}$ $\parallel$ $c$ [$H_{c2,\parallel c}$(0)] is far less than the Pauli limit, which is very different from that at $H_{0}$ $\perp$ $c$. The $H_{c2}(0)$ character along with the evaluated zero $T$ coherence length $[ξ_{ab(c)}(0)]$ and penetration depth $[λ_{ab(c)}(0)]$ is compared with those of the hole-doped cuprate HTSCs and typical Fe-based superconductors. We find that the low temperature anisotropic character of PCCO is rather similar to that of hole-doped cuprate HTSCs, but apparently larger than that of typical Fe-based superconductors. This study also proves a new sensitive probe of detecting rich properties of unconventional superconductors with the use of the resonant frequency of a NMR probe circuit. △ Less

Submitted 20 February, 2014; v1 submitted 23 May, 2013; originally announced May 2013.

Comments: 7 pages and 7 figures. Peer-reviewed. To appear in J. Appl. Phys. in 2014

Absence of static orbital current magnetism at the apical oxygen site in HgBa$_2$CuO$_{4+δ}$ from NMR

Authors: A. M. Mounce, Sangwon Oh, Jeongseop A. Lee, W. P. Halperin, A. P. Reyes, P. L. Kuhns, M. K. Chan, C. Dorow, L. Ji, D. Xia, X. Zhao, M. Greven

Abstract: The simple structure of HgBa$_2$CuO$_{4+δ}$ (Hg1201) is ideal among cuprates for study of the pseudogap phase as a broken symmetry state. We have performed $^{17}$O nuclear magnetic resonance (NMR) on an underdoped Hg1201 crystal with transition temperature of 74 K to look for circulating orbital currents proposed theoretically and inferred from neutron scattering. The narrow spectra preclude stat… ▽ More The simple structure of HgBa$_2$CuO$_{4+δ}$ (Hg1201) is ideal among cuprates for study of the pseudogap phase as a broken symmetry state. We have performed $^{17}$O nuclear magnetic resonance (NMR) on an underdoped Hg1201 crystal with transition temperature of 74 K to look for circulating orbital currents proposed theoretically and inferred from neutron scattering. The narrow spectra preclude static local fields in the pseudogap phase at the apical site, suggesting that the moments observed with neutrons are fluctuating. The NMR frequency shifts are consistent with a dipolar field from the Cu$^{+2}$ site. △ Less

Submitted 23 April, 2013; originally announced April 2013.

Comments: 6 pages, 5 figures

Anomalous hysteresis as an evidence for a magnetic field-induced chiral superconducting state in LiFeAs

Authors: G. Li, R. R. Urbano, P. Goswami, C. Tarantini, B. Lv, P. Kuhns, A. P. Reyes, C. W. Chu, L. Balicas

Abstract: Magnetometry measurements in high quality LiFeAs single-crystals reveal a change in the sign of the magnetic hysteresis in the vicinity of the upper critical field $H_{c2}$, from a clear diamagnetic response dominated by the pinning of vortices, to a considerably smaller net hysteretic response of opposite sign, which \emph{disappears} at $H_{c2}$. If the diamagnetic response at high fields result… ▽ More Magnetometry measurements in high quality LiFeAs single-crystals reveal a change in the sign of the magnetic hysteresis in the vicinity of the upper critical field $H_{c2}$, from a clear diamagnetic response dominated by the pinning of vortices, to a considerably smaller net hysteretic response of opposite sign, which \emph{disappears} at $H_{c2}$. If the diamagnetic response at high fields results from pinned vortices and associated screening super-currents, this sign change must result from currents circulating in the opposite sense, which give rise to a small field-dependent magnetic moment \emph{below} $H_{c2}$. This behavior seems to be extremely sensitive to the sample quality or stoichiometry, as we have observed it only in a few fresh crystals, which also display the de Haas van Alphen-effect. We provide arguments against the surface superconductivity, the flux compression, and the random $π$ junction scenarios, which have been previously put forward to explain a paramagnetic Meissner effect, below the lower critical field $H_{c1}$. The observed anomalous hysteresis at high fields will be compatible with the existence of chiral gap wave-functions, which possess a field dependent magnetic moment. Within a Landau-Ginzburg framework, we demonstrate how a $(d_{x^2 - y^2} + id_{xy})$ or a $(p_x+ip_y)$ chiral superconducting component can be stabilized in the mixed state of $s_{\pm}$ superconductor, due to the combined effects of the magnetic field and the presence of competing pairing channels. The realization of a particular chiral pairing depends on the microscopic details of the strengths of the competing pairing channels. △ Less

Submitted 8 January, 2013; v1 submitted 23 August, 2012; originally announced August 2012.

Comments: 12 pages, 10 figures

Journal ref: Phys. Rev. B, 87, 024512 (2013)

Nature of the low energy excitations in the short range ordered region of Cs$_2$CuCl$_4$ as revealed by $^{133}$Cs NMR

Authors: M. -A. Vachon, G. Koutroulakis, V. F. Mitrović, Ookie Ma, J . B. Marston, A. P. Reyes, P. Kuhns, R. Coldea, Z. Tylczynski

Abstract: We report nuclear magnetic resonance measurements of the spin-1/2 anisotropic triangular lattice antiferromagnet Cs$_2$CuCl$_4$ as a function of temperature and applied magnetic field. The observed temperature and magnetic field dependence of the NMR relaxation rate suggests that low energy excitations in the short-range ordered region stabilized over a wide range of intermediate fields and temper… ▽ More We report nuclear magnetic resonance measurements of the spin-1/2 anisotropic triangular lattice antiferromagnet Cs$_2$CuCl$_4$ as a function of temperature and applied magnetic field. The observed temperature and magnetic field dependence of the NMR relaxation rate suggests that low energy excitations in the short-range ordered region stabilized over a wide range of intermediate fields and temperatures of the phase diagram are gapless or nearly gapless fermionic excitations. An upper bound on the size of the gap of 0.037 meV $\approx J/10$ is established. The magnetization and NMR relaxation rate can be qualitatively described either by a quasi-1D picture of weakly coupled chains, or by mean-field theories of specific 2D spin liquids; however, quantitative differences exist between data and theory in both cases. This comparison indicates that 2D interactions are quantitatively important in describing the low-energy physics. △ Less

Submitted 9 March, 2012; originally announced March 2012.

Comments: 14 pages, 8 figures

Journal ref: New J. Phys. 13 093029 (2011)

High--field NMR of the quasi--1D antiferromagnet LiCuVO$_4$

Authors: N. Buttgen, P. Kuhns, A. Prokofiev, A. P. Reyes, L. E. Svistov

Abstract: We report on NMR studies of the quasi one--dimensional (1D) antiferromagnetic $S=1/2$ chain cuprate LiCuVO$_4$ in magnetic fields $H$ up to $μ_0H$ = 30 T ($\approx 70$% of the saturation field $H_{\rm sat}$). NMR spectra in fields higher than $H_{\rm c2}$ ($μ_0H_{\rm c2} \approx 7.5$ T) and temperatures $T<T_{\rm N}$ can be described within the model of a spin-modulated phase in which the magnetic… ▽ More We report on NMR studies of the quasi one--dimensional (1D) antiferromagnetic $S=1/2$ chain cuprate LiCuVO$_4$ in magnetic fields $H$ up to $μ_0H$ = 30 T ($\approx 70$% of the saturation field $H_{\rm sat}$). NMR spectra in fields higher than $H_{\rm c2}$ ($μ_0H_{\rm c2} \approx 7.5$ T) and temperatures $T<T_{\rm N}$ can be described within the model of a spin-modulated phase in which the magnetic moments are aligned parallel to the applied field $H$ and their values alternate sinusoidally along the magnetic chains. Based on theoretical concepts about magnetically frustrated 1D chains, the field dependence of the modulation strength of the magnetic structure is deduced from our experiments. Relaxation time $T_2$ measurements of the $^{51}$V nuclei show that $T_2$ depends on the particular position of the probing $^{51}$V nucleus with respect to the magnetic copper moments within the 1D chains: the largest $T_2$ value is observed for the vanadium nuclei which are very next to the magnetic Cu$^{2+}$ ion with largest ordered magnetic moment. This observation is in agreement with the expectation for the spin-modulated magnetic structure. The $(H,T)$ magnetic phase diagram of LiCuVO$_4$ is discussed. △ Less

Submitted 30 January, 2012; originally announced January 2012.

Journal ref: Phys. Rev. B 85, 214421 (2012)

Magnetic field dependence of spin-lattice relaxation in the s$\pm$ state of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$

Authors: Sangwon Oh, A. M. Mounce, W. P. Halperin, C. L. Zhang, Pengcheng Dai, A. P. Reyes, P. L. Kuhns

Abstract: The spatially averaged density of states, <N(0)>, of an unconventional d-wave superconductor is magnetic field dependent, proportional to $H^{1/2}$, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents[1,2]. This phenomenon, called the Volovik effect, has been predicted to exist for a sign changing $s\pm$ state [3], although it is absent in a single band… ▽ More The spatially averaged density of states, <N(0)>, of an unconventional d-wave superconductor is magnetic field dependent, proportional to $H^{1/2}$, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents[1,2]. This phenomenon, called the Volovik effect, has been predicted to exist for a sign changing $s\pm$ state [3], although it is absent in a single band s-wave superconductor. Consequently, we expect there to be Doppler contributions to the NMR spin-lattice relaxation rate, $1/T_1 \propto <N(0)^2>$, for an $s\pm$ state which will depend on magnetic field. We have measured the $^{75}$As $1/T_1$ in a high-quality, single crystal of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ over a wide range of field up to 28 T. Our spatially resolved measurements show that indeed there are Doppler contributions to $1/T_1$ which increase closer to the vortex core, with a spatial average proportional to $H^2$, inconsistent with recent theory [4] △ Less

Submitted 30 April, 2012; v1 submitted 17 September, 2011; originally announced September 2011.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. B. 85.174508 (2012)

Electronic inhomogeneity in a Kondo lattice

Authors: E. D. Bauer, Yi-feng Yang, C. Capan, R. R. Urbano, C. F. Miclea, H. Sakai, F. Ronning, M. J. Graf, A. V. Balatsky, R. Movshovich, A. D. Bianchi, A. P. Reyes, P. L. Kuhns, J. D. Thompson, Z. Fisk

Abstract: Inhomogeneous electronic states resulting from entangled spin, charge, and lattice degrees of freedom are hallmarks of strongly correlated electron materials; such behavior has been observed in many classes of d-electron materials, including the high-Tc copper-oxide superconductors, manganites, and most recently the iron-pnictide superconductors. The complexity generated by competing phases in the… ▽ More Inhomogeneous electronic states resulting from entangled spin, charge, and lattice degrees of freedom are hallmarks of strongly correlated electron materials; such behavior has been observed in many classes of d-electron materials, including the high-Tc copper-oxide superconductors, manganites, and most recently the iron-pnictide superconductors. The complexity generated by competing phases in these materials constitutes a considerable theoretical challenge-one that still defies a complete description. Here, we report a new manifestation of electronic inhomogeneity in a strongly correlated f-electron system, using CeCoIn5 as an example. A thermodynamic analysis of its superconductivity, combined with nuclear quadrupole resonance measurements, shows that nonmagnetic impurities (Y, La, Yb, Th, Hg and Sn) locally suppress unconventional superconductivity, generating an inhomogeneous electronic "Swiss cheese" due to disrupted periodicity of the Kondo lattice. Our analysis may be generalized to include related systems, suggesting that electronic inhomogeneity should be considered broadly in Kondo lattice materials. △ Less

Submitted 28 April, 2011; originally announced April 2011.

Comments: 27 pages

Journal ref: PNAS 108, 6857 (2011)

Two distinct electronic sites in the Cu-O plane of the (La,Sr)CuO(4) pseudogap state

Authors: Robert X. Smith, Philip L. Kuhns, Arneil P. Reyes, Gregory S. Boebinger

Abstract: The pseudogap state is widely regarded as a precursor to high-temperature superconductivity yet remains poorly understood. Using high field (30 T) NMR, we report two electronically distinct oxygen sites within the copper-oxygen (Cu-O) planes of an underdoped cuprate in the pseudogap state. At temperatures well above the bulk superconducting transition, roughly 25% of the oxygen sites evidence supe… ▽ More The pseudogap state is widely regarded as a precursor to high-temperature superconductivity yet remains poorly understood. Using high field (30 T) NMR, we report two electronically distinct oxygen sites within the copper-oxygen (Cu-O) planes of an underdoped cuprate in the pseudogap state. At temperatures well above the bulk superconducting transition, roughly 25% of the oxygen sites evidence superconducting pair formation, the remaining evidence antiferromagnetic ordering on the nearest neighbor Cu sites. Any superconducting fluctuations are slower than the microsecond timescales of the NMR probe, a confirmation that the (nearly) static bimodality of electronic sites in the Cu-O plane - first reported using scanning tunneling microscopy on surfaces - extends throughout the bulk pseudogap state. △ Less

Submitted 28 January, 2011; originally announced January 2011.

Comments: 4 pages, 4 figures, submitted to Physical Review Letters

$^{75}$As NMR of Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ in High Magnetic Field

Authors: Sangwon Oh, A. M. Mounce, S. Mukhopadhyay, W. P. Halperin, A. B. Vorontsov, S. L. Bud'ko, P. C. Canfield, Y. Furukawa, A. P. Reyes, P. L. Kuhns

Abstract: The superconducting state of an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing slow, possibly higher order than linear, increase with temperature at $T \lesssim 0.5 \, T_c$, wi… ▽ More The superconducting state of an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing slow, possibly higher order than linear, increase with temperature at $T \lesssim 0.5 \, T_c$, with $T_c \approx 23 \, K$. This is consistent with the extended s-wave state with $A_{1g}$ symmetry but the precise details of the gap structure are harder to resolve. Measurements of the NMR spin-spin relaxation time, $T_2$, indicate a strong indirect exchange interaction at all temperatures. Below the superconducting transition temperature vortex dynamics lead to an anomalous dip in $T_2$ at the vortex freezing transition from which we obtain the vortex phase diagram up to $H = 28$ T. △ Less

Submitted 1 June, 2011; v1 submitted 5 January, 2011; originally announced January 2011.

Comments: 13 pages, 9 figures

Journal ref: Phys. Rev. B 83, 214501 (2011)

Spin-Density Wave near the Vortex Cores of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Authors: A. M. Mounce, S. Oh, S. Mukhopadhyay, W. P. Halperin, A. P. Reyes, P. L. Kuhns, K. Fujita, M. Ishikado, S. Uchida

Abstract: Competition with magnetism is at the heart of high temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe using nuclear magnetic resonance. Our spin-lattice-relaxation spectroscopy is spatially resolved both within a conduction plane as well as from one plane to another. With this approach we have found a… ▽ More Competition with magnetism is at the heart of high temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe using nuclear magnetic resonance. Our spin-lattice-relaxation spectroscopy is spatially resolved both within a conduction plane as well as from one plane to another. With this approach we have found a spin-density wave associated with the vortex core in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, which is expected from scanning tunneling microscope observations of "checkerboard" patterns in the local density of electronic states.[1] We determine both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30 T. △ Less

Submitted 5 November, 2010; originally announced November 2010.

Comments: 7 pages, 5 figures

Charge Induced Vortex Lattice Instability

Authors: A. M. Mounce, S. Oh, S. Mukhopadhyay, W. P. Halperin, A. P. Reyes, P. L. Kuhns, K. Fujita, M. Ishikado, S. Uchida

Abstract: It has been predicted that superconducting vortices should be electrically charged and that this effect is particularly enhanced for, high temperature superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic resonance (NMR) experiments\cite{kum01} suggest the existence of vortex charging, but the effects are small and the interpretation controversial. Here we show that the A… ▽ More It has been predicted that superconducting vortices should be electrically charged and that this effect is particularly enhanced for, high temperature superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic resonance (NMR) experiments\cite{kum01} suggest the existence of vortex charging, but the effects are small and the interpretation controversial. Here we show that the Abrikosov vortex lattice, characteristic of the mixed state of superconductors, will become unstable at sufficiently high magnetic field if there is charge trapped on the vortex core. Our NMR measurements of the magnetic fields generated by vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+y}$ single crystals\cite{che07} provide evidence for an electrostatically driven vortex lattice reconstruction with the magnitude of charge on each vortex pancake of $\mathbf{\sim 2}$x$\mathbf{10^{-3} e}$, depending on doping, in line with theoretical estimates.\cite{kho95,kna05} △ Less

Submitted 5 October, 2010; v1 submitted 23 September, 2010; originally announced September 2010.

Comments: to appear in Nature Physics; 6 pages, 7 figures

Carrier-Concentration Dependence of the Pseudogap Ground State of Superconducting Bi2Sr2-xLaxCuO6+delta Revealed by 63,65Cu-Nuclear Magnetic Resonance in Very High Magnetic Fields

Authors: Shinji Kawasaki, Chengtian Lin, Philip L. Kuhns, Arneil P. Reyes, Guo-qing Zheng

Abstract: We report the results of the Knight shift by 63,65Cu-nuclear-magnetic resonance (NMR) measurements on single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely and the pseudogap ground state is revealed. The 63Cu-NMR Knight shift shows that there remains a finite density of states (DOS) at the… ▽ More We report the results of the Knight shift by 63,65Cu-nuclear-magnetic resonance (NMR) measurements on single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely and the pseudogap ground state is revealed. The 63Cu-NMR Knight shift shows that there remains a finite density of states (DOS) at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state decreases with decreasing doping (increasing x) but remains quite large even at the vicinity of the magnetically ordered phase of x > 0.8, which suggests that the DOS plunges to zero upon approaching the Mott insulating phase. △ Less

Submitted 25 August, 2010; originally announced August 2010.

Comments: 4 pages, 5 figures, to appear in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 105, 137002 (2010)

Dynamics of a Heisenberg spin chain in the quantum critical regime: NMR experiment versus effective field theory

Authors: H. Kuehne, A. A. Zvyagin, M. Guenther, A. P. Reyes, P. L. Kuhns, M. M. Turnbull, C. P. Landee, H. -H. Klauss

Abstract: A comprehensive comparison between the magnetic field- and temperature-dependent low frequency spin dynamics in the antiferromagnetic spin-1/2 Heisenberg chain (AFHC) system copper pyrazine dinitrate, probed via the 13C-nuclear magnetic resonance (NMR) relaxation rate 1/T1, and the field theoretical approach in the Luttinger liquid (LL) regime has been performed. We have found a very good agreemen… ▽ More A comprehensive comparison between the magnetic field- and temperature-dependent low frequency spin dynamics in the antiferromagnetic spin-1/2 Heisenberg chain (AFHC) system copper pyrazine dinitrate, probed via the 13C-nuclear magnetic resonance (NMR) relaxation rate 1/T1, and the field theoretical approach in the Luttinger liquid (LL) regime has been performed. We have found a very good agreement between the experiment and theory in the investigated temperature and field range. Our results demonstrate how strongly the quantum critical point affects the spin dynamics of Heisenberg spin chain compounds. △ Less

Submitted 23 July, 2010; v1 submitted 21 July, 2010; originally announced July 2010.

Comments: 5 pages, 3 figures

Distinct high-T transitions in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

Authors: R. R. Urbano, E. L. Green, W. G. Moulton, A. P. Reyes, P. L. Kuhns, E. M. Bittar, C. Adriano, T. M. Garitezi, L. Bufaiçal, P. G. Pagliuso

Abstract: In contrast to the simultaneous structural and magnetic first order phase transition $T_{0}$ previously reported, our detailed investigation on an underdoped Ba$_{0.84}$K$_{0.16}$Fe$_{2}$As$_{2}$ single crystal unambiguously revealed that the transitions are not concomitant. The tetragonal ($τ$: I4/mmm) - orthorhombic ($\vartheta$: Fmmm) structural transition occurs at $T_{S}\simeq$ 110 K, followe… ▽ More In contrast to the simultaneous structural and magnetic first order phase transition $T_{0}$ previously reported, our detailed investigation on an underdoped Ba$_{0.84}$K$_{0.16}$Fe$_{2}$As$_{2}$ single crystal unambiguously revealed that the transitions are not concomitant. The tetragonal ($τ$: I4/mmm) - orthorhombic ($\vartheta$: Fmmm) structural transition occurs at $T_{S}\simeq$ 110 K, followed by an adjacent antiferromagnetic (AFM) transition at $T_{N}\simeq$ 102 K. Hysteresis and coexistence of the $τ$ and $\vartheta$ phases over a finite temperature range observed in our NMR experiments confirm the first order character of the structural transition and provide evidence that both $T_{S}$ and $T_{N}$ are strongly correlated. Our data also show that superconductivity (SC) develops in the $\vartheta$ phase below $T_{c}$ = 20 K and coexists with long range AFM. This new observation, $T_{S}\neq T_{N}$, firmly establishes another similarity between the hole-doped BaFe$_{2}$As$_{2}$ via K substitution and the electron-doped iron-arsenide superconductors. △ Less

Submitted 20 May, 2010; originally announced May 2010.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. Lett. 105, 107001 (2010)