Showing 1–50 of 209 results for author: Liang, R

Wafer-scale hybrid molecular beam epitaxy of BaTiO3 and SrTiO3 on silicon

Authors: Xiaodong Tian, Yan Lin, Hanbin Gao, Han Yu, Yunpeng Ma, Ruiqi Liang, Changfu Chen, Wei Li, Chenguang Deng, Qiang Zheng, Qian Li

Abstract: The integration of epitaxial barium titanate (BTO) on silicon represents a highly promising pathway for next-generation, energy-efficient photonic integrated circuits due to BTO's exceptionally high Pockels coefficients. However, the scalable epitaxy of BTO on Si remains hindered by complex stoichiometric control and slow growth rates. In this work, we demonstrate the continuous, uniform wafer-sca… ▽ More The integration of epitaxial barium titanate (BTO) on silicon represents a highly promising pathway for next-generation, energy-efficient photonic integrated circuits due to BTO's exceptionally high Pockels coefficients. However, the scalable epitaxy of BTO on Si remains hindered by complex stoichiometric control and slow growth rates. In this work, we demonstrate the continuous, uniform wafer-scale growth of high-quality BTO films on SrTiO3 (STO)-buffered 4-inch Si(001) wafers using a fully hybrid molecular beam epitaxy (hMBE) approach. By utilizing titanium tetraisopropoxide as a titanium precursor, we achieve a self-regulating, adsorption-controlled layer-by-layer growth at rates exceeding 75 nm/h, while maintaining an atomically sharp and structurally coherent BTO/STO interface. We systematically compare the structural, ferroelectric, and electro-optic (EO) properties of fully hMBE-grown BTO with those deposited via pulsed laser deposition (PLD) on identical STO/Si templates. While both techniques yield high-quality c-domain dominated films, the optimized hMBE-grown BTO exhibits superior crystallinity and a larger effective EO coefficient of 248 pm/V, surpassing that of the PLD-grown films (220 pm/V). These results highlight the advantages of the fully hMBE approach as a scalable, deterministic, and high-performance materials platform for wafer-scale integrated ferroelectric photonics. △ Less

Submitted 25 April, 2026; originally announced April 2026.

Comments: 22 pages, 4 figures

Persistent short-range charge correlations revealed by ultrafast melting of electronic order in YBa$_2$Cu$_3$O$_{6+x}$

Authors: C. Seo, L. Shen, A. N. Petsch, S. Wandel, V. Esposito, J. D. Koralek, G. L. Dakovski, M-F. Lin, S. P. Moeller, W. F. Schlotter, A. H. Reid, M. P. Minitti, R. Liang, D. Bonn, W. Hardy, A. Damascelli, C. Giannetti, E. H. da Silva Neto, J. J. Turner, F. Boschini, G. Coslovich

Abstract: Charge density waves (CDW) are ubiquitous in the complex phase diagram of cuprate superconductors and exhibit both short- and long-range correlations. Using time-resolved resonant X-ray scattering, we investigate the photo-induced dynamics of CDW in YBa$_2$Cu$_3$O$_{6.67}$. We discover an excitation threshold ($Φ$$_\mathrm{C}$ $\approx$ 65 $μ$J/cm$^2$) above which long-range CDW disappear, reveali… ▽ More Charge density waves (CDW) are ubiquitous in the complex phase diagram of cuprate superconductors and exhibit both short- and long-range correlations. Using time-resolved resonant X-ray scattering, we investigate the photo-induced dynamics of CDW in YBa$_2$Cu$_3$O$_{6.67}$. We discover an excitation threshold ($Φ$$_\mathrm{C}$ $\approx$ 65 $μ$J/cm$^2$) above which long-range CDW disappear, revealing a persistent CDW peak with short-range correlation length. Ultrafast photo-excitation promptly uncovers this residual short-range CDW correlations, appearing within $\approx$ 0.2 ps. Long-range CDW coherence recovers within $\approx$ 0.6 ps, while the peak intensity remains partially suppressed. We rationalize the dichotomic behavior in the fluence and temporal dependencies as the signature of two coexisting CDW peaks, arising from short- and long-range correlations, which we disentangle through their distinct response to photo-excitation. We provide evidence that the collapse of long-range correlations is driven by an electronic process, while short-range correlations are characterized by distinct timescales and stiffness against photo-excitation. This approach establishes ultrafast X-ray scattering as an effective tool for disentangling coexisting density waves and correlations in quantum materials. △ Less

Submitted 11 March, 2026; originally announced March 2026.

Comments: 8 pages, 5 figures

Optically driven thermodynamic transition from free- to locked-epitaxy

Authors: Renhong Liang, Mao Ye, Yiran Ying, Longlong Shu, Renkui Zheng, Haitao Huang, Jianhua Hao, Shuk-Yin Tong, Shanming Ke

Abstract: Controlling crystallographic orientation in quasi-van der Waals (vdW) epitaxy remains a fundamental challenge, especially for material systems located near the boundary between weakly and strongly coupled growth regimes. In such marginal systems, epitaxial selection is governed by a delicate thermodynamic competition between surface-energy penalties and interfacial interaction gains, giving rise t… ▽ More Controlling crystallographic orientation in quasi-van der Waals (vdW) epitaxy remains a fundamental challenge, especially for material systems located near the boundary between weakly and strongly coupled growth regimes. In such marginal systems, epitaxial selection is governed by a delicate thermodynamic competition between surface-energy penalties and interfacial interaction gains, giving rise to two archetypal limits: vdW-dominated free-epitaxy and strong interfacial coupling dominated locked-epitaxy. However, dynamically driving transitions between these regimes has remained elusive. Here, we demonstrate that external light irradiation can deterministically induce such a transition. Using the thermodynamically frustrated Fe4N/mica interface as a model system, we show that photo-excited carriers act as a chemical potentiator, significantly enhancing the interfacial chemical affinity. Within a quantitative thermodynamic description, this optical modulation increases the locking criterion (I_lock)-defined as the ratio of interfacial energy gain to surface-energy cost-beyond its critical threshold. As a result, the system switches from vdW-dominated free-epitaxy with (001) orientation to chemically locked-epitaxy with (111) orientation. Our findings establish light as a non-invasive and switchable control knob to dynamically reconfigure the interfacial energy landscape in quasi-vdW epitaxy, enabling programmable access to distinct epitaxial states beyond intrinsic material limitations. △ Less

Submitted 10 March, 2026; originally announced March 2026.

Comments: 20 pages, 4 figures

Universal thermodynamic framework for quasi-van der Waals epitaxy

Authors: Renhong Liang, Mao Ye, Renkui Zheng, Jianhua Hao, Haitao Huang, Longlong Shu, Shanming Ke

Abstract: van der Waals (vdW) epitaxy is conventionally regarded as a rotation-free and strain-free growth mode driven by weak, isotropic interactions, yet many interfaces paradoxically exhibit strictly locked orientations that defy standard surface-energy models. We resolve this inconsistency by establishing a unified quantitative framework for 2D-3D systems, in which strong electrostatic and chemical inte… ▽ More van der Waals (vdW) epitaxy is conventionally regarded as a rotation-free and strain-free growth mode driven by weak, isotropic interactions, yet many interfaces paradoxically exhibit strictly locked orientations that defy standard surface-energy models. We resolve this inconsistency by establishing a unified quantitative framework for 2D-3D systems, in which strong electrostatic and chemical interactions compete with entropic forces. We introduce a two-tier descriptor set-the predictive index (I_pre) and the thermodynamic locking criterion (I_lock)-to quantify the energetic sufficiency for locked epitaxy. Our theory accurately predicted the competitive interactions at the interface within the 2D-3D system, precisely characterized whether the epitaxial layer underwent free growth or was constrained in a locked growth mode, demonstrating robust consistency with diverse experimental observations. This framework unifies orientation selection in 3D-on-2D films and rotational locking in 2D-on-3D layers within a single-phase diagram. Our work provides a generalizable, predictive route to controlling epitaxial orientation across a broad spectrum of layered heterostructure △ Less

Submitted 25 December, 2025; originally announced December 2025.

Comments: 23 pages, 5 figures

Quasiparticle interference in LiFeAs: Signature of inelastic tunneling through spin fluctuations

Authors: Shun Chi, Carolina A. Marques, Walter N. Hardy, Ruixing Liang, Pinder Dosanjh, Doug A. Bonn, Sarah A. Burke, Peter Wahl

Abstract: Quasiparticle interference (QPI) is a powerful tool to characterize the symmetry of the superconducting order parameter in unconventional superconductors, by mapping the spatial dependence of elastic tunneling of electrons between the tip of a scanning tunneling microscope and a sample. Here, we consider the influence of inelastic tunneling on quasi-particle interference, exemplarily for the iron-… ▽ More Quasiparticle interference (QPI) is a powerful tool to characterize the symmetry of the superconducting order parameter in unconventional superconductors, by mapping the spatial dependence of elastic tunneling of electrons between the tip of a scanning tunneling microscope and a sample. Here, we consider the influence of inelastic tunneling on quasi-particle interference, exemplarily for the iron-based superconductor LiFeAs. We clearly observe replica features in both experimental QPI maps and the dispersion extracted from QPI, which from comparison with theoretical model calculations can be attributed to inelastic tunneling. Analysis of the QPI dispersion shows that the inelastic mode that gives rise to these replica features exhibits a resonance between 8 and 10 meV. Comparison of the energy scale of the resonance energy estimated from QPI with inelastic neutron scattering indicates that the replica features arise from interaction with spin fluctuations. △ Less

Submitted 24 June, 2026; v1 submitted 15 August, 2025; originally announced August 2025.

Comments: 9 pages/7 figures, replaced with published version

Journal ref: Physical Review B 113, 235148 (2026)

Thermal Hall conductivity in the strongest cuprate superconductor: Estimate of the mean free path in the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8 + δ}$

Authors: Munkhtuguldur Altangerel, Quentin Barthélemy, Étienne Lefrançois, Jordan Baglo, Manel Mezidi, Gaël Grissonnanche, Ashvini Vallipuram, Emma Campillo, Anne Forget, Dorothée Colson, Ruixing Liang, D. A. Bonn, W. N. Hardy, Cyril Proust, Louis Taillefer

Abstract: The thermal Hall conductivity of the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ (Hg1223) - the superconductor with the highest critical temperature $T_c$ at ambient pressure - was measured at temperatures down to 2 K for three dopings in the underdoped regime ($p$ = 0.09, 0.10, 0.11). By combining a previously introduced simple model and prior theoretical results, we derive a formula for the i… ▽ More The thermal Hall conductivity of the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ (Hg1223) - the superconductor with the highest critical temperature $T_c$ at ambient pressure - was measured at temperatures down to 2 K for three dopings in the underdoped regime ($p$ = 0.09, 0.10, 0.11). By combining a previously introduced simple model and prior theoretical results, we derive a formula for the inverse mean free path, $1 / \ell$, which allows us to estimate the mean free path of $d$-wave quasiparticles in Hg1223 below $T_c$. We find that $1 / \ell$ grows as $T^3$, in agreement with the theoretical expectation for a clean $d$-wave superconductor. Measurements were also conducted on the single layer mercury-based cuprate HgBa$_2$CuO$_{6+δ}$ (Hg1201), revealing that the mean free path in this compound is roughly half that of its three-layered counterpart at the same doping ($p$ = 0.10). This observation is be attributed to the protective role of the outer planes in Hg1223, which results in a more pristine inner plane. We also report data in an ultraclean crystal of YBa$_2$Cu$_3$O$_y$ (YBCO) with full oxygen content $p$ = 0.18, believed to be the cleanest of any cuprate, and find that $\ell$ is not longer than in Hg1223. △ Less

Submitted 10 August, 2025; v1 submitted 24 April, 2025; originally announced April 2025.

Journal ref: Physical Review B 112, 014522 (2025)

Signatures of two gaps in the spin susceptibility of a cuprate superconductor

Authors: R. Zhou, I. Vinograd, M. Hirata, T. Wu, H. Mayaffre, S. Krämer, W. N. Hardy, R. Liang, D. A. Bonn, T. Loew, J. Porras, B. Keimer, M. -H. Julien

Abstract: A major obstacle to understanding high-Tc cuprates is that superconductivity precludes observing normal-state properties at low temperatures. One prime example is the normal-state spin susceptibility \c{hi}spin: although its decrease upon cooling far above Tc typifies pseudogap behavior, its behavior at low temperatures is generally unknown. Here, our measurements in high magnetic fields expose \c… ▽ More A major obstacle to understanding high-Tc cuprates is that superconductivity precludes observing normal-state properties at low temperatures. One prime example is the normal-state spin susceptibility \c{hi}spin: although its decrease upon cooling far above Tc typifies pseudogap behavior, its behavior at low temperatures is generally unknown. Here, our measurements in high magnetic fields expose \c{hi}spin of YBa2Cu3Oy down to low temperatures. Even though superconductivity is suppressed by the field, we uncover two thermally-activated contributions alongside a residual \c{hi}spin(T=0) due to gapless excitations. We relate these two distinct gaps to short-range charge-density waves and to the formation of singlets as in certain quantum spin systems. Both phenomena thus contribute to the pseudogap at low temperature, supplementing short-lived antiferromagnetism that initiates pseudogap behavior at high temperatures. We therefore propose that the pseudogap ought to be regarded as a composite property and that, when not undergoing spin-stripe ordering, underdoped cuprates tend to form short-ranged spin singlets. △ Less

Submitted 13 February, 2025; v1 submitted 4 February, 2024; originally announced February 2024.

Comments: The final version is only at the publisher's site

Journal ref: Nature Physics 21, 97 (2025)

Absence of Fermi surface reconstruction in pressure-driven overdoped YBCO

Authors: Stanley W. Tozer, William A. Coniglio, Tobias Förster, Doug A. Bonn, Walter N. Hardy, Ruixing Liang, Erik Kampert, Audrey D. Grockowiak

Abstract: The evolution of the critical superconducting temperature and field, quantum oscillation frequencies and effective mass $m^{*}$ in underdoped YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) crystals ($p$ = 0.11, with $p$ the hole concentration per Cu atom) points to a partial suppression of the charge orders with increasing pressure up to 7 GPa, mimicking doping. Application of pressures up to 25 GPa pushes the sam… ▽ More The evolution of the critical superconducting temperature and field, quantum oscillation frequencies and effective mass $m^{*}$ in underdoped YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) crystals ($p$ = 0.11, with $p$ the hole concentration per Cu atom) points to a partial suppression of the charge orders with increasing pressure up to 7 GPa, mimicking doping. Application of pressures up to 25 GPa pushes the sample to the overdoped side of the superconducting dome. Contrary to other cuprates, or to doping studies on YBCO, the frequency of the quantum oscillations measured in that pressure range do not support the picture of a Fermi-surface reconstruction in the overdoped regime, but possibly point to the existence of a new charge order. △ Less

Submitted 3 December, 2023; originally announced December 2023.

Comments: 13 pages, 17 figures

Searching for the signature of a pair density wave in YBa$_2$Cu$_3$O$_{6.67}$ using high energy X-ray diffraction

Authors: Elizabeth Blackburn, Oleh Ivashko, Emma Campillo, Martin von Zimmermann, Ruixing Liang, Douglas A. Bonn, Walter N. Hardy, Johan Chang, Edward M. Forgan, Stephen M. Hayden

Abstract: We have carried out a search for a pair density wave signature using high-energy X-ray diffraction in fields up to 16 T. We do not see evidence for a signal at the predicted wavevector. This is a report on the details of our experiment, with information on where in reciprocal space we looked. We have carried out a search for a pair density wave signature using high-energy X-ray diffraction in fields up to 16 T. We do not see evidence for a signal at the predicted wavevector. This is a report on the details of our experiment, with information on where in reciprocal space we looked. △ Less

Submitted 27 October, 2023; originally announced October 2023.

Comments: 5 pages, report on experimental results

Planar thermal Hall effect from phonons in cuprates

Authors: Lu Chen, Léna Le Roux, Gaël Grissonnanche, Marie-Eve Boulanger, Steven Thériault, Ruixing Liang, D. A. Bonn, W. N. Hardy, S. Pyon, T. Takayama, H. Takagi, Kejun Xu, Zhi-Xun Shen, Louis Taillefer

Abstract: A surprising "planar" thermal Hall effect, whereby the field is parallel to the current, has recently been observed in a few magnetic insulators, and this has been attributed to exotic excitations such as Majorana fermions or chiral magnons. Here we investigate the possibility of a planar thermal Hall effect in three different cuprate materials, in which the conventional thermal Hall conductivity… ▽ More A surprising "planar" thermal Hall effect, whereby the field is parallel to the current, has recently been observed in a few magnetic insulators, and this has been attributed to exotic excitations such as Majorana fermions or chiral magnons. Here we investigate the possibility of a planar thermal Hall effect in three different cuprate materials, in which the conventional thermal Hall conductivity $κ_{\rm {xy}}$ (with an out-of-plane field perpendicular to the current) is dominated by either electrons or phonons. Our measurements show that the planar $κ_{\rm {xy}}$ from electrons in cuprates is zero, as expected from the absence of a Lorentz force in the planar configuration. By contrast, we observe a sizable planar $κ_{\rm {xy}}$ in those samples where the thermal Hall response is due to phonons, even though it should in principle be forbidden by the high crystal symmetry. Our findings call for a careful re-examination of the mechanisms responsible for the phonon thermal Hall effect in insulators. △ Less

Submitted 1 November, 2023; v1 submitted 11 October, 2023; originally announced October 2023.

Journal ref: Phys. Rev. X 14, 041011 (2024)

Flexo-photovoltaic effect and above-bandgap photovoltage in halide perovskites

Authors: Zhiguo Wang, Shengwen Shu, Xiaoyong Wei, Renhong Liang, Shanming Ke, Longlong Shu, Gustau Catalan

Abstract: Halide perovskites have outstanding photovoltaic properties which have been optimized through interfacial engineering. However, as these materials approach the limits imposed by the physics of semiconductor junctions, it is urgent to explore alternatives, such as the bulk photovoltaic effect, whose physical origin is different and not bound by the same limits. In this context, we focus on the flex… ▽ More Halide perovskites have outstanding photovoltaic properties which have been optimized through interfacial engineering. However, as these materials approach the limits imposed by the physics of semiconductor junctions, it is urgent to explore alternatives, such as the bulk photovoltaic effect, whose physical origin is different and not bound by the same limits. In this context, we focus on the flexo-photovoltaic effect, a type of bulk photovoltaic effect that was recently observed in oxides under strain gradients. We have measured the flexo-photovoltaic effect of MAPbBr3 and MAPbI3 crystals under bending and found it to be orders of magnitude larger than for SrTiO3, the benchmark flexo-photovoltaic oxide. For sufficiently large strain gradients, photovoltages bigger than the bandgap can be produced. Bulk photovoltaic effects are additive and, for MAPbI3, the flexo-photovoltage exists on top of a native bulk photovoltage that is hysteretic, consistent with the electrically switchable macroscopic polarization of this material. The results suggest that harnessing the flexo-photovoltaic effect through strain gradient engineering can provide a functional leap forward for halide perovskites. △ Less

Submitted 4 January, 2023; v1 submitted 9 September, 2022; originally announced September 2022.

Comments: 20 pages, 11 figures

No nematicity at the onset temperature of the pseudogap phase in the cuprate superconductor YBCO

Authors: G. Grissonnanche, O. Cyr-Choinière, J. Day, R. Liang, D. A. Bonn, W. N. Hardy, N. Doiron-Leyraud, L. Taillefer

Abstract: Electronic nematicity is the spontaneous loss of rotational symmetry in a metal, without breaking translational symmetry. In the cuprate superconductors, there is experimental evidence for nematicity, but its origin remains unclear. Here we investigate the onset of nematicity in the transport of charge by means of electric and thermoelectric measurements in underdoped YBa$_{\rm 2}$Cu$_{\rm 3}$O… ▽ More Electronic nematicity is the spontaneous loss of rotational symmetry in a metal, without breaking translational symmetry. In the cuprate superconductors, there is experimental evidence for nematicity, but its origin remains unclear. Here we investigate the onset of nematicity in the transport of charge by means of electric and thermoelectric measurements in underdoped YBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm y}$, performed by passing the current (electrical or thermal) first along the $a$ axis then the $b$ axis of the orthorhombic structure in the same crystal, with a hole doping $p = 0.12$. Upon cooling, we observe no additional in-plane anisotropy -- beyond the background anisotropy due to the CuO chains -- in either the resistivity $ρ$ or the Seebeck coefficient $S$ as the temperature $T^{\star}$~for the onset of the pseudogap phase is crossed. We conclude that the pseudogap phase of cuprates is not nematic. However, at temperatures much lower than $T^{\star}$, a strong additional anisotropy is observed, most clearly in the Peltier coefficient $α= S / ρ$. We interpret it as nematicity associated with the development of charge order. △ Less

Submitted 14 February, 2023; v1 submitted 10 May, 2022; originally announced May 2022.

Comments: 7 pages, 4 figures

Journal ref: Physical Review X 13, 031010 (2023)

Emergence of cooperation in a population with bimodal response behaviors

Authors: Lin Ma, Jiqiang Zhang, Guozhong Zheng, Rizhou Liang, Li Chen

Abstract: We human beings show remarkable adaptability in response to complex surroundings, we adopt different behavioral modes at different occasions, such response multimodality is critical to our survival. Yet, how this behavioral multimodality affects the evolution of cooperation remains largely unknown. Here we build a toy model to address this issue by considering a population with bimodal response be… ▽ More We human beings show remarkable adaptability in response to complex surroundings, we adopt different behavioral modes at different occasions, such response multimodality is critical to our survival. Yet, how this behavioral multimodality affects the evolution of cooperation remains largely unknown. Here we build a toy model to address this issue by considering a population with bimodal response behaviors, or specifically, with the Fermi and Tit-for-tat updating rules. While the former rule tends to imitate the strategies of those neighbors who are doing well, the latter repeats what their neighbors did to them. In a structural mixing implementation, where the updating rule is fixed for each individual, we find that a moderate mode mixture unexpectedly boosts the overall cooperation level of the population. The boost is even more pronounced in the probabilistic mixing, where each individual randomly chooses one of the two modes at each step, and full cooperation is seen in a wide range. These findings are robust to the underlying topology of the population. Our mean-field treatment reveals that the cooperation prevalence within the players with the Fermi rule linearly increases with the fraction of TFT players and explains the non-monotonic dependence in the structural mixing. Our study shows that the diversity in response behaviors may help to explain the emergence of cooperation in realistic contexts. △ Less

Submitted 5 January, 2023; v1 submitted 9 May, 2022; originally announced May 2022.

Comments: 23 pages, 9 figures

Journal ref: Chaos, Solitons & Fractals 171, 113452(2023)

Probabilistic fair behaviors spark its boost in the Ultimatum Game: the strength of good Samaritans

Authors: Guozhong Zheng, Jiqiang Zhang, Rizhou Liang, Lin Ma, Li Chen

Abstract: Behavioral experiments on the Ultimatum Game have shown that we human beings have remarkable preference in fair play, contradicting the predictions by the game theory. Most of the existing models seeking for explanations, however, strictly follow the assumption of \emph{Homo economicus} in orthodox Economics that people are self-interested and fully rational to maximize their earnings. Here we rel… ▽ More Behavioral experiments on the Ultimatum Game have shown that we human beings have remarkable preference in fair play, contradicting the predictions by the game theory. Most of the existing models seeking for explanations, however, strictly follow the assumption of \emph{Homo economicus} in orthodox Economics that people are self-interested and fully rational to maximize their earnings. Here we relax this assumption by allowing that people probabilistically choose to be "good Samaritans", acting as fair players from time to time. For well-mixed and homogeneously structured populations, we numerically show that as this probability increases the level of fairness undergoes from the low scenario abruptly to the full fairness state, where occasional fair behaviors ($\sim5\%$) are sufficient to drive the whole population to behave in the half-half split manner. We also develop a mean-field theory, which correctly reproduces the first-order phase transition and points out that the bistability is an intrinsic property of this game and small fair acts lead to dramatical change due to its bifurcation structure. Heterogeneously structured populations, however, display continuous fairness transition; surprisingly, very few hub nodes acting as fair players are able to entrain the whole population to the full fairness state. Our results thus reveal the unexpected strength of "good Samaritans", which may constitute a new explanation for the emergence of fairness in our society. △ Less

Submitted 12 February, 2022; originally announced February 2022.

Comments: 8 pages, 6 figures

Journal ref: J. Phys. Complex. 3 035004(2022)

Acoustically Induced Giant Synthetic Hall Voltages in Graphene

Authors: Pai Zhao, Chithra H. Sharma, Renrong Liang, Christian Glasenapp, Lev Mourokh, Vadim M. Kovalev, Patrick Huber, Marta Prada, Lars Tiemann, Robert H. Blick

Abstract: Any departure from graphene's flatness leads to the emergence of artificial gauge fields that act on the motion of the Dirac fermions through an associated pseudomagnetic field. Here, we demonstrate the tunability of strong gauge fields in non-local experiments using a large planar graphene sheet that conforms to the deformation of a piezoelectric layer by a surface acoustic wave. The acoustic wav… ▽ More Any departure from graphene's flatness leads to the emergence of artificial gauge fields that act on the motion of the Dirac fermions through an associated pseudomagnetic field. Here, we demonstrate the tunability of strong gauge fields in non-local experiments using a large planar graphene sheet that conforms to the deformation of a piezoelectric layer by a surface acoustic wave. The acoustic wave induces a longitudinal and a giant synthetic Hall voltage in the absence of external magnetic fields. The superposition of a synthetic Hall potential and a conventional Hall voltage can annihilate the sample's transversal potential at large external magnetic fields. Surface acoustic waves thus provide a promising and facile avenue for the exploit of gauge fields in large planar graphene systems. △ Less

Submitted 7 July, 2022; v1 submitted 22 December, 2021; originally announced December 2021.

Journal ref: Phys. Rev. Lett. 128, 256601 (2022)

The Three-Dimensional Electronic Structure of LiFeAs: Strong-coupling Superconductivity and Topology in the Iron Pnictides

Authors: Ryan P. Day, MengXing Na, Manuel Zingl, Berend Zwartsenberg, Matteo Michiardi, Giorgio Levy, Michael Schneider, Doug Wong, Pinder Dosanjh, Tor M. Pedersen, Sergey Gorovikov, Shun Chi, Ruixing Liang, Walter N. Hardy, Douglas A. Bonn, Sergey Zhdanovich, Ilya S. Elfimov, Andrea Damascelli

Abstract: Amongst the iron-based superconductors, LiFeAs is unrivalled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here, we explore the challenge of interpret… ▽ More Amongst the iron-based superconductors, LiFeAs is unrivalled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here, we explore the challenge of interpretation in such experiments. By combining comprehensive photon energy- and polarization- dependent angle-resolved photoemission spectroscopy (ARPES) measurements with numerical simulations, we establish the providence of several contradictions in the present understanding of this and related materials. We identify a confluence of surface-related issues which have precluded unambiguous identification of both the number and dimensionality of the Fermi surface sheets. Ultimately, we arrive at a scenario which supports indications of topologically non-trivial states, while also being incompatible with superconductivity as a spin-fluctuation driven Fermi surface instability. △ Less

Submitted 27 September, 2021; originally announced September 2021.

Comments: 8 pages, 4 figures

Report number: Phys. Rev. B 105, 155142

Detecting Carbon Nanotube Orientation with Topological Data Analysis of Scanning Electron Micrographs

Authors: Liyu Dong, Haibin Hang, Jin Gyu Park, Washington Mio, Richard Liang

Abstract: As the aerospace industry becomes increasingly demanding for stronger lightweight materials, the ultra-strong carbon nanotube (CNT) composites with highly aligned CNT network structures could be the answer. In this work, a novel methodology applying topological data analysis (TDA) to the scanning electron microscope (SEM) images was developed to detect CNT orientation. The CNT bundle extensions in… ▽ More As the aerospace industry becomes increasingly demanding for stronger lightweight materials, the ultra-strong carbon nanotube (CNT) composites with highly aligned CNT network structures could be the answer. In this work, a novel methodology applying topological data analysis (TDA) to the scanning electron microscope (SEM) images was developed to detect CNT orientation. The CNT bundle extensions in certain directions were summarized algebraically and expressed as visible barcodes. The barcodes were then calculated and converted into the total spread function $V(X,θ)$, from which the alignment fraction and the preferred direction could be determined. For validation purposes, the random CNT sheets were mechanically stretched at various strain ratios ranging from $0-40\%$, and quantitative TDA analysis was conducted based on the SEM images taken at random positions. The results showed high consistency ($R^2=0.975$) compared to the Herman's orientation factors derived from the polarized Raman spectroscopy and wide-angle X-ray scattering analysis. Additionally, the TDA method presented great robustness with varying SEM acceleration voltages and magnifications, which might alter the scope in alignment detection. With potential applications in nanofiber systems, this study offers a rapid and simple way to quantify CNT alignment, which plays a crucial role in transferring the CNT properties into engineering products. △ Less

Submitted 9 August, 2021; originally announced August 2021.

A multi-component Fermi surface in the vortex state of an underdoped high-Tc superconductor

Authors: Suchitra E. Sebastian, N. Harrison, E. Palm, T. P. Murphy, C. H. Mielke, Ruixing Liang, D. A. Bonn, W. N. Hardy, G. G. Lonzarich

Abstract: In order to understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high Tc cuprate superconductors have revealed the existence of a Fermi surface akin to normal metals, comprising fermionic carriers that undergo orbital quantization. However, the unexpectedly… ▽ More In order to understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high Tc cuprate superconductors have revealed the existence of a Fermi surface akin to normal metals, comprising fermionic carriers that undergo orbital quantization. However, the unexpectedly small size of the observed carrier pocket leaves open a variety of possibilities as to the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we present quantum oscillations in the magnetisation (the de Haas-van Alphen or dHvA effect) observed in superconducting YBa2Cu3O6.51 that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave scenario with spiral or related modulated magnetic order, consistent with experimental observations. △ Less

Submitted 31 March, 2021; originally announced March 2021.

Comments: archiving older manuscript

Journal ref: Nature 454, 200-203 (2008)

Locally commensurate charge-density wave with three-unit-cell periodicity in YBCO

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

Abstract: In order to identify the mechanism responsible for the formation of charge-density waves (CDW) in cuprate superconductors, it is important to understand which aspects of the CDW's microscopic structure are generic and which are material-dependent. Here, we show that, at the local scale probed by NMR, long-range CDW order in YBa2Cu3Oy is unidirectional with a commensurate period of three unit cells… ▽ More In order to identify the mechanism responsible for the formation of charge-density waves (CDW) in cuprate superconductors, it is important to understand which aspects of the CDW's microscopic structure are generic and which are material-dependent. Here, we show that, at the local scale probed by NMR, long-range CDW order in YBa2Cu3Oy is unidirectional with a commensurate period of three unit cells (lambda = 3b), implying that the incommensurability found in X-ray scattering is ensured by phase slips (discommensurations). Furthermore, NMR spectra reveal a predominant oxygen character of the CDW with an out-of-phase relationship between certain lattice sites but no specific signature of a secondary CDW with lambda = 6b associated with a putative pair-density wave. These results shed light on universal aspects of the cuprate CDW. In particular, its spatial profile appears to generically result from the interplay between an incommensurate tendency at long length scales, possibly related to properties of the Fermi surface, and local commensuration effects, due to electron-electron interactions or lock-in to the lattice. △ Less

Submitted 24 March, 2021; originally announced March 2021.

Comments: Original submission (revised version available upon request)

Journal ref: Nat Commun 12, 3274 (2021)

Unconventional short-range structural fluctuations in cuprate high-$T_c$ superconductors

Authors: D. Pelc, R. J. Spieker, Z. W. Anderson, M. J. Krogstad, N. Biniskos, N. G. Bielinski, B. Yu, T. Sasagawa, L. Chauviere, P. Dosanjh, R. Liang, D. A. Bonn, A. Damascelli, S. Chi, Y. Liu, R. Osborn, M. Greven

Abstract: The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic pr… ▽ More The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic properties. Here we use neutron and X-ray diffuse scattering to uncover universal structural fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ and Tl$_2$Ba$_2$CuO$_{6+δ}$, two cuprate superconductors with distinct point disorder effects and optimal superconducting transition temperatures. The fluctuations are present in wide doping and temperature ranges, including compositions that maintain high average structural symmetry, and they exhibit unusual, yet simple scaling behavior. The scaling regime is robust and universal, similar to the well-known critical fluctuations close to second-order phase transitions, but with a distinctly different physical origin. We relate this behavior to pre-transitional phenomena in a broad class of systems with structural and magnetic transitions, and propose an explanation based on rare structural fluctuations caused by intrinsic nanoscale inhomogeneity. We also uncover parallels with superconducting fluctuations, which indicates that the underlying inhomogeneity plays an important role in cuprate physics. △ Less

Submitted 5 December, 2022; v1 submitted 9 March, 2021; originally announced March 2021.

Comments: 24 pages, 4 figures

Journal ref: Sci. Rep. 12, 20483 (2022)

Dynamical reciprocity in interacting games: numerical results and mechanism analysis

Authors: Rizhou Liang, Qinqin Wang, Jiqiang Zhang, Guozhong Zheng, Lin Ma, Li Chen

Abstract: We study the evolution of two mutually interacting games with both pairwise games as well as the public goods game on different topologies. On 2d square lattices, we reveal that the game-game interaction can promote the cooperation prevalence in all cases, and the cooperation-defection phase transitions even become absent and fairly high cooperation is expected when the interaction goes to be very… ▽ More We study the evolution of two mutually interacting games with both pairwise games as well as the public goods game on different topologies. On 2d square lattices, we reveal that the game-game interaction can promote the cooperation prevalence in all cases, and the cooperation-defection phase transitions even become absent and fairly high cooperation is expected when the interaction goes to be very strong. A mean-field theory is developed that points out new dynamical routes arising therein. Detailed analysis shows indeed that there are rich categories of interactions in either individual or bulk scenario: invasion, neutral, and catalyzed types; their combination puts cooperators at a persistent advantage position, which boosts the cooperation. The robustness of the revealed reciprocity is strengthened by the studies of model variants, including asymmetrical or time-varying interactions, games of different types, games with time-scale separation, different updating rules etc. The structural complexities of the underlying population, such as Newman--Watts small world networks, Erdős--Rényi random networks, and Barabási--Albert networks, also do not alter the working of the dynamical reciprocity. In particular, as the number of games engaged increases, the cooperation level continuously improves in general. Our work thus uncovers a new class of cooperation mechanism and indicates the great potential for human cooperation where concurrent issues are so often seen in the real world. △ Less

Submitted 2 June, 2021; v1 submitted 30 January, 2021; originally announced February 2021.

Comments: 22 pages, 19 figures, 2 tables, a companion short paper (arXiv:2101.00359) is also posted. Any comments are welcomed

Journal ref: Phys Rev. E, 105, 054302(2022)

Emergent route towards cooperation in interacting games: the dynamical reciprocity

Authors: Qinqin Wang, Rizhou Liang, Jiqiang Zhang, Guozhong Zheng, Lin Ma, Li Chen

Abstract: The success of modern civilization is built upon widespread cooperation in human society, deciphering the mechanisms behind has being a major goal for centuries. A crucial fact is, however, largely missing in most prior studies that games in the real world are typically played simultaneously and interactively rather than separately as assumed. Here we introduce the idea of interacting games that d… ▽ More The success of modern civilization is built upon widespread cooperation in human society, deciphering the mechanisms behind has being a major goal for centuries. A crucial fact is, however, largely missing in most prior studies that games in the real world are typically played simultaneously and interactively rather than separately as assumed. Here we introduce the idea of interacting games that different games coevolve and influence each other's decision-making. We show that as the game-game interaction becomes important, the cooperation phase transition dramatically improves, a fairly high level of cooperation is reached for all involved games when interaction goes to be strong. A mean-field theory indicates that a new mechanism -- \emph{the dynamical reciprocity}, as a counterpart to the well-known network reciprocity, is at work to foster cooperation, which is confirmed by the detailed analysis. This revealed reciprocity is robust against variations in the game type, the population structure, and the updating rules etc, and more games generally yield a higher level of cooperation. Our findings point out the great potential towards high cooperation for many issues are interwoven with each other in the real world, and also the possibility of sustaining decent cooperation even in extremely adverse circumstances. △ Less

Submitted 2 June, 2021; v1 submitted 30 January, 2021; originally announced February 2021.

Comments: 5 pages, 4 figures, 1 table, a companion long paper (arXiv:2102.00360) is also posted. Comments are welcomed

Thermal Hall conductivity in the cuprate Mott insulators Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$

Authors: Marie-Eve Boulanger, Gaël Grissonnanche, Sven Badoux, Andréanne Allaire, Étienne Lefrançois, Anaëlle Legros, Adrien Gourgout, Maxime Dion, C. H. Wang, X. H. Chen, R. Liang, W. N. Hardy, D. A Bonn, Louis Taillefer

Abstract: The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La$_2$CuO$_4$ were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures… ▽ More The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La$_2$CuO$_4$ were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders - Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$ - and show that two potential mechanisms can be excluded - the scattering of phonons by rare-earth impurities and by structural domains. Our comparative study further reveals that orthorhombicity, apical oxygens, the tilting of oxygen octahedra and the canting of spins out of the CuO$_2$ planes are not essential to the mechanism of chirality. Our findings point to a chiral mechanism coming from a coupling of acoustic phonons to the intrinsic excitations of the CuO$_2$ planes. △ Less

Submitted 9 July, 2020; originally announced July 2020.

Comments: 29 pages, 8 figures

Journal ref: Nature Communications 11, 5325 (2020)

Enhanced charge density wave coherence in a light-quenched, high-temperature superconductor

Authors: S. Wandel, F. Boschini, E. H. da Silva Neto, L. Shen, M. X. Na, S. Zohar, Y. Wang, S. B. Welch, M. H. Seaberg, J. D. Koralek, G. L. Dakovski, W. Hettel, M-F. Lin, S. P. Moeller, W. F. Schlotter, A. H. Reid, M. P. Minitti, T. Boyle, F. He, R. Sutarto, R. Liang, D. Bonn, W. Hardy, R. A. Kaindl, D. G. Hawthorn , et al. (6 additional authors not shown)

Abstract: Superconductivity and charge density waves (CDW) are competitive, yet coexisting orders in cuprate superconductors. To understand their microscopic interdependence, a probe capable of discerning their interaction on its natural length and time scales is necessary. We use ultrafast resonant soft x-ray scattering to track the transient evolution of CDW correlations in YBa$_{2}$Cu$_{3}$O$_{6+x}$ foll… ▽ More Superconductivity and charge density waves (CDW) are competitive, yet coexisting orders in cuprate superconductors. To understand their microscopic interdependence, a probe capable of discerning their interaction on its natural length and time scales is necessary. We use ultrafast resonant soft x-ray scattering to track the transient evolution of CDW correlations in YBa$_{2}$Cu$_{3}$O$_{6+x}$ following the quench of superconductivity by an infrared laser pulse. We observe a non-thermal response of the CDW order characterized by a near doubling of the correlation length within $\approx$ 1 picosecond of the superconducting quench. Our results are consistent with a model in which the interaction between superconductivity and CDW manifests inhomogeneously through disruption of spatial coherence, with superconductivity playing the dominant role in stabilizing CDW topological defects, such as discommensurations. △ Less

Submitted 26 May, 2022; v1 submitted 9 March, 2020; originally announced March 2020.

Comments: Accepted version. 34 pages, 11 figures, Main text and Supplementary Materials

Journal ref: Science 376, 860 (2022)

Many-body recombination in insulating cuprates

Authors: Derek G. Sahota, Ruixing Liang, M. Dion, Patrick Fournier, Hanna A. Dąbkowska, Graeme M. Luke, J. Steven Dodge

Abstract: We study the pump-probe response of three insulating cuprates and develop a model for its recombination kinetics. The dependence on time, fluence, and both pump and probe photon energies imply many-body recombination on femtosecond timescales, characterized by anomalously large trapping and Auger coefficients. The fluence dependence follows a universal form that includes a characteristic volume sc… ▽ More We study the pump-probe response of three insulating cuprates and develop a model for its recombination kinetics. The dependence on time, fluence, and both pump and probe photon energies imply many-body recombination on femtosecond timescales, characterized by anomalously large trapping and Auger coefficients. The fluence dependence follows a universal form that includes a characteristic volume scale, which we associate with the holon-doublon excitation efficiency. This volume varies strongly with pump photon energy and peaks near twice the charge-transfer energy, suggesting that the variation is caused by carrier multiplication through impact ionization. △ Less

Submitted 3 December, 2019; originally announced December 2019.

Journal ref: Phys. Rev. Research 1, 033214 (2019)

Spatially Inhomogeneous Competition between Superconductivity and the Charge Density Wave in YBa$_2$Cu$_3$O$_{6.67}$

Authors: J. Choi, O. Ivashko, E. Blackburn, R. Liang, D. A. Bonn, W. N. Hardy, A. T. Holmes, N. B. Christensen, M. Hücker, S. Gerber, O. Gutowski, U. Rütt, M. v. Zimmermann, E. M. Forgan, S. M. Hayden, J. Chang

Abstract: The charge density wave in the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) is now known to have two different ordering tendencies differentiated by their $c$-axis correlations. These correspond to ferro- (F-CDW) and antiferro- (AF-CDW) couplings between CDW in neighbouring CuO$_2$ bilayers. This discovery has prompted a number of fundamental questions. For example, how does super… ▽ More The charge density wave in the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) is now known to have two different ordering tendencies differentiated by their $c$-axis correlations. These correspond to ferro- (F-CDW) and antiferro- (AF-CDW) couplings between CDW in neighbouring CuO$_2$ bilayers. This discovery has prompted a number of fundamental questions. For example, how does superconductivity adjust to two competing orders and are either of these orders responsible for the electronic reconstruction? Here we use high-energy x-ray diffraction to study YBa$_2$Cu$_3$O$_{6.67}$ as a function of magnetic field and temperature. We show that regions of the sample with F-CDW correlations suppress superconductivity more strongly than those with AF-CDW correlations. This implies that an inhomogeneous superconducting state exists, in which some regions show a weak or fragile form of superconductivity. By comparison of F-CDW and AF-CDW correlation lengths, it is furthermore concluded that F-CDW ordering is sufficiently long-range to modify the electronic structure. Our study thus suggests that F-CDW correlations have an important impact on superconducting and normal state properties of underdoped YBCO. △ Less

Submitted 2 March, 2020; v1 submitted 20 September, 2019; originally announced September 2019.

Comments: 9 pages, 5 figures, and supplementary information

Journal ref: Nature Communications 11, 990 (2020)

NMR study of charge density waves under hydrostatic pressure in YBa2Cu3Oy

Authors: I. Vinograd, R. Zhou, H. Mayaffre, S. Krämer, R. Liang, W. N. Hardy, D. A. Bonn, M. -H. Julien

Abstract: The effect of hydrostatic pressure (P) on charge density waves (CDW) in YBa2Cu3Oy has recently been controversial. Using NMR, we find that both the short-range CDW in the normal state and the long-range CDW in high fields are, at most, slightly weakened at P=1.9 GPa. This result is in contradiction with x-ray scattering results finding complete suppression of the CDW at ~1 GPa and we discuss possi… ▽ More The effect of hydrostatic pressure (P) on charge density waves (CDW) in YBa2Cu3Oy has recently been controversial. Using NMR, we find that both the short-range CDW in the normal state and the long-range CDW in high fields are, at most, slightly weakened at P=1.9 GPa. This result is in contradiction with x-ray scattering results finding complete suppression of the CDW at ~1 GPa and we discuss possible explanations of this discrepancy. Quantitative analysis, however, shows that the NMR data is not inconsistent with a disappearance of the CDW on a larger pressure scale, typically ~10-20 GPa. We also propose a simple model reconciling transport data with such a hypothesis, provided the pressure-induced change in doping is taken into account. We conclude that it is therefore possible that most of the spectacular increase in Tc upon increasing pressure up to ~15~GPa arises from a concomitant decrease of CDW strength. △ Less

Submitted 1 September, 2019; originally announced September 2019.

Journal ref: Phys. Rev. B 100, 094502 (2019)

Orbital Symmetries of Charge Density Wave Order in YBa2Cu3O6+x

Authors: Christopher McMahon, A. J. Achkar, E. H. da Silva Neto, I. Djianto, J. Menard, F. He, R. Sutarto, R. Comin, Ruixing Liang, D. A. Bonn, W. N. Hardy, A. Damascelli, D. G. Hawthorn

Abstract: Charge density wave (CDW) order has been shown to compete and coexist with superconductivity in underdoped cuprates. Theoretical proposals for the CDW order include an unconventional $d$-symmetry form factor CDW, evidence for which has emerged from measurements, including resonant soft x-ray scattering (RSXS) in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO). Here, we revisit RSXS measurements of the CDW symmetry… ▽ More Charge density wave (CDW) order has been shown to compete and coexist with superconductivity in underdoped cuprates. Theoretical proposals for the CDW order include an unconventional $d$-symmetry form factor CDW, evidence for which has emerged from measurements, including resonant soft x-ray scattering (RSXS) in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO). Here, we revisit RSXS measurements of the CDW symmetry in YBCO, using a variation in the measurement geometry to provide enhanced sensitivity to orbital symmetry. We show that the $(0\ 0.31\ L)$ CDW peak measured at the Cu $L$ edge is dominated by an $s$ form factor rather than a $d$ form factor as was reported previously. In addition, by measuring both $(0.31\ 0\ L)$ and $(0\ 0.31\ L)$ peaks, we identify a pronounced difference in the orbital symmetry of the CDW order along the $a$ and $b$ axes, with the CDW along the $a$ axis exhibiting orbital order in addition to charge order. △ Less

Submitted 9 November, 2020; v1 submitted 29 April, 2019; originally announced April 2019.

Comments: 17 pages, 4 figures + supplementary information

Journal ref: Science Advances, 6 (45), eaay0345 (2020)

Resolving the nature of electronic excitations in resonant inelastic x-ray scattering

Authors: M. Kang, J. Pelliciari, Y. Krockenberger, J. Li, D. E. McNally, E. Paris, R. Liang, W. N. Hardy, D. A. Bonn, H. Yamamoto, T. Schmitt, R. Comin

Abstract: The study of elementary bosonic excitations is essential toward a complete description of quantum electronic solids. In this context, resonant inelastic X-ray scattering (RIXS) has recently risen to becoming a versatile probe of electronic excitations in strongly correlated electron systems. The nature of the radiation-matter interaction endows RIXS with the ability to resolve the charge, spin and… ▽ More The study of elementary bosonic excitations is essential toward a complete description of quantum electronic solids. In this context, resonant inelastic X-ray scattering (RIXS) has recently risen to becoming a versatile probe of electronic excitations in strongly correlated electron systems. The nature of the radiation-matter interaction endows RIXS with the ability to resolve the charge, spin and orbital nature of individual excitations. However, this capability has been only marginally explored to date. Here, we demonstrate a systematic method for the extraction of the character of excitations as imprinted in the azimuthal dependence of the RIXS signal. Using this novel approach, we resolve the charge, spin, and orbital nature of elastic scattering, (para-)magnon/bimagnon modes, and higher energy dd excitations in magnetically-ordered and superconducting copper-oxide perovskites (Nd2CuO4 and YBa2Cu3O6.75). Our method derives from a direct application of scattering theory, enabling us to deconstruct the complex scattering tensor as a function of energy loss. In particular, we use the characteristic tensorial nature of each excitation to precisely and reliably disentangle the charge and spin contributions to the low energy RIXS spectrum. This procedure enables to separately track the evolution of spin and charge spectral distributions in cuprates with doping. Our results demonstrate a new capability that can be integrated into the RIXS toolset, and that promises to be widely applicable to materials with intertwined spin, orbital, and charge excitations. △ Less

Submitted 19 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. B 99, 045105 (2019), https://link.aps.org/doi/10.1103/PhysRevB.99.045105

Logarithmic Upturn in Low-Temperature Electronic Transport as a Signature of d-Wave Order in Cuprate Superconductors

Authors: Xiaoqing Zhou, D. C. Peets, Benjamin Morgan, W. A. Huttema, N. C. Murphy, E. Thewalt, C. J. S. Truncik, P. J. Turner, A. J. Koenig, J. R. Waldram, A. Hosseini, Ruixing Liang, D. A. Bonn, W. N. Hardy, D. M. Broun

Abstract: In cuprate superconductors, high magnetic fields have been used extensively to suppress superconductivity and expose the underlying normal state. Early measurements revealed insulating-like behavior in underdoped material versus temperature $T$, in which resistivity increases on cooling with a puzzling $\log(1/T)$ form. We instead use microwave measurements of flux-flow resistivity in YBa$_2$Cu… ▽ More In cuprate superconductors, high magnetic fields have been used extensively to suppress superconductivity and expose the underlying normal state. Early measurements revealed insulating-like behavior in underdoped material versus temperature $T$, in which resistivity increases on cooling with a puzzling $\log(1/T)$ form. We instead use microwave measurements of flux-flow resistivity in YBa$_2$Cu$_3$O$_{6+y}$ and Tl$_2$Ba$_2$CuO$_{6+δ}$ to study charge transport deep inside the superconducting phase, in the low temperature and low field regime. Here, the transition from metallic low-temperature resistivity ($dρ/dT>0$) to a $\log(1/T)$ upturn persists throughout the superconducting doping range, including a regime at high carrier dopings in which the field-revealed normal-state resistivity is Fermi-liquid-like. The $\log(1/T)$ form is thus likely a signature of $d$-wave superconducting order, and the field-revealed normal state's $\log(1/T)$ resistivity may indicate the free-flux-flow regime of a phase-disordered $d$-wave superconductor. △ Less

Submitted 29 November, 2018; originally announced November 2018.

Comments: 6 pages, 3 figures + Supplementary Material

Probing itinerant antiferromagnetism with $d$-wave Andreev reflection spectroscopy

Authors: C. R. Granstrom, R. -X. Liang, Y. Li, P. Li, Z. -H. Lu, E. Svanidze, E. Morosan, J. Y. T. Wei

Abstract: To study how Andreev reflection (AR) is affected by itinerant antiferromagnetism, we perform $d$-wave AR spectroscopy with superconducting YBa$_2$Cu$_3$O$_{7-δ}$ on TiAu and on variously-oxidized Nb (NbO$_x$) samples. X-ray photoelectron spectroscopy is also used on the latter to measure their surface oxide composition. Below the Néel temperatures ($T_N$) of both TiAu and NbO$_x$, the conductance… ▽ More To study how Andreev reflection (AR) is affected by itinerant antiferromagnetism, we perform $d$-wave AR spectroscopy with superconducting YBa$_2$Cu$_3$O$_{7-δ}$ on TiAu and on variously-oxidized Nb (NbO$_x$) samples. X-ray photoelectron spectroscopy is also used on the latter to measure their surface oxide composition. Below the Néel temperatures ($T_N$) of both TiAu and NbO$_x$, the conductance spectra show a dip-like structure instead of a zero-bias peak within the superconducting energy gap; for NbO$_x$, higher-oxidized samples show a stronger spectral dip at zero bias. These observations indicate that itinerant antiferromagnetic order suppresses the AR process. Interestingly, the spectral dip persists above $T_N$ for both TiAu and NbO$_x$, implying that spin fluctuations can also suppress AR. Our results suggest that $d$-wave AR spectroscopy may be used to probe the degree of spin ordering in itinerant antiferromagnets. △ Less

Submitted 31 October, 2018; originally announced November 2018.

Reply to "Comment on `No evidence for orbital loop currents in charge-ordered YBa$_2$Cu$_3$O$_{6+x}$ from polarized neutron diffraction' "

Authors: T. P. Croft, E. Blackburn, J. Kulda, Ruixing Liang, D. A. Bonn, W. N Hardy, S. M. Hayden

Abstract: The issues raised in the preceding comment of Bourges et al. [arXiv:1710.08173, Phys. Rev. B 98, 016501 (2018)] are shown to be unfounded. We highlight the complications caused by inhomogeneous beam polarization that can occur when using polarized neutron diffraction to detect small magnetic moments. The issues raised in the preceding comment of Bourges et al. [arXiv:1710.08173, Phys. Rev. B 98, 016501 (2018)] are shown to be unfounded. We highlight the complications caused by inhomogeneous beam polarization that can occur when using polarized neutron diffraction to detect small magnetic moments. △ Less

Submitted 17 July, 2018; originally announced July 2018.

Comments: 8 pages, supplementary material included

Journal ref: Phys. Rev. B 98, 016502 (2018)

Unusual interplay between superconductivity and field-induced charge order in YBa2Cu3Oy

Authors: J. Kacmarcik, I. Vinograd, B. Michon, A. Rydh, A. Demuer, R. Zhou, H. Mayaffre, R. Liang, W. Hardy, D. A. Bonn, N. Doiron-Leyraud, L. Taillefer, M. -H. Julien, C. Marcenat, T. Klein

Abstract: We present a detailed study of the temperature (T) and magnetic field (H) dependence of the electronic density of states (DOS) at the Fermi level, as deduced from specific heat and Knight shift measurements in underdoped YBa2Cu3Oy. We find that the DOS becomes field-independent above a characteristic field H_{DOS} and that the H_{DOS}(T) line displays an unusual inflection near the onset of the lo… ▽ More We present a detailed study of the temperature (T) and magnetic field (H) dependence of the electronic density of states (DOS) at the Fermi level, as deduced from specific heat and Knight shift measurements in underdoped YBa2Cu3Oy. We find that the DOS becomes field-independent above a characteristic field H_{DOS} and that the H_{DOS}(T) line displays an unusual inflection near the onset of the long range 3D charge-density wave order. The unusual S-shape of H_{DOS}(T) is suggestive of two mutually-exclusive orders that eventually establish a form of cooperation in order to coexist at low T. On theoretical grounds, such a collaboration could result from the stabilisation of a pair-density wave state, which calls for further investigations in this region of the phase diagram △ Less

Submitted 17 May, 2018; originally announced May 2018.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 121, 167002 (2018)

Discovery of a strain-stabilised smectic electronic order in LiFeAs

Authors: Chi Ming Yim, Christopher Trainer, Ramakrishna Aluru, Shun Chi, Walter N. Hardy, Ruixing Liang, Doug Bonn, Peter Wahl

Abstract: In many high temperature superconductors, small orthorhombic distortions of the lattice structure result in surprisingly large symmetry breaking of the electronic states and macroscopic properties, an effect often referred to as nematicity. To directly study the impact of symmetry-breaking lattice distortions on the electronic states, using low-temperature scanning tunnelling microscopy we image a… ▽ More In many high temperature superconductors, small orthorhombic distortions of the lattice structure result in surprisingly large symmetry breaking of the electronic states and macroscopic properties, an effect often referred to as nematicity. To directly study the impact of symmetry-breaking lattice distortions on the electronic states, using low-temperature scanning tunnelling microscopy we image at the atomic scale the influence of strain-tuned lattice distortions on the correlated electronic states in the iron-based superconductor LiFeAs, a material which in its ground state is tetragonal, with four-fold ($C_4$) symmetry. Our experiments uncover a new strain-stabilised modulated phase which exhibits a smectic order in LiFeAs, an electronic state which not only breaks rotational symmetry but also reduces translational symmetry. We follow the evolution of the superconducting gap from the unstrained material with $C_4$ symmetry through the new nematic phase with two-fold ($C_2$) symmetry and charge-density-wave order to a state where superconductivity is completely suppressed. △ Less

Submitted 5 July, 2018; v1 submitted 14 February, 2018; originally announced February 2018.

Comments: replaced with published version

Journal ref: Nature Communications 9, 2602 (2018)

Influence of Spin Orbit Coupling in the Iron-Based Superconductors

Authors: R. P. Day, G. Levy, M. Michiardi, B. Zwartsenberg, M. Zonno, F. Ji, E. Razzoli, F. Boschini, S. Chi, R. Liang, P. K. Das, I. Vobornik, J. Fujii, D. A. Bonn, W. N. Hardy, I. S. Elfimov, A. Damascelli

Abstract: We report on the influence of spin-orbit coupling (SOC) in the Fe-based superconductors (FeSCs) via application of circularly-polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides and Fe-chalcogenides with ab initio density functional theory and tight-binding calculations to establish an ubiquitous modification of… ▽ More We report on the influence of spin-orbit coupling (SOC) in the Fe-based superconductors (FeSCs) via application of circularly-polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides and Fe-chalcogenides with ab initio density functional theory and tight-binding calculations to establish an ubiquitous modification of the electronic structure in these materials imbued by SOC. The influence of SOC is found to be concentrated on the hole pockets where the superconducting gap is generally found to be largest. This result contests descriptions of superconductivity in these materials in terms of pure spin-singlet eigenstates, raising questions regarding the possible pairing mechanisms and role of SOC therein. △ Less

Submitted 20 July, 2018; v1 submitted 17 November, 2017; originally announced November 2017.

Comments: For supplementary information, see http://qmlab.ubc.ca/ARPES/PUBLICATIONS/articles.html

Journal ref: Phys. Rev. Lett. 121, 076401 (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)

Determination of the Superconducting Order Parameter from Defect Bound State Quasiparticle Interference

Authors: Shun Chi, W. N. Hardy, Ruixing Liang, P. Dosanjh, Peter Wahl, S. A. Burke, D. A. Bonn

Abstract: The superconducting order parameter is directly related to the pairing interaction, with the amplitude determined by the interaction strength, while the phase reflects the spatial structure of the interaction. However, given the large variety of materials and their rich physical properties within the iron-based high-Tc superconductors, the structure of the order parameter remains controversial in… ▽ More The superconducting order parameter is directly related to the pairing interaction, with the amplitude determined by the interaction strength, while the phase reflects the spatial structure of the interaction. However, given the large variety of materials and their rich physical properties within the iron-based high-Tc superconductors, the structure of the order parameter remains controversial in many cases. Here, we introduce Defect Bound State Quasi Particle Interference (DBS-QPI) as a new method to determine the superconducting order parameter. Using a low-temperature scanning tunneling microscope, we image in-gap bound states in the stoichiometric iron-based superconductor LiFeAs and show that the bound states induced by defect scattering are formed from Bogoliubov quasiparticles that have significant spatial extent. Quasiparticle interference from these bound states has unique signatures from which one can determine the phase of the order parameter as well as the nature of the defect, i.e. whether it is better described as a magnetic vs a nonmagnetic scatterer. DBS-QPI provides an easy but general method to characterize the pairing symmetry of superconducting condensates. △ Less

Submitted 25 October, 2017; originally announced October 2017.

Extracting phase information about the superconducting order parameter from defect bound states

Authors: Shun Chi, W. N. Hardy, Ruixing Liang, P. Dosanjh, Peter Wahl, S. A. Burke, D. A. Bonn

Abstract: Impurity bound states and quasi-particle scattering from these can serve as sensitive probes for identifying the pairing state of a superconducting condensate. We introduce and discuss defect bound state quasi-particle interference (DBS-QPI) imaging as a tool to extract information about the symmetry of the order parameter from spatial maps of the density of states around magnetic and non-magnetic… ▽ More Impurity bound states and quasi-particle scattering from these can serve as sensitive probes for identifying the pairing state of a superconducting condensate. We introduce and discuss defect bound state quasi-particle interference (DBS-QPI) imaging as a tool to extract information about the symmetry of the order parameter from spatial maps of the density of states around magnetic and non-magnetic impurities. We show that the phase information contained in the scattering patterns around impurities can provide valuable information beyond what is obtained through conventional QPI imaging. Keeping track of phase, rather than just magnitudes, in the Fourier transforms is achieved through phase-referenced Fourier transforms that preserve both real and imaginary parts of the QPI images. We further compare DBS-QPI to other approaches which have been proposed to use either QPI or defect scattering to distinguish different symmetries of the order parameter. △ Less

Submitted 25 October, 2017; originally announced October 2017.

Nanoscale Bandgap Tuning across an Inhomogeneous Ferroelectric Interface

Authors: Jing Wang, Houbing Huang, Wangqiang He, Qinghua Zhang, Danni Yang, Yuelin Zhang, Renrong Liang, Chuanshou Wang, Xingqiao Ma, Lin Gu, Longqing Chen, Ce-Wen Nan, Jinxing Zhang

Abstract: We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovolatic effects and the spectral response of the photocurrent were explore to illustrate the reversible bandgap variation (~0.3eV). This local-strain-engineered bandgap has been further revealed by in situ probe… ▽ More We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovolatic effects and the spectral response of the photocurrent were explore to illustrate the reversible bandgap variation (~0.3eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the opticallyrelated behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters. △ Less

Submitted 25 September, 2017; originally announced September 2017.

Journal ref: ACS Applied Materials & Interfaces, 2017, 9(29): 24704-24710

No Evidence for Orbital Loop Currents in Charge Ordered YBa$_2$Cu$_3$O$_{6+x}$ from Polarized Neutron Diffraction

Authors: T. P. Croft, E. Blackburn, J. Kulda, Ruixing Liang, D. A. Bonn, W. N. Hardy, S. M. Hayden

Abstract: It has been proposed that the pseudogap state of underdoped cuprate superconductors may be due to a transition to a phase which has circulating currents within each unit cell. Here, we use polarized neutron diffraction to search for the corresponding orbital moments in two samples of underdoped YBa$_2$Cu$_3$O$_{6+x}$ with doping levels $p=0.104$ and 0.123. In contrast to some other reports using p… ▽ More It has been proposed that the pseudogap state of underdoped cuprate superconductors may be due to a transition to a phase which has circulating currents within each unit cell. Here, we use polarized neutron diffraction to search for the corresponding orbital moments in two samples of underdoped YBa$_2$Cu$_3$O$_{6+x}$ with doping levels $p=0.104$ and 0.123. In contrast to some other reports using polarized neutrons, but in agreement with nuclear magnetic resonance and muon spin rotation measurements, we find no evidence for the appearance of magnetic order below 300 K. Thus, our experiment suggests that such order is not an intrinsic property of high-quality cuprate superconductor single crystals. Our results provide an upper bound for a possible orbital loop moment which depends on the pattern of currents within the unit cell. For example, for the CC-$θ_{II}$ pattern proposed by Varma, we find that the ordered moment per current loop is less than 0.013 $μ_B$ for $p=0.104$. △ Less

Submitted 15 December, 2017; v1 submitted 18 September, 2017; originally announced September 2017.

Comments: Comments in arXiv:1710.08173v1 fully addressed

Journal ref: Phys. Rev. B 96, 214504 (2017)

Crossover from Collective to Incoherent Spin Excitations in Superconducting Cuprates Probed by Detuned Resonant Inelastic X-ray Scattering

Authors: M. Minola, Y. Lu, Y. Y. Peng, G. Dellea, H. Gretarsson, M. W. Haverkort, Y. Ding, X. Sun, X. J. Zhou, D. C. Peets, L. Chauviere, P. Dosanjh, D. A. Bonn, R. Liang, A. Damascelli, N. B. Brookes, F. Yakhou, J. Pelliciari, M. Dantz, X. Lu, T. Schmitt, L. Braicovich, G. Ghiringhelli, B. Keimer, M. Le Tacon

Abstract: Spin excitations in the overdoped high temperature superconductors Tl$_2$Ba$_2$CuO$_{6+δ}$ and (Bi,Pb)$_2$(Sr,La)$_{2}$CuO$_{6+δ}$ were investigated by resonant inelastic x-ray scattering (RIXS) as functions of doping and detuning of the incoming photon energy above the Cu-$L_3$ absorption peak. The RIXS spectra at optimal doping are dominated by a paramagnon feature with peak energy independent o… ▽ More Spin excitations in the overdoped high temperature superconductors Tl$_2$Ba$_2$CuO$_{6+δ}$ and (Bi,Pb)$_2$(Sr,La)$_{2}$CuO$_{6+δ}$ were investigated by resonant inelastic x-ray scattering (RIXS) as functions of doping and detuning of the incoming photon energy above the Cu-$L_3$ absorption peak. The RIXS spectra at optimal doping are dominated by a paramagnon feature with peak energy independent of photon energy, similar to prior results on underdoped cuprates. Beyond optimal doping, the RIXS data indicate a sharp crossover to a regime with a strong contribution from incoherent particle/hole excitations whose maximum shows a fluorescence-like shift upon detuning. The spectra of both compound families are closely similar, and their salient features are reproduced by exact-diagonalization calculations of the single-band Hubbard model on a finite cluster. The results are discussed in the light of recent transport experiments indicating a quantum phase transition near optimal doping. △ Less

Submitted 2 August, 2017; originally announced August 2017.

Comments: 6 pages, 3 figures, accepted in Physical Review Letters

Journal ref: Phys. Rev. Lett. 119, 097001 (2017)

Anisotropy of the Seebeck Coefficient in the Cuprate Superconductor YBa$_{2}$Cu$_{3}$O$_{y}$: Fermi-Surface Reconstruction by Bidirectional Charge Order

Authors: O. Cyr-Choinière, S. Badoux, G. Grissonnanche, B. Michon, S. A. A. Afshar, S. Fortier, D. LeBoeuf, D. Graf, J. Day, D. A. Bonn, W. N. Hardy, R. Liang, N. Doiron-Leyraud, Louis Taillefer

Abstract: The Seebeck coefficient $S$ of the cuprate YBa$_{2}$Cu$_{3}$O$_{y}$ was measured in magnetic fields large enough to suppress superconductivity, at hole dopings $p = 0.11$ and $p = 0.12$, for heat currents along the $a$ and $b$ directions of the orthorhombic crystal structure. For both directions, $S/T$ decreases and becomes negative at low temperature, a signature that the Fermi surface undergoes… ▽ More The Seebeck coefficient $S$ of the cuprate YBa$_{2}$Cu$_{3}$O$_{y}$ was measured in magnetic fields large enough to suppress superconductivity, at hole dopings $p = 0.11$ and $p = 0.12$, for heat currents along the $a$ and $b$ directions of the orthorhombic crystal structure. For both directions, $S/T$ decreases and becomes negative at low temperature, a signature that the Fermi surface undergoes a reconstruction due to broken translational symmetry. Above a clear threshold field, a strong new feature appears in $S_{\rm b}$, for conduction along the $b$ axis only. We attribute this feature to the onset of 3D-coherent unidirectional charge-density-wave modulations seen by x-ray diffraction, also along the $b$ axis only. Because these modulations have a sharp onset temperature well below the temperature where $S/T$ starts to drop towards negative values, we infer that they are not the cause of Fermi-surface reconstruction. Instead, the reconstruction must be caused by the quasi-2D bidirectional modulations that develop at significantly higher temperature. △ Less

Submitted 12 April, 2017; originally announced April 2017.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. X 7, 031042 (2017)

Unravelling local spin polarization of Zhang-Rice singlet in lightly hole-doped cuprates using high-energy optical conductivity

Authors: Iman Santoso, Wei Ku, Tomonori Shirakawa, Gerd Neuber, Xinmao Yin, M. Enoki, Masaki Fujita, Ruixing Liang, T. Venkatesan, George A. Sawatzky, Aleksei Kotlov, Seiji Yunoki, Michael Rübhausen, Andrivo Rusydi

Abstract: Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlat… ▽ More Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05}$O$_{4}$. The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at ~7.2 eV, with an anomaly at the magnetic stripe phase (~25 K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at ~2.5 ~8.7, ~9.7, ~11.3 and ~21.8 eV. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using high- energy optical conductivity in correlated electron systems. △ Less

Submitted 8 April, 2017; originally announced April 2017.

Comments: 38 pages, 11 figures

Journal ref: Phys. Rev. B 95, 165108 (2017)

Imaging the Real Space Structure of the Spin Fluctuations in an Iron-based superconductor

Authors: Shun Chi, Ramakrishna Aluru, Stephanie Grothe, A. Kreisel, Udai Raj Singh, Brian M. Andersen, W. N. Hardy, Ruixing Liang, D. A. Bonn, S. A. Burke, Peter Wahl

Abstract: Spin fluctuations are a leading candidate for the pairing mechanism in high temperature superconductors, supported by the common appearance of a distinct resonance in the spin susceptibility across the cuprates, iron-based superconductors and many heavy fermion materials. The information we have about the spin resonance comes almost exclusively from neutron scattering. Here we demonstrate that by… ▽ More Spin fluctuations are a leading candidate for the pairing mechanism in high temperature superconductors, supported by the common appearance of a distinct resonance in the spin susceptibility across the cuprates, iron-based superconductors and many heavy fermion materials. The information we have about the spin resonance comes almost exclusively from neutron scattering. Here we demonstrate that by using low-temperature scanning tunnelling microscopy and spectroscopy we can characterize the spin resonance in real space. We show that inelastic tunnelling leads to the characteristic dip-hump feature seen in tunnelling spectra in high temperature superconductors and that this feature arises from excitations of the spin fluctuations. Spatial mapping of this feature near defects allows us to probe non-local properties of the spin susceptibility and to image its real space structure. △ Less

Submitted 29 June, 2017; v1 submitted 20 March, 2017; originally announced March 2017.

Comments: published version (7 pages, 5 figures)

Report number: NBI CMT 2017

Journal ref: Nat. Commun. 8, 15996 (2017)

Pseudogap temperature $T^\star$ of cuprate superconductors from the Nernst effect

Authors: O. Cyr-Choinière, R. Daou, F. Laliberté, C. Collignon, S. Badoux, D. LeBoeuf, J. Chang, B. J. Ramshaw, D. A. Bonn, W. N. Hardy, R. Liang, J. -Q. Yan, J. -G. Cheng, J. -S. Zhou, J. B. Goodenough, S. Pyon, T. Takayama, H. Takagi, N. Doiron-Leyraud, Louis Taillefer

Abstract: We use the Nernst effect to delineate the boundary of the pseudogap phase in the temperature-doping phase diagram of cuprate superconductors. New data for the Nernst coefficient $ν(T)$ of YBa$_{2}$Cu$_{3}$O$_{y}$ (YBCO), La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ (Eu-LSCO) and La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ (Nd-LSCO) are presented and compared with previous data including La$_{2-x}$Sr$_x$CuO$_4$ (LS… ▽ More We use the Nernst effect to delineate the boundary of the pseudogap phase in the temperature-doping phase diagram of cuprate superconductors. New data for the Nernst coefficient $ν(T)$ of YBa$_{2}$Cu$_{3}$O$_{y}$ (YBCO), La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ (Eu-LSCO) and La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ (Nd-LSCO) are presented and compared with previous data including La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). The temperature $T_ν$ at which $ν/T$ deviates from its high-temperature behaviour is found to coincide with the temperature at which the resistivity deviates from its linear-$T$ dependence, which we take as the definition of the pseudogap temperature $T^\star$- in agreement with gap opening detected in ARPES data. We track $T^\star$ as a function of doping and find that it decreases linearly vs $p$ in all four materials, having the same value in the three LSCO-based cuprates, irrespective of their different crystal structures. At low $p$, $T^\star$ is higher than the onset temperature of the various orders observed in underdoped cuprates, suggesting that these orders are secondary instabilities of the pseudogap phase. A linear extrapolation of $T^\star(p)$ to $p=0$ yields $T^\star(p\to 0)\simeq T_N(0)$, the Néel temperature for the onset of antiferromagnetic order at $p=0$, suggesting that there is a link between pseudogap and antiferromagnetism. With increasing $p$, $T^\star(p)$ extrapolates linearly to zero at $p\simeq p_{\rm c2}$, the critical doping below which superconductivity emerges at high doping, suggesting that the conditions which favour pseudogap formation also favour pairing. We also use the Nernst effect to investigate how far superconducting fluctuations extend above $T_{\rm c}$, as a function of doping, and find that a narrow fluctuation regime tracks $T_{\rm c}$, and not $T^\star$. This confirms that the pseudogap phase is not a form of precursor superconductivity. △ Less

Submitted 1 February, 2018; v1 submitted 20 March, 2017; originally announced March 2017.

Comments: 24 pages and 26 figures including Appendix

Journal ref: Phys. Rev. B 97, 064502 (2018)

A global inversion-symmetry-broken phase inside the pseudogap region of YBa$_2$Cu$_3$O$_y$

Authors: L. Zhao, C. A. Belvin, R. Liang, D. A. Bonn, W. N. Hardy, N. P. Armitage, D. Hsieh

Abstract: The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low energy electronic excitations. Polarized neutron diffraction, Nernst effect, THz polarimetery and ultrasound measurements on YBa$_2$Cu$_3$O$_y$ suggest that the pseudogap onset below a temperature T* coincides with a bona fide thermodynamic phas… ▽ More The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low energy electronic excitations. Polarized neutron diffraction, Nernst effect, THz polarimetery and ultrasound measurements on YBa$_2$Cu$_3$O$_y$ suggest that the pseudogap onset below a temperature T* coincides with a bona fide thermodynamic phase transition that breaks time-reversal, four-fold rotation and mirror symmetries respectively. However, the full point group above and below T* has not been resolved and the fate of this transition as T* approaches the superconducting critical temperature T$_c$ is poorly understood. Here we reveal the point group of YBa$_2$Cu$_3$O$_y$ inside its pseudogap and neighboring regions using high sensitivity linear and second harmonic optical anisotropy measurements. We show that spatial inversion and two-fold rotational symmetries are broken below T* while mirror symmetries perpendicular to the Cu-O plane are absent at all temperatures. This transition occurs over a wide doping range and persists inside the superconducting dome, with no detectable coupling to either charge ordering or superconductivity. These results suggest that the pseudogap region coincides with an odd-parity order that does not arise from a competing Fermi surface instability and exhibits a quantum phase transition inside the superconducting dome. △ Less

Submitted 25 November, 2016; originally announced November 2016.

Comments: 14 pages main text, 4 figures, 11 pages supplementary information, Nature Physics (2016)

Journal ref: Nature Physics 13, 250-254 (2017)

Transient terahertz photoconductivity of insulating cuprates

Authors: J. C. Petersen, A. Farahani, D. G. Sahota, Ruixing Liang, J. S. Dodge

Abstract: We establish a detailed phenomenology of photocarrier transport in the copper oxide plane by studying the transient terahertz photoconductivity of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$. The peak photoconductivity saturates with fluence, decays on multiple picosecond timescales, and evolves into a state characterized by activated transport. The time dependence shows little change with fluence,… ▽ More We establish a detailed phenomenology of photocarrier transport in the copper oxide plane by studying the transient terahertz photoconductivity of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$. The peak photoconductivity saturates with fluence, decays on multiple picosecond timescales, and evolves into a state characterized by activated transport. The time dependence shows little change with fluence, indicating that the decay is governed by first-order recombination kinetics. We find that most photocarriers make a negligible contribution to the dc photoconductivity, and we estimate the intrinsic photocarrier mobility to be 0.6-0.7 cm$^2$/V s at early times, comparable to the mobility in chemically doped materials. △ Less

Submitted 19 September, 2017; v1 submitted 31 October, 2016; originally announced October 2016.

Journal ref: Phys. Rev. B 96, 115133 (2017)

Anomalous Thermal Diffusivity in Underdoped YBa$_2$Cu$_3$O$_{6+x}$

Authors: J. -C. Zhang, E. M. Levenson-Falk, B. J. Ramshaw, D. A. Bonn, R. Liang, W. N. Hardy, S. A. Hartnoll, A. Kapitulnik

Abstract: We present local optical measurements of thermal diffusivity in the $ab$ plane of underdoped YBCO crystals. We find that the diffusivity anisotropy is comparable to reported values of the electrical resistivity anisotropy, suggesting that the anisotropies have the same origin. The anisotropy drops sharply below the charge order transition. We interpret our results through a strong electron-phonon… ▽ More We present local optical measurements of thermal diffusivity in the $ab$ plane of underdoped YBCO crystals. We find that the diffusivity anisotropy is comparable to reported values of the electrical resistivity anisotropy, suggesting that the anisotropies have the same origin. The anisotropy drops sharply below the charge order transition. We interpret our results through a strong electron-phonon scattering picture and find that both electronic and phononic contributions to the diffusivity saturate a proposed bound. Our results suggest that neither well-defined electron nor phonon quasiparticles are present in this material. △ Less

Submitted 15 May, 2017; v1 submitted 18 October, 2016; originally announced October 2016.

Comments: 8 pages + 4 pages supporting information

Towards a quantitative description of tunneling conductance of superconductors: application to LiFeAs

Authors: A. Kreisel, R. Nelson, T. Berlijn, W. Ku, Ramakrishna Aluru, Shun Chi, Haibiao Zhou, Udai Raj Singh, Peter Wahl, Ruixing Liang, Walter N. Hardy, D. A. Bonn, P. J. Hirschfeld, Brian M. Andersen

Abstract: Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles electronic structure calculations, makes predictions for the superconduct… ▽ More Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles electronic structure calculations, makes predictions for the superconducting gap. Within the same framework, the surface wave functions may also be calculated, allowing, e.g., for detailed comparisons between theoretical results and measured scanning tunneling topographs and spectra. Here we present such a comparison between theory and experiment on the Fe-based superconductor LiFeAs. Results for the homogeneous surface as well as impurity states are presented as a benchmark test of the theory. For the homogeneous system, we argue that the maxima of topographic image intensity may be located at positions above either the As or Li atoms, depending on tip height and the setpoint current of the measurement. We further report the experimental observation of transitions between As and Li-registered lattices as functions of both tip height and setpoint bias, in agreement with this prediction. Next, we give a detailed comparison between the simulated scanning tunneling microscopy images of transition-metal defects with experiment. Finally, we discuss possible extensions of the current framework to obtain a theory with true predictive power for scanning tunneling microscopy in Fe-based systems. △ Less

Submitted 31 December, 2016; v1 submitted 3 October, 2016; originally announced October 2016.

Comments: 14 pages, 15 figures, published version

Report number: NBI CMT 2016

Journal ref: Phys. Rev. B 94, 224518 (2016)

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)