arXiv subjects

X. M. Chen

Publications and source records attributed to X. M. Chen.

13 recordsLinked to original sources

arXiv2021

Charge Condensation and Lattice Coupling Drives Stripe Formation in Nickelates

Revealing the predominant driving force behind symmetry breaking in correlated materials is sometimes a formidable task due to the intertwined nature of different degrees of freedom. This is the case for La2-xSrxNiO4+δ in which coupled incommensurate charge and spin stripes form at low temperatures. Here, we use resonant X-ray photon correlation spectroscopy to study the temporal stability and domain memory of the charge and spin stripes in La2-xSrxNiO4+δ. Although spin stripes are more spatially correlated, charge stripes maintain a better temporal stability against temperature change. More intriguingly, charge order shows robust domain memory with thermal cycling up to 250 K, far above the ordering temperature. These results demonstrate the pinning of charge stripes to the lattice and that charge condensation is the predominant factor in the formation of stripe orders in nickelates.

cond-mat.str-el↗

arXiv2021

Multiferroic order parameters in rhombic antiferromagnets. RCrO$_3$

In this paper, we explore magneteoelectricity of rare earth orthochromites from the symmetry point of view. We determine the principal structural order parameters and find their couplings with ferroelectric and magnetic orderings. Our calculations showed that electric dipole moments emerge in the vicinity of Cr3+ ions in the unit cell of RCrO3 due to the displacements of oxygen ions from their highly symmetric positions in the parent perovskite phase (structural instability). We find that the electric dipole moments are arranged in an antiferroelectric mode, so, in essence, RCrO3 are antiferroelectric materials. By classifying the order parameters according to the irreducible representations of the RCrO3 symmetry group (D2h16), we determine the possible couplings between distortive, ferroelectric and magnetic orderings and explore the emerging magnetoelectric structures in these terms. Our analysis makes it possible to explain experimentally observed polarization reversal and the concomitant reorientation of spins in a series of RCrO3 compounds and to predict the possible scenarios of phase transitions in RCrO3.

cond-mat.mtrl-sci↗

arXiv2019

Spontaneous Magnetic Superdomain Wall Fluctuations in an Artificial Antiferromagnet

Collective dynamics often play an important role in determining the stability of ground states for both naturally occurring materials and metamaterials. We studied the temperature dependent dynamics of antiferromagnetically ordered superdomains in a square artificial spin lattice using soft x-ray photon correlation spectroscopy. We observed an exponential slowing down of superdomain wall motion below the AF onset temperature, similar to the behavior of typical bulk antiferromagnets. Using a continuous time random walk model we show that these superdomain walls undergo low-temperature ballistic and high-temperature diffusive motions.

cond-mat.mes-hall↗

arXiv2018

Persistent Charge Density Wave Memory in a Cuprate Superconductor

Although charge density wave (CDW) correlations appear to be a ubiquitous feature of the superconducting cuprates, their disparate properties suggest a crucial role for coupling or pinning of the CDW to lattice deformations and disorder. While diffraction intensities can demonstrate the occurrence of CDW domain formation, the lack of scattering phase information has limited our understanding of this process. Here, we report coherent resonant x-ray speckle correlation analysis, which directly determines the reproducibility of CDW domain patterns in La1.875Ba0.125CuO4 (LBCO 1/8) with thermal cycling. While CDW order is only observed below 54 K, where a structural phase transition results in equivalent Cu-O bonds, we discover remarkably reproducible CDW domain memory upon repeated cycling to temperatures well above that transition. That memory is only lost on cycling across the transition at 240(3) K that restores the four-fold symmetry of the copper-oxide planes. We infer that the structural-domain twinning pattern that develops below 240 K determines the CDW pinning landscape below 54 K. These results open a new view into the complex coupling between charge and lattice degrees of freedom in superconducting cuprates.

cond-mat.supr-con↗

arXiv2018

Remarkable Stability of Charge Density Wave Order in La$_{1.875}$Ba$_{0.125}$CuO$_4$

The occurrence of charge-density-wave (CDW) order in underdoped cuprates is now well established, although the precise nature of the CDW and its relationship with superconductivity is not. Theoretical proposals include contrasting ideas such as that pairing may be driven by CDW fluctuations or that static CDWs may intertwine with a spatially-modulated superconducting wave function. We test the dynamics of CDW order in La$_{1.825}$Ba$_{0.125}$CuO$_4$ by using x-ray photon correlation spectroscopy (XPCS) at the CDW wave vector, detected resonantly at the Cu $L_3$-edge. We find that the CDW domains are strikingly static, with no evidence of significant fluctuations up to 2\,\nicefrac{3}{4} hours. We discuss the implications of these results for some of the competing theories.

cond-mat.supr-con↗

arXiv2017

Structural, magnetic, and electronic properties of GdTiO3 Mott insulator thin films grown by pulsed laser deposition

We report on the optimization process to synthesize epitaxial thin films of GdTiO3 on SrLaGaO4 substrates by pulsed laser deposition. Optimized films are free of impurity phases and are fully strained. They possess a magnetic Curie temperature TC = 31.8 K with a saturation magnetization of 4.2 muB per formula unit at 10 K. Transport measurements reveal an insulating response, as expected. Optical spectroscopy indicates a band gap of 0.7 eV, comparable to the bulk value. Our work adds ferrimagnetic orthotitanates to the palette of perovskite materials for the design of emergent strongly correlated states at oxide interfaces using a versatile growth technique such as pulsed laser deposition.

cond-mat.str-el↗

arXiv2017

Static Charge Density Wave Order in the Superconducting State of La2-xBaxCuO4

Charge density wave (CDW) correlations feature prominently in the phase diagram of the cuprates, motivating competing theories of whether fluctuating CDW correlations aid superconductivity or whether static CDW order coexists with superconductivity in inhomogeneous or spatially modulated states. Here we report Cu $L$-edge resonant x-ray photon correlation spectroscopy (XPCS) measurements of CDW correlations in superconducting La$_{2-x}$Ba$_x$CuO$_4$ $x=0.11$. Static CDW order is shown to exist in the superconducting state at low temperatures and to persist up to at least 85\% of the CDW transition temperature. We discuss the implications of our observations for how \emph{nominally} competing order parameters can coexist in the cuprates.

cond-mat.supr-con↗

arXiv2016

Ultrafast energy and momentum resolved dynamics of magnetic correlations in photo-doped Mott insulator Sr$_2$IrO$_4$

Measuring how the magnetic correlations throughout the Brillouin zone evolve in a Mott insulator as charges are introduced dramatically improved our understanding of the pseudogap, non-Fermi liquids and high $T_C$ superconductivity. Recently, photoexcitation has been used to induce similarly exotic states transiently. However, understanding how these states emerge has been limited because of a lack of available probes of magnetic correlations in the time domain, which hinders further investigation of how light can be used to control the properties of solids. Here we implement magnetic resonant inelastic X-ray scattering at a free electron laser, and directly determine the magnetization dynamics after photo-doping the Mott insulator Sr$_2$IrO$_4$. We find that the non-equilibrium state 2~ps after the excitation has strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. The magnetism recovers its two-dimensional (2D) in-plane Néel correlations on a timescale of a few ps, while the three-dimensional (3D) long-range magnetic order restores over a far longer, fluence-dependent timescale of a few hundred ps. The dramatic difference in these two timescales, implies that characterizing the dimensionality of magnetic correlations will be vital in our efforts to understand ultrafast magnetic dynamics.

cond-mat.str-el↗

arXiv2015

Influence of Ti doping on the incommensurate charge density wave in 1T-TaS2

We report temperature-dependent transport and x-ray diffraction measurements of the influence of Ti hole doping on the charge density wave (CDW) in 1T-Ta(1-x)Ti(x)S(2). Confirming past studies, we find that even trace impurities eliminate the low-temperature commensurate (C) phase in this system. Surprisingly, the magnitude of the in-plane component of the CDW wave vector in the nearly commensurate (NC) phase does not change significantly with Ti concentration, as might be expected from a changing Fermi surface volume. Instead, the angle of the CDW in the basal plane rotates, from 11.9 deg at x=0 to 16.4 deg at x=0.12. Ti substitution also leads to an extended region of coexistence between incommensurate (IC) and NC phases, indicating heterogeneous nucleation near the transition. Finally, we explain a resistive anomaly originally observed by DiSalvo [F. J. DiSalvo, et al., Phys. Rev. B {\bf 12}, 2220 (1975)] as arising from pinning of the CDW on the crystal lattice. Our study highlights the importance of commensuration effects in the NC phase, particularly at x ~ 0.08.

cond-mat.str-el↗

arXiv2013

Emergence of charge density wave domain walls above the superconducting dome in TiSe2

Superconductivity (SC) in so-called "unconventional superconductors" is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between SC and fluctuations in some other order parameter. To better understand this connection, we used high-pressure x-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe2, which was previously shown to exhibit SC when the CDW is suppressed by pressure [1] or intercalation of Cu atoms [2]. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at Pc = 5.1 +/- 0.2 GPa, which is more than 1 GPa beyond the end of the SC region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that SC in TiSe2 may not be connected to the QCP itself, but to the formation of CDW domain walls.

cond-mat.supr-con↗

arXiv2011

Pressure- and Field-Tuning the Magnetostructural Phases of Mn3O4: Raman Scattering and X-Ray Diffraction Studies

We present temperature-, magnetic-field-, and pressure-dependent Raman scattering studies of single crystal Mn3O4, combined with temperature- and field-dependent x-ray diffraction studies, revealing the novel magnetostructural phases in Mn3O4. Our temperature-dependent studies showed that the commensurate magnetic transition at T2=33K in the binary spinel Mn3O4 is associated with a structural transition from tetragonal to orthorhombic structures. Field-dependent studies showed that the onset and nature of this structural transition can be controlled with an applied magnetic field, and revealed evidence for a field-tuned quantum phase transition to a tetragonal spin-disordered phase for H||[1-10]. Pressure-dependent Raman measurements showed that the magnetic easy axis direction in Mn3O4 can be controlled---and the ferrimagnetic transition temperature increased---with applied pressure. Finally, combined pressure- and magnetic-field-tuned Raman measurements revealed a rich magnetostructural phase diagram---including a pressure- and field-induced magnetically frustrated tetragonal phase in the PH phase diagram---that can be generated in Mn3O4 with applied pressure and magnetic field.

cond-mat.str-el↗

arXiv2011

Resonant soft x-ray scattering from La(1-x)Sr(x)MnO(3) quantum wire arrays

We describe a strategy for using resonant soft x-ray scattering (RSXS) to study the electronic structure of transition metal oxide quantum wires. Using electron beam lithography and ion milling, we have produced periodic, patterned arrays of colossal magnetoresistance (CMR) phase La(1-x)Sr(x)MnO(3) consisting of ~ 5000 wires, each of which is 80 nm in width. The scattered intensity exhibits a series of peaks that can be interpreted as Bragg reflections from the periodic structure or, equivalently, diffraction orders from the grating-like structure. RSXS measurements at the Mn L(2,3) edge, which has a large magnetic cross section, show clear evidence for a magnetic superstructure with a commensurate period of five wires, which we interpret as commensurately modulated antiferromagnetism. This superstructure, which is accompanied by non-trivial reorganization of the magnetization within each wire, likely results from classical dipole interactions among the wires. We introduce a simple, exactly soluble, analytic model of the scattering that captures, semi-quantitatively, the primary features in the RSXS data; this model will act as a foundation for forthcoming, detailed studies of the magnetic structure in these systems.

cond-mat.str-el↗

arXiv2009

Mapping the magneto-structural quantum phases of Mn3O4

We present temperature-dependent x-ray diffraction and temperature- and field-dependent Raman scattering studies of single crystal Mn3O4, which reveal the novel magnetostructural phases that evolve in the spinels due to the interplay between strong spin-orbital coupling, geometric frustration, and applied magnetic field. We observe a structural transition from tetragonal to monoclinic structures at the commensurate magnetic transition at T2=33K, show that the onset and nature of this structural transition can be controlled with an applied magnetic field, and find evidence for a field-tuned quantum phase transition to a tetragonal incommensurate or spin glass phase.

cond-mat.str-el↗