arXiv subjects

J. Orenstein

Publications and source records attributed to J. Orenstein.

At least 19 recordsLinked to original sources

arXiv2025

Linear magneto-birefringence as a probe of altermagnetism

Altermagnets are a class of collinear magnets that exhibit non-relativistic spin splitting (NRSS) of electronic bands in the absence of net magnetization. Their potential to generate large spin polarization without spin-orbit coupling has created strong interest in probes that access the underlying order parameter directly. In this Perspective, we show that linear magneto-birefringence (LMB) provides a natural and broadly applicable route to detecting altermagnetic order. Building on the correspondence between the momentum-space structure of NRSS and the ferroic ordering of magnetic multipoles in real space, we demonstrate how $d$-wave and $g$-wave NRSS textures yield distinct LMB responses. We present a symmetry-based framework that identifies the optical geometries and field configurations required to isolate specific multipole components, enabling domain imaging and providing benchmarks for theoretical models of LMB.

cond-mat.mtrl-sci↗

arXiv2025

Strain-induced multiferroicity in Cr1/3NbS2

Multiferroic materials, in which electric polarization and magnetic order coexist and couple, offer rich opportunities for both fundamental discovery and technology. However, multiferroicity remains rare due to conflicting electronic requirements for ferroelectricity and magnetism. One route to circumvent this challenge is to exploit the noncollinear ordering of spin cycloids, whose symmetry permits the emergence of polar order. In this work, we introduce another pathway to multiferroic order in which strain generates polarization in materials that host nonpolar spin spirals. To demonstrate this phenomenon, we chose the spin spiral in the well-studied helimagnet Cr1/3NbS2. To detect the induced polarization, we introduce the technique of magnetoelectric birefringence (MEB), an optical probe that enables spatially-resolved and unambiguous detection of polar order. By combining MEB imaging with strain engineering, we confirm the onset of a polar vector at the magnetic transition, establishing strained Cr1/3NbS2 as a type-II multiferroic.

cond-mat.mtrl-sci↗

arXiv2025

Magneto-optical Kerr effect in an A-type antiferromagnet

Magneto-optic Kerr effect (MOKE) is a powerful probe of broken time-reversal symmetry ($\mathcal{T}$), typically used to study ferromagnets. While MOKE has been observed in some antiferromagnets (AFMs) with vanishing magnetization, it is often associated with structures whose symmetry is lower than basic collinear, bipartite order. In contrast, theory predicts a mechanism for MOKE intrinsic to all AFMs of A-type, i.e. layered AFMs in which ferromagnetic layers are antiferromagnetically aligned. Here we report the first experimental confirmation of this mechanism in a bulk AFM. We achieve this by measuring the imaginary component of MOKE as a function of photon energy in MnBi$_2$Te$_4$, an A-type AFM where $\mathcal{T}$ is preserved in combination with a translation. By comparing the experimental results with model calculations, we demonstrate that observable MOKE should be expected in all collinear A-type AFMs with out-of-plane spin order, thus enabling optical detection of AFM domains and expanding the scope of MOKE to few-layer AFMs.

cond-mat.mtrl-sci↗

arXiv2023

Symmetry-breaking pathway towards the unpinned broken helix

One of the prime material candidates to host the axion insulator state is EuIn$_{2}$As$_{2}$. First-principles calculations predicted the emergence of this exotic topological phase based on the assumption of a collinear antiferromagnetic structure. However, neutron scattering measurements revealed a more intricate magnetic ground state, characterized by two coexisting magnetic wavevectors, reached by successive thermal phase transitions. The proposed high and low temperature phases were a spin helix and a state with interpenetrating helical and antiferromagnetic order, termed a broken helix, respectively. Despite its complexity, the broken helix still protects the axion state because the product of time-reversal and a rotational symmetry is preserved. Here we identify the magnetic structure associated with these two phases using a multimodal approach that combines symmetry-sensitive optical probes, scattering, and group theoretical analysis. We find that the higher temperature phase hosts a nodal structure rather than a helix, characterized by a variation of the magnetic moment amplitude from layer to layer, with the moment vanishing entirely in every third Eu layer. The lower temperature structure is similar to the broken helix, with one important difference: the relative orientation of the magnetic structure and the lattice is not fixed, resulting in an `unpinned broken helix'. As a result of the breaking of rotational symmetry, the axion phase is not generically protected. Nevertheless, we show that it can be restored if the magnetic structure is tuned with externally-applied uniaxial strain. Finally, we present a spin Hamiltonian that identifies the spin interactions needed to account for the complex magnetic order in EuIn$_{2}$As$_{2}$. Our work highlights the importance of the multimodal approach in determining the symmetry of complex order-parameters.

cond-mat.str-el↗

arXiv2019

Observation of Topological Photocurrents in the Chiral Weyl Semimetal RhSi

Weyl semimetals are crystals in which electron bands cross at isolated points in momentum space. Associated with each crossing point (or Weyl node) is an integer topological invariant known as the Berry monopole charge. The discovery of new classes of Weyl materials is driving the search for novel properties that derive directly from the Berry charge. The circular photogalvanic effect (CPGE), whereby circular polarized light generates a current whose direction depends on the helicity of the absorbed photons, is a striking example of a macroscopic property that emerges from Weyl topology. Recently, it was predicted that the rate of current generation associated with optical transitions near a Weyl node is proportional to its monopole charge and independent of material-specific parameters. In Weyl semimetals that retain mirror symmetry this universal photogalvanic current is strongly suppressed by opposing contributions from energy equivalent nodes of opposite charge. However, when all mirror symmetries are broken, as in chiral Weyl systems, nodes with opposite topological charge are no longer degenerate, opening a window of photon energies where the topological CPGE can emerge. In this work we test this theory through measurement of the photon-energy dependence of the CPGE in the chiral Weyl semimetal RhSi. The spectrum is fully consistent with a topological CPGE, as it reveals a response in a low-energy window that closes at 0.65 eV, in quantitative agreement with the theoretically-derived bandstucture.

cond-mat.mes-hall↗

arXiv2019

Electrical switching in a magnetically intercalated transition metal dichalcogenide

Recent advances in tuning the correlated behavior of graphene and transition-metal dichalcogenides (TMDs) have opened a new frontier in the study of many-body physics in two dimensions and promise exciting possibilities for new quantum technologies. An emerging field where these materials have yet to make a deep impact is the study of antiferromagnetic (AFM) spintronics - a relatively new research direction that promises technologies that are insensitive to external magnetic fields, fast switching times, and reduced crosstalk. In this study we present measurements on the intercalated TMD Fe1/3NbS2 which exhibits antiferromagnetic ordering below 42K. We find that current densities on the order of 10^4 A/cm^2 can reorient the magnetic order, the response of which can be detected in the sample's resistance. This demonstrates that Fe1/3NbS2 can be used as an antiferromagnetic switch with electronic "write-in" and "read-out". This switching is found to be stable over time and remarkably robust to external magnetic fields. Fe1/3NbS2 is a rare example of an AFM system that exhibits fully electronic switching behavior in single crystal form, making it appealing for low-power, low-temperature memory storage applications. Moreover, Fe1/3NbS2 is part of a much larger family of magnetically intercalated TMDs, some of which may exhibit the switching behavior at higher temperatures and form a platform from which to build tunable AFM spintronic devices.

cond-mat.str-el↗

arXiv2018

Resonance-enhanced optical nonlinearity in the Weyl semimetal TaAs

While all media can exhibit first-order conductivity describing current linearly proportional to electric field, $E$, the second-order conductivity, $σ^{(2)}$ , relating current to $E^2$, is nonzero only when inversion symmetry is broken. Second order nonlinear optical responses are powerful tools in basic research, as probes of symmetry breaking, and in optical technology as the basis for generating currents from far-infrared to X-ray wavelengths. The recent surge of interest in Weyl semimetals with acentric crystal structures has led to the discovery of a host of $σ^{(2)}$ -related phenomena in this class of materials, such as polarization-selective conversion of light to dc current (photogalvanic effects) and the observation of giant second-harmonic generation (SHG) efficiency in TaAs at photon energy 1.5 eV. Here, we present measurements of the SHG spectrum of TaAs revealing that the response at 1.5 eV corresponds to the high-energy tail of a resonance at 0.7 eV, at which point the second harmonic conductivity is approximately 200 times larger than seen in the standard candle nonlinear crystal, GaAs. This remarkably large SHG response provokes the question of ultimate limits on $σ^{(2)}$ , which we address by a new theorem relating frequency-integrated nonlinear response functions to the third cumulant (or "skewness") of the polarization distribution function in the ground state. This theorem provides considerable insight into the factors that lead to the largest possible second-order nonlinear response, specifically showing that the spectral weight is unbounded and potentially divergent when the possibility of next-neighbor hopping is included.

cond-mat.mtrl-sci↗

arXiv2017

Antiferromagnetic resonance and terahertz continuum in $α-$RuCl$_3$

We report measurements of optical absorption in the zig-zag antiferromagnet $α$-RuCl$_3$ as a function of temperature, $T$, magnetic field, $B$, and photon energy, $\hbarω$ in the range $\sim$ 0.3 to 8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that 3-fold rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at $B=0$ that we identify as magnetic-dipole excitation of a zero-wavevector magnon, or antiferromagnetic resonance (AFMR). With application of $B$, the AFMR broadens and shifts to lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the $dc$ susceptibility from a magnetic excitation continuum.

cond-mat.str-el↗

arXiv2017

Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals

Although Weyl fermions have proven elusive in high-energy physics, their existence as emergent quasiparticles has been predicted in certain crystalline solids in which either inversion or time-reversal symmetry is broken\cite{WanPRB2011,BurkovPRL2011, WengPRX2015,HuangNatComm2015}. Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention \cite{XuScience2015, LvPRX2015, YangNatPhys2015}. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic properties. The TMMPs are polar metals, a rare subset of inversion-breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons \cite{anderson1965symmetry,shi2013ferroelectric,kim2016polar}. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, $χ^{(2)}$, leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic $χ^{(2)}$ in the TMMPs TaAs, TaP, and NbAs. With the fundamental and second harmonic fields oriented parallel to the polar axis, the value of $χ^{(2)}$ is larger by almost one order of magnitude than its value in the archetypal electro-optic materials GaAs \cite{bergfeld2003second} and ZnTe \cite{wagner1998dispersion}, and in fact larger than reported in any crystal to date.

cond-mat.mtrl-sci↗

arXiv2016

The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors

The condensation of an electron superfluid from a conventional metallic state at a critical temperature $T_c$ is described well by the BCS theory. In the underdoped copper-oxides, high-temperature superconductivity condenses instead from a nonconventional metallic "pseudogap" phase that exhibits a variety of non-Fermi liquid properties. Recently, it has become clear that a charge density wave (CDW) phase exists within the pseudogap regime, appearing at a temperature $T_{CDW}$ just above $T_c$. The near coincidence of $T_c$ and $T_{CDW}$, as well the coexistence and competition of CDW and superconducting order below $T_c$, suggests that they are intimately related. Here we show that the condensation of the superfluid from this unconventional precursor is reflected in deviations from the predictions of BSC theory regarding the recombination rate of quasiparticles. We report a detailed investigation of the quasiparticle (QP) recombination lifetime, $τ_{qp}$, as a function of temperature and magnetic field in underdoped HgBa$_{2}$CuO$_{4+δ}$ (Hg-1201) and YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that $τ_{qp}(T)$ exhibits a local maximum in a small temperature window near $T_c$ that is prominent in underdoped samples with coexisting charge order and vanishes with application of a small magnetic field. We explain this unusual, non-BCS behavior by positing that $T_c$ marks a transition from phase-fluctuating SC/CDW composite order above to a SC/CDW condensate below. Our results suggest that the superfluid in underdoped cuprates is a condensate of coherently-mixed particle-particle and particle-hole pairs.

cond-mat.supr-con↗

arXiv2015

Resonant magneto-optic Kerr effect in the magnetic topological insulator Cr:(Sb$_x$,Bi$_{1-x}$)$_2$Te$_3$

We report measurements of the polar Kerr effect, proportional to the out-of-plane component of the magnetization, in thin films of the magnetically doped topological insulator $(\text{Cr}_{0.12}\text{Bi}_{0.26}\text{Sb}_{0.62})_2\text{Te}_3$. Measurements of the complex Kerr angle, $Θ_K$, were performed as a function of photon energy in the range $0.8\text{ eV}<\hbarω<3.0\text{ eV}$. We observed a peak in the real part of $Θ_K(ω)$ and zero crossing in the imaginary part that we attribute to resonant interaction with a spin-orbit avoided crossing located $\approx$ 1.6 eV above the Fermi energy. The resonant enhancement allows measurement of the temperature and magnetic field dependence of $Θ_K$ in the ultrathin film limit, $d\geq2$ quintuple layers. We find a sharp transition to zero remanent magnetization at 6 K for $d<8$~QL, consistent with theories of the dependence of impurity spin interactions on film thickness and their location relative to topological insulator surfaces.

cond-mat.mes-hall↗

arXiv2015

Photoexcited states of the harmonic honeycomb iridate γ-Li$_2$IrO$_3$

We report equilibrium and nonequilibrium optical measurements on the recently synthesized "harmonic" honeycomb iridate γ-Li$_2$IrO$_3$ (LIO), as well as the layered honeycomb iridate Na$_2$IrO$_3$ (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition we measured the photoinduced changed in reflectance, ΔR, as a function of time, t, temperature, T, and probe field polarization in both LIO and NIO. In LIO, ΔR(t,T) is anisotropic and comprised of three T dependent components. Two of these components are related to the onset of magnetic order and the third is related to a photoinduced population of metastable electronic excited states. In NIO, ΔR(t,T) has a single T dependent component that is strikingly similar to the electronic excitation component of ΔR in LIO. Through analysis and comparison of ΔR(t,T) for two compounds, we extract information on the onset of magnetic correlations at and above the transition temperature in LIO, the bare spin-flip scattering rate in equilibrium, the lifetime of low-lying quasiparticle excitations, and the polarization dependence of optical transitions that are sensitive to magnetic order.

cond-mat.str-el↗

arXiv2015

Optical evidence of broken C$_4$ symmetry across optimal doping in BaFe$_{2}$(As$_{1-x}$P$_x$)$_2$

The isovalently doped high-$T_{\text{c}}$ superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ is characterized by a rich temperature-doping phase diagram, which includes structural, antiferromagnetic, and superconducting phase transitions. In this work, we use photomodulated reflectance at 1.5 eV to detect the onset of linear birefringence in the normal state, and by inference, the breaking of four-fold rotational (C$_4$) symmetry. We find that C$_4$ breaking takes place in the normal state over a range of $x$ that spans the optimal P concentration, unlike the line of orthorhombic-tetragonal and magnetic phase transitions, which terminates at, or just below, the optimal value of $x$.

cond-mat.supr-con↗

arXiv2015

Terahertz time-domain spectroscopy of transient metallic and superconducting states

Time-resolved terahertz time-domain spectroscopy (THz-TDS) is an ideal tool for probing photoinduced nonequilibrium metallic and superconducting states. Here, we focus on the interpretation of the two-dimensional response function $Σ(ω;t)$ that it measures, examining whether it provides an accurate snapshot of the instantaneous optical conductivity, $σ(ω;t)$. For the Drude model with a time-dependent carrier density, we show that $Σ(ω;t)$ is not simply related to $σ(ω;t)$. The difference in the two response functions is most pronounced when the momentum relaxation rate of photocarriers is long, as would be the case in a system that becomes superconducting following pulsed photoexcitation. From the analysis of our model, we identify signatures of photoinduced superconductivity that could be seen by time-resolved THz-TDS.

cond-mat.supr-con↗

arXiv2015

Phase transition beneath the superconducting dome in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

We present a theory for the large suppression of the superfluid-density, $ρ_s$, in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ in the vicinity of a putative spin-density wave quantum critical point at a P-doping, $x=x_{c}$. We argue that the transition becomes weakly first-order in the vicinity of $x_{c}$, and disorder induces puddles of superconducting and antiferromagnetic regions at short length-scales; thus the system becomes an electronic micro-emulsion. We propose that frustrated Josephson couplings between the superconducting grains suppress $ρ_s$. In addition, the presence of `normal' quasiparticles at the interface of the frustrated Josephson junctions will give rise to a highly non-trivial feature in the low frequency response in a narrow vicinity around $x_c$. We propose a number of experiments to test our theory.

cond-mat.str-el↗

arXiv2014

Kerr effect as evidence of gyrotropic order in the cuprates - revisited

Recent analysis has confirmed earlier general arguments that the Kerr response vanishes in any time-reversal invariant system which satisfies the Onsager relations. Thus, the widely cited relation between natural optical activity (gyrotropy) and the Kerr response, employed in Hosur \textit{et al}, Phys. Rev. B \textbf{87}, 115116 (2013), is incorrect. However, there is increasingly clear experimental evidence that, as argued in our paper, the onset of an observable Kerr-signal in the cuprates reflects point-group symmetry rather than time-reversal symmetry breaking.

cond-mat.supr-con↗

arXiv2013

Tuning the electronic properties of J_eff=1/2 correlated semimetal in epitaxial perovskite SrIrO3

We investigated the electronic properties of epitaxially stabilized perovskite SrIrO3 and demonstrated the effective strain-control on its electronic structure. Comprehensive transport measurements showed that the strong spin-orbit coupling renders a novel semimetallic phase for the J_eff=1/2 electrons rather than an ordinary correlated metal, elucidating the nontrivial mechanism underlying the dimensionality-controlled metal-insulator transition in iridates. The electron-hole symmetry of this correlated semimetal was found to exhibit drastic variation when subject to bi-axial strain. Under compressive strain, substantial electron-hole asymmetry is observed in contrast to the tensile side, where the electron and hole effective masses are comparable, illustrating the susceptivity of the J_eff=1/2 to structural distortion. Tensile strain also shrinks the Fermi surface, indicative of an increasing degree of correlation which is consistent with optical measurements. These results pave a pathway to investigate and manipulate the electronic states in spin-orbit-coupled correlated oxides, and lay the foundation for constructing 5d transition metal heterostructures.

cond-mat.str-el↗

arXiv2013

A new collective mode in YBCO observed by time-domain reflectometry

We report the observation of coherent oscillations associated with charge density wave (CDW) order in the underdoped cuprate superconductor YBa2Cu3O6+x by time-resolved optical reflectivity. Oscillations with frequency 1.87 THz onset at approximately 105 K and 130 K for dopings of x = 0.67 (ortho-VIII) and x = 0.75 (ortho-III), respectively. Upon cooling below the superconducting critical temperature (T_c), the oscillation amplitude is enhanced, the phase shifts by π, and the frequency softens by δν/ ν~7%. A bi-quadratically coupled Landau-Ginzburg model qualitatively describes this behavior as arising from competition between superconducting and CDW orders.

cond-mat.supr-con↗