@article{21469, abstract = {Terahertz (THz) spectroscopy is a powerful probe of low-energy excitations in complex materials. Extending it into the nonlinear regime broadens its scope and can provide valuable insight into interactions among these modes. However, interpreting nonlinear spectra is challenging because resonant features in this case do not always reflect intrinsic material dynamics. Here, we study nonlinear THz-induced Kerr effect in a generic material LaAlO3. After detailed analysis of temporal oscillations of the Kerr signal, we identify an 𝐸𝑔 Raman mode at 1.1 THz excited through a two-photon process, while two additional peaks (0.86 and 0.36 THz) arise from phase matching of the near-infrared probe beam with co- and counterpropagating THz pump fields, mediated by off-resonant electronic hyperpolarizability. These results demonstrate the crucial role of kinematic effects in shaping THz-induced Kerr response and establish a framework for interpreting nonlinear spectroscopies in complex materials.}, author = {Shen, Chao and Frenzel, Maximilian and Maehrlein, Sebastian F. and Alpichshev, Zhanybek}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {10}, publisher = {American Physical Society}, title = {{Disentangling electronic and ionic nonlinear polarization effects in bulk THz Kerr response}}, doi = {10.1103/1c5k-9z82}, volume = {136}, year = {2026}, } @article{21382, abstract = {The exceptional energy-harvesting efficiency of lead-halide perovskites arises from unusually long photocarrier diffusion lengths and recombination lifetimes that persist even in defect-rich, solution-grown samples. Paradoxically, perovskites are also known for having very short exciton decay times. Here, we resolve this apparent contradiction by showing that key optoelectronic properties of perovskites can be explained by localized flexoelectric polarization confined to interfaces between domains of spontaneous strain. Using birefringence imaging, electrochemical staining, and zero-bias photocurrent measurements, we visualize the domain structure and directly probe the associated internal fields in nominally cubic single crystals of methylammonium lead bromide. We demonstrate that localized flexoelectric fields spatially separate electrons and holes to opposite sides of domain walls, exponentially suppressing recombination. Domain walls thus act as efficient mesoscopic transport channels for long-lived photocarriers, microscopically linking structural heterogeneity to charge transport and offering mechanistically informed design principles for perovskite solar-energy technologies.}, author = {Rak, Dmytro and Lorenc, Dusan and Balazs, Daniel and Zhumekenov, Ayan A. and Bakr, Osman M. and Alpichshev, Zhanybek}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Flexoelectric domain walls enable charge separation and transport in cubic perovskites}}, doi = {10.1038/s41467-026-68660-5}, volume = {17}, year = {2026}, } @article{20405, abstract = {Dielectric breakdown of physical vacuum (Schwinger effect) is the textbook demonstration of compatibility of Relativity and Quantum theory. Although observing this effect is still practically unachievable, its analogue generalizations have been shown to be more readily attainable. This paper demonstrates that a gapped Dirac semiconductor, methylammonium lead-bromide perovskite (MAPbBr3), exhibits analogue dynamic Schwinger effect. Tunneling ionization under deep subgap mid-infrared irradiation leads to intense photoluminescence in the visible range, in full agreement with quasi-adiabatic theory. In addition to revealing a gapped extended system suitable for studying the analogue Schwinger effect, this observation holds great potential for nonperturbative field sensing, i.e., sensing electric fields through nonperturbative light-matter interactions. First, this paper illustrates this by measuring the local deviation from the nominally cubic phase of a perovskite single crystal, which can be interpreted in terms of frozen-in fields. Next, it is shown that analogue dynamic Schwinger effect can be used for nonperturbative amplification of nonparametric upconversion process in perovskites driven simultaneously by multiple optical fields. This discovery demonstrates the potential for material response beyond perturbation theory in the tunneling regime, offering extremely sensitive light detection and amplification across an ultrabroad spectral range not accessible by conventional devices.}, author = {Lorenc, Dusan and Volosniev, Artem and Zhumekenov, Ayan A. and Lee, Seungho and Ibáñez, Maria and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek}, issn = {2330-4022}, journal = {ACS Photonics}, number = {9}, pages = {5220--5230}, publisher = {American Chemical Society}, title = {{Observation of analogue dynamic Schwinger effect and non-perturbative light sensing in lead halide perovskites}}, doi = {10.1021/acsphotonics.5c01360}, volume = {12}, year = {2025}, } @article{20705, abstract = {Optical tweezers are widely used as a highly sensitive tool to measure forces on micron-scale particles. One such application is the measurement of the electric charge of a particle, which can be done with high precision in liquids, air, or vacuum. We experimentally investigate how the trapping laser itself can electrically charge such a particle, in our case a ∼1  μ⁢m SiO2 sphere in air. We model the charging mechanism as a two-photon process which reproduces the experimental data with high fidelity.}, author = {Stöllner, Andrea and Lenton, Isaac C and Volosniev, Artem and Millen, James and Shibuya, Renjiro and Ishii, Hisao and Rak, Dmytro and Alpichshev, Zhanybek and David, Grégory and Signorell, Ruth and Muller, Caroline J and Waitukaitis, Scott R}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {21}, publisher = {American Physical Society}, title = {{Using optical tweezers to simultaneously trap, charge, and measure the charge of a microparticle in air}}, doi = {10.1103/5xd9-4tjj}, volume = {135}, year = {2025}, } @article{19531, abstract = {In standard quantum electrodynamics (QED), the so-called non-minimal (Pauli) coupling is suppressed for elementary particles and has no physical implications. Here, we show that the Pauli term naturally appears in a known family of Dirac materials—the lead-halide perovskites, suggesting a novel playground for the study of analog QED effects. We outline measurable manifestations of the Pauli term in the phenomena pertaining to (i) relativistic corrections to bound states (ii) the Klein paradox, and (iii) spin effects in scattering. In particular, we demonstrate that (a) the binding energy of an electron in the vicinity of a positively charged defect is noticeably decreased due to the polarizability of lead ions and the appearance of a Darwin-like term, (b) strong spin-orbit coupling due to the Pauli term affects the exciton states, and (c) scattering of an electron off an energy barrier with broken mirror symmetry produces spin polarization in the outgoing current. Our study adds to the understanding of quantum phenomena in lead-halide perovskites and paves the way for tabletop simulations of analog Dirac-Pauli equations.}, author = {Shiva Kumar, Abhishek and Maslov, Mikhail and Lemeshko, Mikhail and Volosniev, Artem and Alpichshev, Zhanybek}, issn = {2397-4648}, journal = {npj Quantum Materials}, publisher = {Springer Nature}, title = {{Massive Dirac-Pauli physics in lead-halide perovskites}}, doi = {10.1038/s41535-025-00754-7}, volume = {10}, year = {2025}, } @article{14886, abstract = {It is a basic principle that an effect cannot come before the cause. Dispersive relations that follow from this fundamental fact have proven to be an indispensable tool in physics and engineering. They are most powerful in the domain of linear response where they are known as Kramers-Kronig relations. However, when it comes to nonlinear phenomena the implications of causality are much less explored, apart from several notable exceptions. Here in this paper we demonstrate how to apply the dispersive formalism to analyze the ultrafast nonlinear response in the context of the paradigmatic nonlinear Kerr effect. We find that the requirement of causality introduces a noticeable effect even under assumption that Kerr effect is mediated by quasi-instantaneous off-resonant electronic hyperpolarizability. We confirm this by experimentally measuring the time-resolved Kerr dynamics in GaAs by means of a hybrid pump-probe Mach-Zehnder interferometer and demonstrate the presence of an intrinsic lagging between amplitude and phase responses as predicted by dispersive analysis. Our results describe a general property of the time-resolved nonlinear processes thereby highlighting the importance of accounting for dispersive effects in the nonlinear optical processes involving ultrashort pulses.}, author = {Lorenc, Dusan and Alpichshev, Zhanybek}, issn = {2643-1564}, journal = {Physical Review Research}, number = {1}, publisher = {American Physical Society}, title = {{Dispersive effects in ultrafast nonlinear phenomena: The case of optical Kerr effect}}, doi = {10.1103/PhysRevResearch.6.013042}, volume = {6}, year = {2024}, } @article{17476, abstract = {Lead halide perovskites have recently been reported to demonstrate an exceptionally high nonlinear (Kerr) refractive index n2 of up to 10−8cm2/W in CH3⁢NH3⁢PbBr3. Other researchers, however, observe different, substantially more conservative numbers. In order to resolve this disagreement, the nonlinear Kerr index of a bulk sample of lead halide perovskite was measured directly by means of an interferometer. This approach has many advantages as compared to the more standard z-scan technique. In particular, this method allows studying the induced changes to the refractive index in a time-resolved manner, thus enabling to separate the different contributions to 𝑛2. The extracted 𝑛2 values for CsPbBr3 and MAPbBr3 at 𝜆≈1µ⁢m are 𝑛2=+2.1×10−14cm2/W and 𝑛2=+6×10−15cm2/W, respectively. Hence, these values are substantially lower than what has been indicated in most of the previous reports, implying the latter one should be regarded with great care.}, author = {Lorenc, Dusan and Zhumekenov, Ayan and Bakr, Osman M. and Alpichshev, Zhanybek}, issn = {2475-9953}, journal = {Physical Review Materials}, number = {8}, publisher = {American Physical Society}, title = {{No extraordinary χ(3) in lead-halide perovskites: Placing an upper bound on Kerr nonlinearity by means of time-resolved interferometry}}, doi = {10.1103/PhysRevMaterials.8.085403}, volume = {8}, year = {2024}, } @article{14342, abstract = {We propose a simple method to measure nonlinear Kerr refractive index in mid-infrared frequency range that avoids using sophisticated infrared detectors. Our approach is based on using a near-infrared probe beam which interacts with a mid-IR beam via wavelength-non-degenerate cross-phase modulation (XPM). By carefully measuring XPM-induced spectral modifications in the probe beam and comparing the experimental data with simulation results, we extract the value for the non-degenerate Kerr index. Finally, in order to obtain the value of degenerate mid-IR Kerr index, we use the well-established two-band formalism of Sheik-Bahae et al., which is shown to become particularly simple in the limit of low frequencies. The proposed technique is complementary to the conventional techniques, such as z-scan, and has the advantage of not requiring any mid-infrared detectors.}, author = {Lorenc, Dusan and Alpichshev, Zhanybek}, issn = {0003-6951}, journal = {Applied Physics Letters}, number = {9}, publisher = {AIP Publishing}, title = {{Mid-infrared Kerr index evaluation via cross-phase modulation with a near-infrared probe beam}}, doi = {10.1063/5.0161713}, volume = {123}, year = {2023}, } @article{12723, abstract = {Lead halide perovskites enjoy a number of remarkable optoelectronic properties. To explain their origin, it is necessary to study how electromagnetic fields interact with these systems. We address this problem here by studying two classical quantities: Faraday rotation and the complex refractive index in a paradigmatic perovskite CH3NH3PbBr3 in a broad wavelength range. We find that the minimal coupling of electromagnetic fields to the k⋅p Hamiltonian is insufficient to describe the observed data even on the qualitative level. To amend this, we demonstrate that there exists a relevant atomic-level coupling between electromagnetic fields and the spin degree of freedom. This spin-electric coupling allows for quantitative description of a number of previous as well as present experimental data. In particular, we use it here to show that the Faraday effect in lead halide perovskites is dominated by the Zeeman splitting of the energy levels and has a substantial beyond-Becquerel contribution. Finally, we present general symmetry-based phenomenological arguments that in the low-energy limit our effective model includes all basis coupling terms to the electromagnetic field in the linear order.}, author = {Volosniev, Artem and Shiva Kumar, Abhishek and Lorenc, Dusan and Ashourishokri, Younes and Zhumekenov, Ayan A. and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek}, issn = {1079-7114}, journal = {Physical Review Letters}, keywords = {General Physics and Astronomy}, number = {10}, publisher = {American Physical Society}, title = {{Spin-electric coupling in lead halide perovskites}}, doi = {10.1103/physrevlett.130.106901}, volume = {130}, year = {2023}, } @article{12724, abstract = {We use general symmetry-based arguments to construct an effective model suitable for studying optical properties of lead halide perovskites. To build the model, we identify an atomic-level interaction between electromagnetic fields and the spin degree of freedom that should be added to a minimally coupled k⋅p Hamiltonian. As a first application, we study two basic optical characteristics of the material: the Verdet constant and the refractive index. Beyond these linear characteristics of the material, the model is suitable for calculating nonlinear effects such as the third-order optical susceptibility. Analysis of this quantity shows that the geometrical properties of the spin-electric term imply isotropic optical response of the system, and that optical anisotropy of lead halide perovskites is a manifestation of hopping of charge carriers. To illustrate this, we discuss third-harmonic generation.}, author = {Volosniev, Artem and Shiva Kumar, Abhishek and Lorenc, Dusan and Ashourishokri, Younes and Zhumekenov, Ayan and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek}, issn = {2469-9969}, journal = {Physical Review B}, number = {12}, publisher = {American Physical Society}, title = {{Effective model for studying optical properties of lead halide perovskites}}, doi = {10.1103/physrevb.107.125201}, volume = {107}, year = {2023}, } @article{13251, abstract = {A rotating organic cation and a dynamically disordered soft inorganic cage are the hallmark features of organic-inorganic lead-halide perovskites. Understanding the interplay between these two subsystems is a challenging problem, but it is this coupling that is widely conjectured to be responsible for the unique behavior of photocarriers in these materials. In this work, we use the fact that the polarizability of the organic cation strongly depends on the ambient electrostatic environment to put the molecule forward as a sensitive probe of the local crystal fields inside the lattice cell. We measure the average polarizability of the C/N–H bond stretching mode by means of infrared spectroscopy, which allows us to deduce the character of the motion of the cation molecule, find the magnitude of the local crystal field, and place an estimate on the strength of the hydrogen bond between the hydrogen and halide atoms. Our results pave the way for understanding electric fields in lead-halide perovskites using infrared bond spectroscopy.}, author = {Wei, Yujing and Volosniev, Artem and Lorenc, Dusan and Zhumekenov, Ayan A. and Bakr, Osman M. and Lemeshko, Mikhail and Alpichshev, Zhanybek}, issn = {1948-7185}, journal = {The Journal of Physical Chemistry Letters}, keywords = {General Materials Science, Physical and Theoretical Chemistry}, number = {27}, pages = {6309--6314}, publisher = {American Chemical Society}, title = {{Bond polarizability as a probe of local crystal fields in hybrid lead-halide perovskites}}, doi = {10.1021/acs.jpclett.3c01158}, volume = {14}, year = {2023}, } @article{11737, abstract = {Spin-orbit coupling in thin HgTe quantum wells results in a relativistic-like electron band structure, making it a versatile solid state platform to observe and control nontrivial electrodynamic phenomena. Here we report an observation of universal terahertz (THz) transparency determined by fine-structure constant α≈1/137 in 6.5-nm-thick HgTe layer, close to the critical thickness separating phases with topologically different electronic band structure. Using THz spectroscopy in a magnetic field we obtain direct evidence of asymmetric spin splitting of the Dirac cone. This particle-hole asymmetry facilitates optical control of edge spin currents in the quantum wells.}, author = {Dziom, Uladzislau and Shuvaev, A. and Gospodarič, J. and Novik, E. G. and Dobretsova, A. A. and Mikhailov, N. N. and Kvon, Z. D. and Alpichshev, Zhanybek and Pimenov, A.}, issn = {2469-9969}, journal = {Physical Review B}, number = {4}, publisher = {American Physical Society}, title = {{Universal transparency and asymmetric spin splitting near the Dirac point in HgTe quantum wells}}, doi = {10.1103/PhysRevB.106.045302}, volume = {106}, year = {2022}, } @article{394, abstract = {The valley pseudospin in monolayer transition metal dichalcogenides (TMDs) has been proposed as a new way to manipulate information in various optoelectronic devices. This relies on a large valley polarization that remains stable over long time scales (hundreds of nanoseconds). However, time-resolved measurements report valley lifetimes of only a few picoseconds. This has been attributed to mechanisms such as phonon-mediated intervalley scattering and a precession of the valley pseudospin through electron-hole exchange. Here we use transient spin grating to directly measure the valley depolarization lifetime in monolayer MoSe2. We find a fast valley decay rate that scales linearly with the excitation density at different temperatures. This establishes the presence of strong exciton-exciton Coulomb exchange interactions enhancing the valley depolarization. Our work highlights the microscopic processes inhibiting the efficient use of the exciton valley pseudospin in monolayer TMDs. }, author = {Mahmood, Fahad and Alpichshev, Zhanybek and Lee, Yi and Kong, Jing and Gedik, Nuh}, journal = {Nano Letters}, number = {1}, pages = {223 -- 228}, publisher = {American Chemical Society}, title = {{Observation of exciton-exciton interaction mediated valley Depolarization in Monolayer MoSe2}}, doi = {10.1021/acs.nanolett.7b03953}, volume = {18}, year = {2018}, } @article{391, abstract = {Three-dimensional topological insulators are bulk insulators with Z 2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport. }, author = {Xu, Yishuai and Chiu, Janet and Miao, Lin and He, Haowei and Alpichshev, Zhanybek and Kapitulnik, Aharon and Biswas, Rudro and Wray, Lewis}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Disorder enabled band structure engineering of a topological insulator surface}}, doi = {10.1038/ncomms14081}, volume = {8}, year = {2017}, } @article{393, abstract = {We use a three-pulse ultrafast optical spectroscopy to study the relaxation processes in a frustrated Mott insulator Na2IrO3. By being able to independently produce the out-of-equilibrium bound states (excitons) of doublons and holons with the first pulse and suppress the underlying antiferromagnetic order with the second one, we were able to elucidate the relaxation mechanism of quasiparticles in this system. By observing the difference in the exciton dynamics in the magnetically ordered and disordered phases we found that the mass of this quasiparticle is mostly determined by its interaction with the surrounding spins. }, author = {Alpichshev, Zhanybek and Sie, Edbert and Mahmood, Fahad and Cao, Gang and Gedik, Nuh}, journal = {Physical Review B}, number = {23}, publisher = {American Physical Society}, title = {{Origin of the exciton mass in the frustrated Mott insulator Na2IrO3}}, doi = {10.1103/PhysRevB.96.235141}, volume = {96}, year = {2017}, } @article{392, abstract = {We used femtosecond optical pump-probe spectroscopy to study the photoinduced change in reflectivity of thin films of the electron-doped cuprate La2-xCexCuO4 (LCCO) with dopings of x=0.08 (underdoped) and x=0.11 (optimally doped). Above Tc, we observe fluence-dependent relaxation rates that begin at a temperature similar to the one where transport measurements first show signatures of antiferromagnetic correlations. Upon suppressing superconductivity with a magnetic field, it is found that the fluence and temperature dependence of relaxation rates are consistent with bimolecular recombination of electrons and holes across a gap (2ΔAF) originating from antiferromagnetic correlations which comprise the pseudogap in electron-doped cuprates. This can be used to learn about coupling between electrons and high-energy (ω>2ΔAF) excitations in these compounds and set limits on the time scales on which antiferromagnetic correlations are static.}, author = {Vishik, Inna and Mahmood, Fahad and Alpichshev, Zhanybek and Gedik, Nuh and Higgins, Joshu and Greene, Richard}, issn = {2469-9969}, journal = {Physical Review B}, number = {11}, publisher = {American Physical Society}, title = {{Ultrafast dynamics in the presence of antiferromagnetic correlations in electron doped cuprate La2 xCexCuO4±δ}}, doi = {10.1103/PhysRevB.95.115125}, volume = {95}, year = {2017}, } @article{390, abstract = {In the underdoped copper-oxides, high-temperature superconductivity condenses 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. This CDW coexists and competes with superconductivity (SC) below the transition temperature Tc, suggesting that these two orders 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, τqp, as a function of temperature and magnetic field in underdoped HgBa2CuO4+δ (Hg-1201) and YBa2Cu3O6+x (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that τqp(T) exhibits a local maximum in a small temperature window near Tc 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 Tc 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.}, author = {Hinton, James and Thewalt, E and Alpichshev, Zhanybek and Mahmood, Fahad and Koralek, Jake and Chan, Mun and Veit, Michael and Dorow, Chelsey and Barišić, Neven and Kemper, Alexander and Bonn, Doug and Hardy, Walter and Liang, Ruixing and Gedik, Nuh and Greven, Martin and Lanzara, Alessandra and Orenstein, Joseph}, issn = {2045-2322}, journal = {Scientific Reports}, publisher = {Nature Publishing Group}, title = {{The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors}}, doi = {10.1038/srep23610}, volume = {6}, year = {2016}, } @article{389, abstract = {The coherent optical manipulation of solids is emerging as a promising way to engineer novel quantum states of matter. The strong time-periodic potential of intense laser light can be used to generate hybrid photon-electron states. Interaction of light with Bloch states leads to Floquet-Bloch states, which are essential in realizing new photo-induced quantum phases. Similarly, dressing of free-electron states near the surface of a solid generates Volkov states, which are used to study nonlinear optics in atoms and semiconductors. The interaction of these two dynamic states with each other remains an open experimental problem. Here we use time- and angle-resolved photoemission spectroscopy (Tr-ARPES) to selectively study the transition between these two states on the surface of the topological insulator Bi2Se3. We find that the coupling between the two strongly depends on the electron momentum, providing a route to enhance or inhibit it. Moreover, by controlling the light polarization we can negate Volkov states to generate pure Floquet-Bloch states. This work establishes a systematic path for the coherent manipulation of solids via light-matter interaction.}, author = {Mahmood, Fahad and Chan, Ching and Alpichshev, Zhanybek and Gardner, Dillon and Lee, Young and Lee, Patrick and Gedik, Nuh}, issn = {1745-2481}, journal = {Nature Physics}, pages = {306 -- 310}, publisher = {Springer nature}, title = {{Selective scattering between Floquet Bloch and Volkov states in a topological insulator}}, doi = {10.1038/nphys3609}, volume = {12}, year = {2016}, } @article{388, abstract = {We use ultrafast optical spectroscopy to observe binding of charged single-particle excitations (SE) in the magnetically frustrated Mott insulator Na2IrO3. Above the antiferromagnetic ordering temperature (TN) the system response is due to both Hubbard excitons (HE) and their constituent unpaired SE. The SE response becomes strongly suppressed immediately below TN. We argue that this increase in binding energy is due to a unique interplay between the frustrated Kitaev and the weak Heisenberg-type ordering term in the Hamiltonian, mediating an effective interaction between the spin-singlet SE. This interaction grows with distance causing the SE to become trapped in the HE, similar to quark confinement inside hadrons. This binding of charged particles, induced by magnetic ordering, is a result of a confinement-deconfinement transition of spin excitations. This observation provides evidence for spin liquid type behavior which is expected in Na2IrO3.}, author = {Alpichshev, Zhanybek and Mahmood, Fahad and Cao, Gang and Gedik, Nuh}, journal = {Physical Review Letters}, number = {1}, publisher = {American Physical Society}, title = {{Confinement deconfinement transition as an indication of spin liquid type behavior in Na2IrO3}}, doi = {10.1103/PhysRevLett.114.017203}, volume = {114}, year = {2015}, } @article{387, abstract = {In this Letter we present detailed study of the density of states near defects in Bi 2Se 3. In particular, we present data on the commonly found triangular defects in this system. While we do not find any measurable quasiparticle scattering interference effects, we do find localized resonances, which can be well fitted by theory once the potential is taken to be extended to properly account for the observed defects. The data together with the fits confirm that while the local density of states around the Dirac point of the electronic spectrum at the surface is significantly disrupted near the impurity by the creation of low-energy resonance state, the Dirac point is not locally destroyed. We discuss our results in terms of the expected protected surface state of topological insulators. © 2012 American Physical Society.}, author = {Alpichshev, Zhanybek and Biswas, Rudro and Balatsky, Alexander and Analytis, James and Chu, Jiunhaw and Fisher, Ian and Kapitulnik, Aharon}, journal = {Physical Review Letters}, number = {20}, publisher = {American Physical Society}, title = {{STM imaging of impurity resonances on Bi 2Se 3}}, doi = {10.1103/PhysRevLett.108.206402}, volume = {108}, year = {2012}, }