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In the Kohn-Sham orbital basis imaginary-time path integral for electrons in a semiconductor nanoparticle has a mild Fermion sign problem and is amenable to evaluation by the standard stochastic methods. This is evidenced by the simulations of silicon hydrogen-passivated nanocrystals, such as $Si_{35}H_{36},~Si_{87}H_{76},~Si_{147}H_{100}$ and $Si_{293}H_{172},$ which contain $176$ to $1344$ valence electrons and range in size $1.0 - 2.4~nm$, utilizing the output of density functional theory simulations. We find that approximating Fermion action with just the leading order polarization term results in a positive-definite integrand in the functional integral, and that it is a good approximation of the full action. We compute imaginary-time electron propagators in these nanocrystals and extract the energies of low-lying electron and hole levels. Our quasiparticle gap predictions agree with the results of high-precision calculations using $G_0W_0$ technique. This formalism can be extended to calculations of more complex excited states, such as excitons and trions.

We use the Boltzmann transport equation (BE) to study time evolution of a photoexcited state, including phonon-mediated exciton relaxation, multiple exciton generation (MEG), and energy-transfer processes. BE collision integrals are derived using Kadanoff–Baym–Keldysh many-body perturbation theory (MBPT) based on density functional theory (DFT) simulations, including exciton effects. We apply the method to a nanostructured p–n junction composed of a 1 nm hydrogen-terminated Si quantum dot (QD) doped with two phosphorus atoms (Si36P2H42) adjacent to the (6, 2) single-wall carbon nanotube (CNT) with two chlorine atoms per two unit cells adsorbed to the surface. We find that an initial excitation localized on either the QD or CNT evolves into a transient charge-transfer (CT) state where either electron or hole transfer has taken place. The CT state lifetime is about 40 fs. Also, we study MEG in this system by computing internal quantum efficiency (QE), which is the number of excitons generated from an absorb...

Methylammonium lead iodide perovskite (MAPbI3) is a promising material for photovoltaic devices. A modification of MAPbI3 into confined nanostructures is expected to further increase efficiency of solar energy conversion. Photoexcited dynamic processes in a MAPbI3 quantum dot (QD) have been modeled by many-body perturbation theory and nonadiabatic dynamics. A photoexcitation is followed by either exciton cooling (EC), its radiative (RR) or nonradiative recombination (NRR), or multiexciton generation (MEG) processes. Computed times of these processes fall in the order of MEG < EC < RR < NRR, where MEG is on the order of a few femtoseconds, EC is in the picosecond range, while RR and NRR are on the order of nanoseconds. Computed time scales indicate which electronic transition pathways can contribute to increase in charge collection efficiency. Simulated mechanisms of relaxation and their rates show that quantum confinement promotes MEG in MAPbI3 QDs.

Multiple exciton generation (MEG) in nanometer-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbative many-body quantum mechanics calculations based on the density functional theory simulations. Working to the second order in the electron–photon coupling and in the screened Coulomb interaction, we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, in the Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, simple cubic three-dimensional arrays constructed from these QDs, crystalline and amorphous NWs, and quasi two-dimensional silicon nanofilms, also both crystalline and amorphous. Efficient MEG with QE ranging from 1.3 up to 1.8 at the photon energy of about 3Eg, where Eg is the electronic gap, is predicted in these nanoparticles except for the crystalline NW and crystalline film where QE ≃ 1. MEG in t...

Intrinsic constraints on efficient photoluminescence (PL) from smaller alkene-capped silicon nanocrystals (SiNCs) put limits on potential applications, but the root cause of such effects remains elusive. Here, plasma-synthesized colloidal SiNCs separated into monodisperse fractions reveal an abrupt size-dependent partitioning of multilevel PL relaxation, which we study as a function of temperature. Guided by theory and simulation, we explore the potential role of resonant phonon interactions with “minigaps” that emerge in the electronic density of states (DOS) under strong quantum confinement. Such higher-order structures can be very sensitive to SiNC surface chemistry, which we suggest might explain the common implication of surface effects in both the emergence of multimodal PL relaxation and the loss of quantum yield with decreasing nanocrystal size. Our results have potentially profound implications for optimizing the radiative recombination kinetics and quantum yield of smaller ligand-passivated SiNCs.

We use Boltzmann transport equation (BE) to study time evolution of a photo-excited state in a nanoparticle including phonon-mediated exciton relaxation and the multiple exciton generation (MEG) processes, such as exciton-to-biexciton multiplication and biexciton-to-exciton recombination. BE collision integrals are computed using Kadanoff-Baym-Keldysh many-body perturbation theory (MBPT) based on density functional theory (DFT) simulations, including exciton effects. We compute internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon in the course of the relaxation. We apply this approach to chiral single-wall carbon nanotubes (SWCNTs), such as (6,2), and (6,5). We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the $2 E_g$ energy threshold and with QE reaching $\sim 1.6$ at about $3 E_g,$ where $E_g$ is the electronic gap., Comment: 20 pages, 6 figures. 7/20/2017: arXiv admin note: substantial text overlap with arXiv:1703.04693. In v2 text changed to avoid text overlap with our earlier work on the same subject arXiv:1703.04693

Singlet fission (SF) process, where a singlet exciton decays into a pair of spin one exciton states which are in the total spin singlet state, is one of the possible channels for multiple exciton generation (MEG). In chiral single-wall carbon nanotubes (SWCNTs) efficient SF is present within the solar spectrum energy range which is shown by the many-body perturbation theory (MBPT) calculations based on the density functional theory (DFT) simulations. We calculate SF exciton-to-biexction decay rates ${\rm R}_{1\to 2}$ and biexciton-to-exction rates ${\rm R}_{2\to1}$ in the (6,2), (6,5), (10,5) SWCNTs, and in (6,2) SWCNT functionalized with Cl atoms. Within the solar energy range, we predict ${\rm R}_{1\to2}\sim 10^{14}-10^{15}~s^{-1}$, while biexciton-to-exction recombination is weak with ${\rm R}_{2\to 1}/{\rm R}_{1\to 2}\leq 10^{-2}.$ SF MEG strength in pristine SWCNTs varies strongly with the excitation energy, which is due to highly non-uniform density of states at low energy. However, our results for (6,2) SWCNT with chlorine atoms adsorbed to the surface suggest that MEG in the chiral SWCNTs can be enhanced by altering the low-energy electronic states via surface functionalization.

We use the density functional theory (DFT) combined with the many-body perturbation theory to derive expressions for the rates of the optical photon→exciton and photon→bi-exciton processes in nanoparticles, and for quantum efficiency, all to the leading order in the screened Coulomb interaction between Kohn–Sham quasiparticles. Also, we calculate exciton→bi-exciton rates due to the impact ionisation (II) mechanism in Si29H36 quantum dots (QDs) with both crystalline and amorphous core structures, and in quasi-one dimensional (1-D) arrays constructed from these QDs. We observe significant dependence of the carrier multiplication rates on the structure’s morphology and structural disorder. Amorphous silicon QD arrays are predicted to have more efficient bi-exciton generation rates as a function of exciton energy compared to their crystalline counterparts, and the isolated QDs of both kinds.

Using the $\ensuremath{\varepsilon}$-expansion technique, we compute $\ensuremath{\eta}/s$, where $\ensuremath{\eta}$ is the shear viscosity and $s$ is the entropy density of the normal phase of unitary Fermi gas in $d=4\ensuremath{-}\ensuremath{\varepsilon}$ dimensions to leading order (LO) in $\ensuremath{\varepsilon}$. We use the kinetic theory approach and solve transport equations for a medium perturbed by a shear hydrodynamic flow. The collision integrals are calculated to ${\ensuremath{\varepsilon}}^{2}$, which is LO. The LO result is temperature independent with $\ensuremath{\eta}/s\ensuremath{\simeq}(0.11/{\ensuremath{\varepsilon}}^{2})(\ensuremath{\hbar}/{k}_{B})$. The $d=3$ prediction for $\ensuremath{\eta}/s$ exceeds the $\ensuremath{\hbar}/4\ensuremath{\pi}{k}_{B}$ bound by a factor of about $1.4$.

We use Density Functional Theory (DFT) combined with the many body perturbation theory to calculate spatially non-uniform field response in several hydrogen-passivated silicon nanosystems, such as Si29H36 quantum dots (QDs) with crystalline and amorphous structures, the quasi-one dimensional (1-D) arrays constructed from these QDs. We model response to a spatially non uniform time dependent electric field that vanishes on every other QD. Current density induced by such electric field configurations serves as a characteristic of QD coupling in the arrays and, in particular, of the inter QD exciton transport. We observe rapid decay of the QD-QD coupling with the increase of inter QD separation and its complete diminishing at the QD-QD distance comparable with the size of the QD.

Efficient multiple exciton generation (MEG) in chiral single-wall carbon nanotubes (SWCNTs) is present within the solar spectrum range as shown by the many-body perturbation theory calculations combined with the density functional theory simulations. To describe the impact ionization process, we calculate exciton-to-biexciton decay rates R1→2 and biexciton-to-exciton rates R2→1 in the (6,2) and (10,5) SWCNTs. Within the solar energy range, we predict R1→2 ∼ 1014 s-1, while biexciton-to-exciton recombination is weak with R2→1/R1→2 ≤ 10-2. Also we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, for which we find QE ≃ 1.2-1.6, that is 20%-60%. However, MEG strength in these SWCNTs varies strongly with the excitation energy due to highly non-uniform density of states at the low energy. We hypothesize that MEG efficiency in the chiral SWCNTs can be enhanced by altering the low-energy electronic spectrum via surface functionalization, or by mixing SWCNTs of different chiralities.

Using the $\ensuremath{\varepsilon}$ expansion proposed in Y. Nishida and D. T. Son [Phys. Rev. Lett. 97, 050403 (2006)], we calculate the density correlation function and the dynamic structure function of the unitary Fermi gas at zero temperature to next-to-leading order in $\ensuremath{\varepsilon}$ for excitation energies below the quasiparticle threshold.

Using $\ensuremath{\epsilon}$ expansion technique proposed by Nishida and Son [Phys. Rev. Lett. 97, 050403 (2006)] we derive an effective Lagrangian (Ginzburg-Landau-type functional) of the degenerate unitary Fermi gas to the next-to-leading order in $\ensuremath{\epsilon}$. It is demonstrated that for many realistic situations it is sufficient to retain leading order terms in the derivative expansion. The functional is used to study vortex structure in the symmetric gas, and interface between normal and superfluid phases in the polarized gas. Typical vortex size, ${r}_{0}{k}_{F}=0.92$, where ${k}_{F}$ is Fermi momentum, agrees well with results of previous work [A. Bulgac and Y. Yu, Phys. Rev. Lett. 91, 190404 (2003)]. Surface free energy is about 3 times larger than the value previously quoted in the literature.

We find that for a large enough strange quark mass, ${m}_{s}^{2}/4\ensuremath{\mu}\ensuremath{\Delta}g2/3(1\ensuremath{-}0.023)$ ($\ensuremath{\mu}$ is the quark number chemical potential, $\ensuremath{\Delta}$ is the superconducting gap), the kaon condensed color flavor locked (CFL) phase of asymptotically dense strongly interacting 3 flavor quark matter is unstable with respect to spontaneous generation of currents of Nambu Goldstone bosons due to spontaneous breaking of baryon number symmetry and hypercharge symmetry in the $\mathrm{CFL}{K}^{0}$ ground state. The total baryon and hypercharge currents vanish in the ground state. We find that $\mathrm{CFL}{K}^{0}$ and the new state are separated by a first order phase transition. The result is derived in the mean field approximation of high density effective theory with electromagnetic interactions turned off.

We study the stability of the kaon condensed color-flavor locked (CFL) phase of dense quark matter with regard to the formation of a nonzero Goldstone boson current. In the kaon condensed phase there is an electrically charged fermion which becomes gapless near {mu}{sub s}{sup (1)}{approx_equal}1.35{delta} and a neutral fermion which becomes gapless near {mu}{sub s}{sup (2)}{approx_equal}1.61{delta}. Here, {mu}{sub s}=m{sub s}{sup 2}/(2p{sub F}) is the shift in the Fermi energy due to the strange quark mass m{sub s} and {delta} is the gap in the chiral limit. The transition to the gapless phase is continuous at {mu}{sub s}{sup (1)} and first order at {mu}{sub s}{sup (2)}. We find that the magnetic screening masses are real in the regime {mu}{sub s} {mu}{sub s}{sup (2)}. We show that there is a very weak current instability for {mu}{sub s}>{mu}{sub s}{sup (1)} and a more robust instability in a small window near {mu}{sub s}{sup (2)}. We show that in the Goldstone boson current phase all components of the magnetic screening mass are real. There is a range of values of {mu}{sub s} below 2{delta} in which the magnetic gluon screening masses are imaginary but themore » phase is stable with respect to electrically neutral fluctuations of the gauge field.« less

We consider a polarized Fermi gas in the unitarity limit. Results are calculated analytically up to next-to-leading order in an expansion about d=4 spatial dimensions. We find a first order transition from superfluid to normal phase. The critical chemical potential asymmetry for this phase transition is delta_mu_c= 2/(mu epsilon)*(1-0.467\epsilon), where epsilon=4-d is the expansion parameter and 'mu' is the average chemical potential of the two fermion species. Stability of the superfluid phase in the presence of supercurrents is also studied., Comment: 5 pages, 5 figures, LaTeX2e; minor changes, note added at the end, to be published in PRA

We consider $O({\ensuremath{\alpha}}_{s})$ corrections to the squared masses of the pseudo-Goldstone excitations about the ground state of dense quark matter. We show that these contributions tend to destabilize the vacuum, leading to a surprisingly complex phase structure for quark matter as a function of quark mass, even for small ${\ensuremath{\alpha}}_{s}$. In particular we find two new phases of CFL quark matter possibly relevant for the real world, for which ${\overline{\ensuremath{\theta}}}_{\mathrm{Q}\mathrm{C}\mathrm{D}}=\ensuremath{\pi}/2$.

We compute free energy of quark matter at asymptotically high baryon number density in the presence of non zero strange quark mass including dynamics of pseudo Nambu-Goldstone bosons due to chiral symmetry breaking, extending previously existing analysis based on perturbative expansion in $m_s^2/4\mu\Delta.$ We demonstrate that the CFL$K^0$ state has lower free energy than the symmetric CFL state for $0

The scalar heavy quark content of the nucleon, ${\cal M}_q = \bra{N} m_q \bar q q \ket{N},$ is relevant for computing the interaction of dark matter candidates with ordinary matter, while ${\cal M}_s$ is important for predicting the properties of dense matter. We compute ${\cal M}_q$ in perturbative QCD to ${\cal{O}}{(\alpha^3)}.$ As one goes from ${\cal M}_t$ to ${\cal M}_c$ the leading order contribution decreases as the number of light quarks is dropping, while the radiative corrections grow and are all positive. The leading source of uncertainty in the calculation is due to the poorly known value of ${\cal M}_s.$ A related calculation suggests that a large value for ${\cal M}_s$ may be reasonable., Comment: 8 pages, 3 figures; a typo in the results corrected

We investigate the issue of charge neutrality of the CFL$K^0$ phase of dense quark matter using the low energy effective theory of high density QCD. We show that the local electric and color charge neutrality of the ground state in a homogeneous color superconducting medium follows from its dynamics. We also consider the situation of a spatially inhomogeneous medium, such as may be found in a neutron star core. We find that spatial inhomogeneity results in the generation of electric fields, and positrons/electrons may be present in the ground state. We estimate the concentration of charged leptons in the ground state to be $n_{e}\sim{10^2}{cm}^{-3}$ and consider their influence on the opacity of the medium with respect to the modified photons., Comment: typos corrected, this version to appear in PRD

Previous work on high-density QCD suggests that the ground state configuration in the presence of a non-zero neutrino chemical potential consists of CFL matter plus a homogenous condensate of neutral and positive kaons. We consider here the stability of this homogenous configuration toward the production of a charge-separated mixed phase, and find that the system is indeed stable against the production of such a heterogenous configuration. We identify the critical value of the fine structure constant which would allow for phase separation. We also derive dispersion relations for the low-lying excitations above the kaon-condensed ground state and discuss their possible phenomenological implications.

We show that a central $1/r^n$ singular potential (with $n\geq 2$) is renormalized by a one-parameter square-well counterterm; low-energy observables are made independent of the square-well width by adjusting the square-well strength. We find a closed form expression for the renormalization-group evolution of the square-well counterterm., 15 pages LaTex, 5 eps figures, error in figures and text corrected

Semiconductor nanomaterials allow tuning their optical and electronic properties as a function of their morphology and structural disorder. Quantum dots (QD), arrays of quantum dots, and nanowires demonstrate strong dependence of absorption spectra on the morphological variables. Organization of QDs into arrays leads to increase in the oscillator strengths and overall brightening of the optical transitions. Electronic structure calculations support search of efficient nanomaterials in several amorphous and crystalline silicon nanosystems.

We study the fermion sign problem in a theory of non-relativistic fermions with a spin-independent repulsive interaction. We work in polar co-ordinates in momentum space, which makes it straightforward to keep only the low-energy degrees of freedom close to the Fermi surface. This is sufficient for the purpose of calculating many physically important low-energy observables. We find indications that the sign problem in this effective theory will be weaker than in the full theory, so low-energy properties of the theory could be calculated by modifying the action to make it positive semi-definite and including reweighting factors in the observables. We discuss suitable modifications of the action, and describe a possible lattice realization of the polar momentum space formulation of the theory., 20 pages; new appendix and clarification of range of validity

We study the effective field theory for fermions in the color-flavor locked (CFL) phase of high density QCD. The effective theory contains a flavor nonet of baryons interacting with a nonet of pseudoscalar Goldstone bosons as well as a singlet scalar Goldstone boson. The theory is similar to chiral perturbation theory in the one-baryon sector. We explain how to incorporate quark mass terms and study the spectrum as a function of the strange quark mass. Without meson condensation gapless baryon modes appears if the strange quark mass exceeds a critical value m_s^2/(2p_F)=Delta, where p_F is the Fermi momentum and Delta is the gap in the chiral limit. We show that kaon condensation leads to a rearrangement of the baryon spectrum and shifts the critical strange quark mass for the appearance of a gapless mode to higher values., Comment: 12 pages, 2 figures; corrected Fig. 2; clarified eq.(23)