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Andrei Kryjevski

Publications and source records attributed to Andrei Kryjevski.

At least 19 recordsLinked to original sources
arXiv2025

Phonon-mediated relaxation in nanomaterials from combining Density Functional Theory based non-adiabatic molecular dynamics with Kadanoff-Baym-Keldysh technique

Boltzmann transport equation (BE) is a potent approach to dynamics of a photoexcited (nano)material. BE collision integrals for different relaxation channels can be systematically computed using the Kadanoff-Baym-Keldysh (KBK) formalism (also called NEGF) utilizing the Density Functional Theory (DFT) simulation output. However, accurate description of phonon-mediated relaxation in a general class of (nano)materials that includes exciton effects is still an outstanding problem. The approach proposed here is based on the observation that the non-adiabatic couplings of the DFT-based non-adiabatic molecular dynamics (NAMD) play the role of a time-dependent external potential coupled to the electrons. This allows application of the Keldysh approach resulting in the exciton-phonon BE collision integral, which incorporates exciton wave functions and energies obtained from Bethe-Salpeter equation. As an application, we augment BE with radiative recombination and photon-mediated exciton-exciton transition terms and then use it to calculate photoluminescence (PL) spectrum for several 1.5-$nm$ semiconductor chalcogenide nanocrystals, such as $Cd_{37}Pb_{31}Se_{68},~Cd_{31}Pb_{37}Se_{68},$ which are Janus-type, and for $Pb_{68}Se_{68}.$

cond-mat.mes-hall↗
arXiv2020

Biexciton State Energies from Many-Body Perturbation Theory Based on Density Functional Theory Simulation

We develop a method for computing self-energy of a biexciton state in a semiconductor nanostructure using many-body perturbation theory (MBPT) based on the density functional theory (DFT) simulation. We compute energies of low-energy biexciton states composed of singlet excitons in the chiral single-wall carbon nanotubes (SWCNT), such as (6,2), (6,5) and (10,5). In all cases we find a small decrease in the biexciton gap: -0.045 $eV$ in (6,2), which is 4.59\% of the non-interacting biexciton gap; -0.041 $eV$ in (6,5), which is 4.47\% of the non-interacting gap and -0.036 $eV$ in (10,5), which is 4.31\%.

cond-mat.mes-hall↗
arXiv2020

Electronic structure of semiconductor nanoparticles from stochastic evaluation of imaginary-time path integral

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.

cond-mat.str-el↗
arXiv2017

Multiple Exciton Generation in Chiral Carbon Nanotubes: Density Functional Theory Based Computation

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.

cond-mat.mes-hall↗
arXiv2017

Singlet Fission in Chiral Carbon Nanotubes: Density Functional Theory Based Computation

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.

cond-mat.mes-hall↗
arXiv2014

Enhanced Multiple Exciton Generation in Amorphous Silicon Nanoparticles

Multiple exciton generation (MEG) in nanometer-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms strongly depends on the degree of the core structural disorder as shown by the many-body perturbation theory (MBPT) calculations based on the density functional theory (DFT) 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 ${\rm Si}_{29}{\rm H}_{36}$ 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 of 1.3 up to 1.8 at the photon energy of about $3E_g$, where $E_g$ is the electronic gap, is predicted in these nanoparticles except for the crystalline NW and crystalline film where $QE\simeq 1.$ MEG in the amorphous nanoparticles is enhanced by the electron localization due to structural disorder. Combined with the lower gaps, the nanometer-sized amorphous silicon NWs and films are predicted to have effective carrier multiplication within the solar spectrum range.

cond-mat.mes-hall↗
arXiv2012

$η/s$ of the Normal Phase of Unitary Fermi Gas from $\varepsilon$ Expansion

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

cond-mat.quant-gas↗
arXiv2009

Mitigating the sign problem for non-relativistic fermions on the lattice

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.

hep-lat↗
arXiv2007

Effective Lagrangian of unitary Fermi gas from $\varepsilon$ expansion

Using $\varepsilon$ expansion technique proposed in \cite{Nishida:2006br} we derive an effective Lagrangian (Ginzburg-Landau-like functional) of the degenerate unitary Fermi gas to the next-to-leading (NLO) order in $\varepsilon.$ It is demonstrated that for many realistic situations it is sufficient to retain leading order (LO) 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. The resulting surface free energy is about four times larger than the value previously quoted in the literature.

nucl-th↗
arXiv2007

Spontaneous Superfluid Current Generation in CFL at Nonzero Strange Quark Mass

We find that for large enough strange quark mass, $m_s^2/4 μΔ>2/3(1-0.024),$ kaon condensed CFL phase of asymptotically dense 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 CFL$K^0$ ground state. The total baryon and hypercharge currents still vanish in the ground state. We find that CFL$K^0$ and the new state are separated by the first order phase transition.

hep-ph↗
arXiv2006

Goldstone boson currents in a kaon condensed CFL phase

We study the stability of the kaon condensed color-flavor locked (CFL) phase of dense quark matter with regard to the formation of a non-zero Goldstone boson current. In the kaon condensed phase there is an electrically charged fermion which becomes gapless near μ_s^(1) \simeq 1.35Δand a neutral fermion which becomes gapless near μ_s^(2)\simeq 1.61Δ. Here, μ_s=m_s^2/(2p_F) is the shift in the Fermi energy due to the strange quark mass m_s and Δis the gap in the chiral limit. The transition to the gapless phase is continuous at μ_s^(1) and first order at μ_s^(2). We find that the magnetic screening masses are real in the regime μ_s< μ_s^(2), but some screening masses are imaginary for μ_s> μ_s^(2). We show that there is a very weak current instability for μ_s>μ_s^(1) and a more robust instability in a small window near μ_s^(2). We also show that in the Goldstone boson current phase all components of the magnetic screening mass are real. There is a range of values of μ_s below 2Δin which the magnetic gluon screening masses are imaginary but the phase is stable with respect to electrically neutral fluctuations of the gauge field.

hep-ph↗
arXiv2006

Polarized fermions in the unitarity limit

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)*(1-0.467ε), 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.

cond-mat.other↗
arXiv2004

CFL Phase of High Density QCD at Non Zero Strange Quark Mass

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 $m_s^2/4μΔ.$ We demonstrate that the CFL$K^0$ state has lower free energy than the symmetric CFL state for $0<m_s^2/4μΔ<2/3$. We also calculate the spectrum of the fermionic quasiparticle excitations about the kaon condensed ground state in the regime $m_s^2/4μΔ\sim 1$ and find that $(m_s^2/4μΔ)_{crit}=2/3$ for the CFL-gCFL phase transition, the leading order result reported in [1], is not modified.

hep-ph↗
arXiv2004

An Effective Theory for Baryons in the CFL Phase

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.

hep-ph↗
arXiv2004

New phases in CFL quark matter

We consider order $α_s$ 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 $α_s$. In particular we find two new phases of CFL quark matter possibly relevant for the real world, for which the QCD angle theta-bar equals pi/2.

hep-ph↗
arXiv2003

Heavy Quark $\bar{q}q$ Matrix Elements in the Nucleon from Perturbative QCD

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 ${\cal{O}}{(α^3)}.$ 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.

hep-ph↗
arXiv2003

Charge Neutrality of the Color-Flavor Locked Phase from the Low Energy Effective Theory

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.

hep-ph↗